Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tps.2020.3011469

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1585/pfr.14.2406008

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DOI： 10.1585/pfr.14.2406015

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DOI： 10.1585/pfr.13.3406107

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

We experimentally studied an intense Smith-Purcell (SP) radiation based on a cylindrical corrugated waveguide driven by a weakly relativistic electron beam. The corrugated waveguide has a rectangular periodic structure, and an annular electron beam with energy less than 100 keV and current of the order of 100 A is injected. We observe both intense surface-wave radiation at 25 GHz and intense SP radiations up to 150 GHz. The observed SP radiations are enhanced by the sixth and fourth temporal harmonics of the surface wave. The characteristics of the enhanced SP radiation depend on the periodic number of the structure. In the experiment with the use of the 60-period structure, the observed power of the SP radiation at the fourth harmonic is higher than that of the radiation at the sixth harmonic. In the case of the 30 period-structure, the power level of the radiation at the sixth harmonic is comparable to that at the fourth harmonic.

DOI： 10.1063/1.5035139

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We studied the operation of an oversized backward wave oscillator driven by an electron beam with energy less than 100 keV for terahertz radiation. Cylindrical slow wave structures (SWSs) with rectangular corrugations and annular electron beam are used to generate broadband terahertz radiation. The starting conditions for intense terahertz radiation are examined by varying the beam parameter and the SWS length. Operation frequencies in the range of 0.26-0.34 THz and 0.31-0.39 THz are obtained using two types of SWSs with different corrugation amplitudes. The obtained maximum output power is at the kilowatt level. Characteristics of the beam instabilities in the terahertz-SWSs are examined by comparison between the experimental and numerical results.

DOI： 10.1109/TPS.2018.2796559

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We numerically and experimentally studied the operation of a W-band (75-110 GHz) surface-wave oscillator driven by the electron beam energy of less than 100 keV. Two numerical methods of the real wavenumber analysis and the saddle point analysis are studied to examine the operation of the oscillator. We analyzed the Cherenkov instability for the interaction of the space charge modes and the structure modes EH01. We showed that the instabilities are of absolute instabilities when the oscillator operates around the p-point region. In the experiment, radiations due to the absolute instability are detected around the pi-point region when the length of the slow-wave structure (SWS) is short enough. On the other hand, the absolute instability generates no meaningful radiation in the p-point region for the long length of SWS.

DOI： 10.1109/TPS.2016.2633550

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical Engineers of Japan  

We studied an oversized W-band (75-110 GHz) surface wave oscillator driven by a weakly relativistic electron beam. Surface wave is formed on slow wave structure (SWS) with rectangular corrugations and has an upper-cutoff frequency of about 100 GHz. To excite the surface wave, an annular electron beam is generated by a disc type cold cathode with beam energy between about 10 and 80 keV. The oscillator is driven in the backward wave oscillator (BWO) region for beam energy less than 30 keV and in the traveling wave tube (TWT) region for beam energy more than 30 keV. In the BWO region, microwaves are generated due to Cherenkov interaction between electron beam and surface wave in the W-band. In the TWT region, two types of microwaves are generated. One is due to the W-band surface wave as in the case of the BWO region. The other is due to higher order modes above 140 GHz.

DOI： 10.1541/ieejfms.137.165

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Beam interactions with surface waves and higher order modes in oversized G-band slow-wave structures (SWSs) are studied in a coaxial SWS. The coaxial SWS reduces the number of higher order mode and eases the mode competition problem. The SWS is excited by an annular electron beam. The slow cyclotron (SC) interaction as well as the Cherenkov interaction occurs due to 3-D beam perturbations of the electron beam. Growth rates for the surface waves are larger than those for the higher order modes and increase with increasing beam current. The SC interaction merges with the Cherenkov interaction for a sufficiently high beam current and a relatively low magnetic field. And the merged growth rate also increases with the beam current.

DOI： 10.1109/TPS.2016.2514380

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have studied an atmospheric-pressure plasma generated using a quartz tube, helium gas, and a copper foil electrode by the application of a radio-frequency high voltage. The atmospheric-pressure plasma, which is generated in the form of a bullet, is released as a plume into the atmosphere. To study the properties of plasma plume formation, we examine the relationships among the plasma plume length and charge and the applied voltage waveform. The dependences of the waveform on the plasma plume length and charge are not clear. However, the plasma plume length increases with increasing plasma plume charge in laminar flow. Consequently, we find that the plasma plume length depends on the plasma plume charge becoming an energy source so that electric charge travels the plasma.

DOI： 10.1109/TPS.2015.2506784

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

A W-band (75 - 110 GHz) oversized surface wave oscillator driven by weakly relativistic electron beams with energy in the range of 10 - 80 keV is studied. Rectangular corrugations are used as slow-wave structures (SWS) having surface waves with an upper cutoff frequency of approximately 100 GHz (W-band). Uniformly distributed annular electron beams are generated by a disk-type cold cathode and then are injected into theW-band oscillator. A longer SWS length causes the oscillator to function in both backward wave oscillator (BWO) and travelling wave tube (TWT) operations, and no meaningful oscillation occurs at the π-point or the Bragg condition. When the SWS length is short enough, oscillation occurs in all regions: BWO, π-point and TWT. The operations of the oscillator are strongly affected by the structure length. The maximum radiation power is estimated to be approximately 20kW with the figure of merit of about 2 � 102MW.GHz2.

DOI： 10.1585/pfr.11.2406085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

We present a measurement method of the plasma current and density in an atmospheric-pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying a high RF voltage. The plasma in the form of a bullet is released as a plume or jet into the atmosphere. To study the characteristics of the atmospheric-pressure plasma, the plasma current is measured using a current probe, and the drift velocity of plasma plume is measured using a photodetector. The current of the plasma plume is estimated by subtracting the ground line current from the power line current in the circuit. The density of plasma plume n is estimated from the plasma plume current I and the drift velocity v as I = envS, where S is the cross section of plasma plume. The density of the released plasma into the atmosphere is estimated as similar to 10(18) m(-3) by the method. (C) 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

DOI： 10.1002/tee.22127

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

An oversized G-band backward wave oscillator (BWO) driven by a weakly relativistic electron beam less than 100 kV is studied. Rectangular corrugations are used as the slow-wave structure (SWS), which have surface waves with upper cutoff frequencies of above 150 GHz [G-band (140-220 GHz)]. In this paper, we examine the starting conditions for intense G-band BWO operation by varying the beam parameters and SWS lengths. It is shown that a starting energy exists for the G-band BWO and that the starting energy is more critical than the starting current in the oversized BWO. Above the starting energy, very intense output powers on the order of 10 kW are obtained. The oscillation-starting condition for such intense operation may play an important role in high-intensity terahertz wave sources.

DOI： 10.1109/TPS.2015.2424993

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Periodical corrugations structured on a cylindrical conductor have cylindrical surface waves (CSWs), which are reflected at the corrugation ends and form a CSW-resonator. In this paper, intense radiations in terahertz region based on the CSW-resonator are reported. The CSW-resonators with upper cut off frequencies in the modern IEEE G-band (110-300 GHz) are excited by a coaxially injected annular beam in a weakly relativistic region less than 100 kV. It is shown that there exists an oscillation starting energy for the CSW-resonator. Above the starting energy, very intense terahertz radiations on the order of kW are obtained. The operation frequencies in the range of 166-173 GHz and 182-200 GHz are obtained using two types of CSW-resonator with the different corrugation amplitude. Electromagnetic properties of the CSW-resonator can be controlled by the artificial structure and may play an important role in high-intensity terahertz generations and applications. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4931371

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have studied atmospheric pressure plasma generated using a quartz tube, helium gas, and a foil electrode by applying RF high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. The helium gas flowing out of quartz tube mixes with air, and the flow channel is composed of the regions of flowing helium gas and air. The plasma plume length is equivalent to the reachable distance of flowing helium gas. Although the amount of helium gas on the flow channel increases by increasing the inner diameter of quartz tube at the same gas flow velocity, the plasma plume length peaks at around 8m/s of gas flow velocity, which is the result that a flow of helium gas is balanced with the amount of gas. The plasma plume is formed at the boundary region where the flow of helium gas is kept to the wall of the air. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4921657

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Smith-Purcell (SP) radiations of weakly relativistic oversized backward wave oscillator (BWO) are examined experimentally. Our oversized BWOs are driven by weakly relativistic electron beams less than 100 keV. New element of this work is an enhancement of SP radiation in the oversized BWO using the bunching section. An annular beam is self-bunched in the K-band bunching section and then is injected into the oversized K-band BWO. SP radiations in the BWO are strongly enhanced in the U-band, which is about twice of the bunching frequency. Moreover, the oversized BWO is developed to G-band. By using the G-band bunching, the SP radiations are enhanced in the H-band. The enhancement of SP-radiation in the oversized BWO by the self-bunched electron beam is attractive to develop THz wave sources.

DOI： 10.1109/PLASMA.2014.7012770

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：John Wiley and Sons Inc.  

We present atmospheric pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying a radio frequency high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. To study the formation of the plasma plume, the dependences of the plasma plume charge and the plasma plume length on the gas flow rate are examined. In a quartz tube with inner diameter 1.5 mm, which has more helium than air in the cross section of the flow channel, the distributions of the length and charge on the gas flow rate show similar tendencies. Consequently, when more helium gas is present than air, the travel length of the plasma plume correlates with the plasma plume charge.

DOI： 10.1002/tee.22039

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

Smith-Purcell (SP) radiations of weakly relativistic oversized backward wave oscillator (BWO) are examined experimentally. A bunching section composed of a coaxial slow wave structure with inner K-band corrugation is placed upstream from the oversized BWO. An annular beam less than 100 kV is self-bunched in the bunching section and then excites the oversized BWO. SP radiations are examined by using 25.5- and 45.0-GHz oversized BWOs. The beam is too far away from the walls of the oversized BWO for SP radiation to occur without the self-bunching. When the corrugation parameters of the bunching section are the same as those of the oversized BWO, enhanced SP radiations in the frequency region above the surface wave are observed even when the beam propagates outside the interaction region of the surface wave.

DOI： 10.3938/jkps.65.1196

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this paper, the electromagnetic properties of a beam free trapezoidally corrugated slow wave structure (TCSWS) are numerically and experimentally investigated. The dimensions of the TCSWS are selected so that it can be deployed in X-band backward wave oscillators (BWOs). The theoretical dispersion characteristics for the axisymmetric TM01-TM03 modes are obtained by solving the dispersion equation numerically. In order to verify the numerical calculation, a cold test is conducted using a 6-period TCSWS cavity and a vector network analyzer (VNA). The cavity is excited by a small needle-type antenna and frequency of the VNA is swept over the passbands. The dispersion characteristics are obtained from frequency values of the discrete resonant peaks of reflection measurement of each TM mode. Experimental results show an excellent agreement with numerical ones. The numerical and cold test results reveal that TCSWS can be used as an alternative to sinusoidally corrugated slow wave structure in BWO experiments with an advantage of ease of fabrication.

DOI： 10.1109/TPS.2014.2308434

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have studied atmospheric pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying RF high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. To study the properties of the plasma plume, the plasma plume current is estimated from the difference in currents on the circuit, and the drift velocity is measured using a photodetector. The relation of the plasma plume density n(plu), which is estimated from the current and the drift velocity, and the gas flow velocity v(gas) is examined. It is found that the dependence of the density on the gas flow velocity has relations of n(plu) proportional to log(v(gas)). However, the plasma plume density in the laminar flow is higher than that in the turbulent flow. Consequently, in the laminar flow, the density increases with increasing the gas flow velocity. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4873384

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7566/JPSCP.1.015084

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Korean Physical Society  

Beam interactions with surface waves and higher-order modes in an oversized backward wave oscillator (BWO) are studied. In addition to the well-known Cherenkov interaction, the slow cyclotron interaction occurs due to transverse perturbations of the electron beam. The Cherenkov interaction dominates the slow cyclotron interaction. Growth rates of both the interactions for the higher order modes are small compared with those for the surface-wave modes in an oversized BWO. The coaxial slow-wave structure exhibits a reduced number of higher-order modes, which consequently reduces the mode competition problem and improves beam interactions with higher-order modes. For higher values of beam currents, the slow cyclotron wave grows at a faster rate than the Cherenkov waves.

DOI： 10.3938/jkps.65.1209

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

We studied a G-band oversized backward wave oscillator (BWO) driven by a weakly relativistic electron beam of less than 100 kV. Rectangular corrugations are used as slow-wave structures having surface waves with upper cutoff frequencies above 150 GHz (G-band). We examine how dispersion characteristics of surface waves are affected by accuracy in machining the corrugation amplitude, width, and period length. Of these, accuracy in the amplitude has the largest effect. Uniformly distributed annular electron beams are generated by a disk-type cold cathode and injected into the G-band BWO. G-band BWO operations in 137-173 GHz and above 173 GHz are achieved by changing the corrugation amplitude. The radiation patterns are fairly broad, and the estimated radiation power is at kW level. © 2014 The Japan Society of Plasma Science and Nuclear Fusion Research.

DOI： 10.1585/pfr.9.3406032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

Surface waves in oversized G-band slow-wave structure with rectangularly corrugated wall are analyzed numerically. The inner corrugation generates cylindrical surface wave. The outer corrugation also generates transverse magnetic surface wave. The upper cut-offs of surface waves are controlled by corrugation amplitude. In excitation of the surface waves by an annular electron beam, the slow cyclotron interaction as well as the Cherenkov interaction occur due to there-dimensional beam perturbations. The slow cyclotron interaction merges with the Cherenkov interaction at lower magnetic field. The merged growth rate is enhanced by 13%as compared to the isolated Cherenkov growth rate. The surface waves on inner and outer corrugations can have different frequencies and can be excited selectively by adjusting the beam radius of the electron beam. © 2014 The Japan Society of Plasma Science and Nuclear Fusion Research.

DOI： 10.1585/pfr.9.3406022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Normal modes and slow-wave instabilities in an oversized coaxial slow-wave structure (SWS) are numerically analyzed for the case of a weakly relativistic annular beam. The coaxial SWS has outer and inner rectangular corrugations with K-band dimensions. The inner corrugations have a cylindrical surface wave, which is the fundamental mode and exists even if the outer conductor is removed. The outer corrugations generate higher-order waveguide modes. Both transverse and longitudinal beam perturbations are considered in the beam interactions, leading to slow cyclotron instabilities as well as Cherenkov instabilities. The instabilities of axisymmetric and nonaxisymmetric modes have almost the same growth rates. The relative position between the outer and inner corrugations affects the normal modes and may allow wide frequency tunability.

DOI： 10.1109/TPS.2013.2256932

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The temporal growth rate (TGR) in a trapezoidally corrugated slow-wave structure for a backward-wave oscillator is theoretically studied. An intense relativistic annular electron is used as the energy source for the device. The annular electron beam is assumed to be infinitesimally thin in the radial extent and guided by an infinitely strong magnetic field. The trapezoidal profile of the structure is approximated by a sinusoidal function using Fourier approximation, and the dispersion relation of the system is derived using the Rayleigh-Fourier method. To study the TGR of the electromagnetic wave inside the system, the dispersion equation is solved for different values of the beam parameters. The dimensions of sinusoidally corrugated comparable trapezoidal structure are determined by comparing their dispersion characteristics. For the TM01 mode, TGR of instability that gives a qualitative measure of the microwave generation is calculated. The peak TGR of the proposed structure is found to be on average 1.5% higher than that of the sinusoidally corrugated slow-wave structure for the same set of beam parameters. Apart from its improved growth rate, the proposed structure has an added advantage of easy fabrication.

DOI： 10.1109/TPS.2013.2258406

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

DOI： 10.1585/pfr.8.2401085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOC  

Cylindrical surface waves (CSWs) and slow-wave instabilities of a rectangularly corrugated cylinder are numerically examined. CSWs are slow waves with upper cutoffs at the pi point. The upper cutoff frequency increases with increasing cylindrical radius R-0. There are two types of higher-order CSWs: one is due to azimuthal standing waves and the other is due to radial standing waves in the corrugation. Both higher-order types of SWSs have lower cutoffs as well as upper cutoffs leading to pass and stop bands. Slow space charge and slow cyclotron modes of an annular beam exist, which excite the Cherenkov and slow cyclotron instabilities of CSWs, respectively. The growth rates of the higher-order CSWs are comparable to those of the fundamental SWSs.

DOI： 10.13182/FST13-A16893

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOC  

Smith-Purcell (SP) radiations based on cylindrical surface wave are examined. Cylindrical surface waves are formed on metal cylinders having a periodically corrugated wall. Corrugation parameters are those used in K-band backward wave oscillators (BWOs). The corrugated metal cylinders are excited by an axially injected coaxial annular beam in a weakly relativistic region less than 100 kV. Cylindrical surface wave excitations due to BWO are observed at about 23 GHz. In addition, SP radiations are observed in the higher frequency regions up to about 90-100 GHz, which is about 4 times higher than the frequency of cylindrical surface wave.

DOI： 10.13182/FST13-A16921

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Electron beam generation in a weakly relativistic region of less than 100 kV is investigated utilizing rectangular and disk cold cathodes made of copper. Electron emission areas are located on the edges of the cathodes. For uniform electron emission, some surface roughness is required on the emission surface, which is obtained by scratching the surface with sandpaper. A uniform sheet beam with a size of about 2×40 mm and an annular beam with a diameter of about 25 mm and a thickness of about 2 mm are demonstrated in the weakly relativistic region. Stable electron beams of sheet and annular shapes are experimentally demonstrated. © 1973-2012 IEEE.

DOI： 10.1109/TPS.2013.2262505

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Publisher：Committee of Joint Conference of Electrical, Electronics and Information Engineers in Kyushu  

DOI： 10.11527/jceeek.2013.0_397

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Language：English   Publishing type：Research paper (scientific journal)  

We present a numerical and experimental study of the dispersion characteristics of cylindrical surface waves on metallic cylinders with rectangular corrugations. In actual devices, reflections at both ends quantize the electromagnetic modes into resonant axial modes. A cavity method based on axial mode measurements is applied to study the properties of the cylindrical surface waves. The resonances are relatively sharp near the upper cutoff, but at low frequencies far from the cutoff, the resonances broaden to resemble those of a Sommerfeld wave. © 2012 The Japan Society of Plasma Science and Nuclear Fusion Research.

DOI： 10.1585/pfr.7.2406022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

Experimental studies of coaxial oversized backward wave oscillators (BWOs) are presented. The beam voltage is weakly relativistic, less than 100 kV. The slow-wave structure (SWS) consists of a rectangularly corrugated oversized hollow waveguide and rectangularly corrugated inner conductor and has a target operating frequency due to Cherenkov interaction is in the K-band. The coaxial SINS can have two surface waves caused by the inner and the outer corrugations. The oversized BWO operations under the influences of two surface waves are examined. The oscillation start voltage is decreased by introducing a rectangularly corrugated inner conductor, The output. power is increased greatly as compared with those without; the corrugated inner conductor. The inner corrugation improves the oversized BWO operation.

DOI： 10.3938/jkps.59.3573

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

In this paper, slow-wave instabilities in oversized coaxial slow-wave structures (SWSs) are numerically analyzed. An infinitesimally thin annular beam with transverse perturbations and guided by a finite axial magnetic field is considered as an energy source for creating the instabilities. The coaxial SWSs consist of an outer oversized hollow waveguide and a central conductor and have sinusoidal corrugations. An electromagnetic slow-wave is generated by the inner corrugation and is called the inner surface wave (ISW). The ISW is a cylindrical surface wave and can exist even if the outer conductor is removed. The higher-order modes are produced by the outer waveguide. The annular beam interacts with the ISW and higher-order modes. A slow cyclotron instability occurs due to the transversely modulated beam boundary, in addition to the conventional Cherenkov instability. The slow-wave instabilities can be controlled by using the beam radius and the relative positions of inner and outer corrugations.

DOI： 10.3938/jkps.59.3555

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Backward wave oscillation based on a cylindrical surface wave of Smith-Purcell free electron laser (SPFEL) is demonstrated. The SP-FEL is composed of a metal cylinder having a periodically corrugated wall and a surrounding hollow straight waveguide. Corrugation parameters are those used in K-band backward wave oscillators (BWOs). The metal cylinder has a surface wave due to the corrugation. The cylindrical surface wave is excited by an axially injected coaxial annular beam. Radiations due to the backward wave oscillation based on the cylindrical surface wave are examined in a weakly relativistic region less than 100 kV. An oscillation starting voltage exists for the backward wave oscillation as in the case of hollow oversized BWO. The frequencies are in K-band and are determined by the cylindrical corrugation. Radiations up to tens of kW are obtained. © 2011 The Japan Society of Plasma Science and Nuclear Fusion Research.

DOI： 10.1585/pfr.6.2402039

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Language：English   Publishing type：Research paper (bulletin of university, research institution)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

Three kinds of models are used for beam instability analyses: those based on a solid beam, an infinitesimally thin annular beam, and a finitely thick annular beam. In high-power experiments, the electron beam is an annulus of finite thickness. In this paper, a numerical code for a sinusoidally corrugated waveguide with a finitely thick annular beam is presented and compared with other models. Our analysis is based on a new version of the self-consistent linear theory that takes into account three-dimensional beam perturbations. Slow-wave instabilities in a K-band oversized sinusoidally corrugated waveguide are analyzed. The dependence of the Cherenkov and slow cyclotron instabilities on the annular thickness and guiding magnetic field are examined.

DOI： 10.1585/pfr.5.S1047

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

Studies of coaxial oversized backward wave oscillators (BWOs) are reported. The beam voltage is weakly relativistic (less than 100 kV). The slow-wave structure consists of a periodically corrugated oversized waveguide and periodically corrugated inner conductor, whose target operating frequency due to Cherenkov interaction is in the K-band. A starting energy exists for the coaxial oversized BWO as it does for a hollow oversized BWO. The coaxial slow-wave structure has two surface wave modes caused by the inner and outer corrugations. Operation based on these surface modes can be controlled by the beam diameter. The phase difference between the inner and outer corrugations has little effect on operation of the oversized BWO.

DOI： 10.1585/pfr.5.S2093

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Other Link： https://jlc.jst.go.jp/DN/JALC/00368733091?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

Studies of slow cyclotron maser operation of a slow-wave device are presented. The beam voltage is weakly relativistic, less than 100 kV. The slow-wave structure is a periodically corrugated oversized waveguide for K-band operation. When using rectangular corrugation having a relatively small ratio of corrugation width to periodic length of about 20 %, the dispersion curve near the upper cut-off becomes flat and second harmonic slow cyclotron maser operation is observed in the low-energy region near 30 kV. Another type of combined resonance of Cherenkov and slow cyclotron interaction is also demonstrated in which an absolute instability is driven by the slow cyclotron interaction. By using sinusoidal corrugation and rectangular corrugation having a 50 % ratio of corrugation width to periodic length, amplification effects due to the fundamental and second harmonic slow cyclotron interactions are observed.

DOI： 10.1585/pfr.5.S1049

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Other Link： https://jlc.jst.go.jp/DN/JALC/00363491071?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

The dispersion characteristics and slow-wave instabilities of a sinusoidally corrugated waveguide with coaxial slow-wave structure (SWS) are analyzed. In SWS, a central cylindrical conductor is surrounded by an outer cylindrical conductor. Sinusoidal corrugation is formed on either conductor or both conductors. The corrugation parameters are those used for an X-band SWS. The relative phase between the sinusoidal corrugations on the inner and outer conductors affects the dispersion characteristics. Instabilities due to beam interactions with the slow -waves are examined by considering three-dimensional beam perturbations. The slow cyclotron instability occurs in addition to the Cherenkov instability, since transverse as well as longitudinal perturbations are included.

DOI： 10.1585/pfr.5.S2091

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

The cherenkov and slow cyclotron instabilities driven by an axially injected electron beam in a cylindrical waveguide are studied using a new version of the self-consistent linear theory considering three-dimensional beam perturbations. There are three kinds of models for beam instability analysis, which are based on a cylindrical solid beam, an infinitesimally thin annular beam, and a finitely thick annular beam. Among these models, the beam shape properly representing the often used actual annular electron beams is the finitely thick annulus. We develop a numerical code for a cylindrical waveguide with a finitely thick annular beam. Our theory is valid for any beam velocity. We present eigen-modes of the cylindrical system with the plasma and beam. Instabilities driven by the annular beam in a dielectric-loaded waveguide are also examined.

DOI： 10.1585/pfr.3.S1020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

Studies on slow-wave devices with a novel disk cathode and two types of rectangular corrugation are reported. The beam voltage is weakly relativistic, which is less than 100 kV. A disk cathode can generate a uniformly distributed annular beam in the weakly relative region. Rectangular corrugation having the ratio of the corrugation width to the periodic length of 50 or 20% is used. Using the former, output powers of about 200 kW are obtained at around 100 kV. For the latter, the effect of slow cyclotron resonance is observed in the low-energy region of around 30 kV. Output powers of slow cyclotron masers are in the range of a few hundred W. The operation mode of a slow cyclotron maser can be controlled between axisymmetric and nonaxisymmetric modes by changing the end condition of rectangular corrugation.

DOI： 10.1585/pfr.3.S1078

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electrical Engineers of Japan  

Studies of an oversized BWO with a novel cathode and a rectangular corrugation are presented. The beam voltage is weakly relativistic, less than 100 kV. We propose to use a disk cathode made of metal only. The disk cathode can generate a uniformly distributed annular electron beam even in the weakly relativistic case. By using the annular electron beam, a weakly relativistic oversized BWO consisting of rectangularly corrugated cylindrical waveguide is demonstrated. The oscillation starting energy is about 60 kV for a 10-period rectangular corrugation. The maximum output power is estimated about 200 kW. The operation mode between axisymmetric and nonaxisymmetric modes can be controlled by changing the end condition of BWO.

DOI： 10.1541/ieejfms.127.681

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

We report an oversized K-band backward wave oscillator (BWO) operating above 20 GHz in the weakly relativistic region less than 100 kV. It is very important to prevent microwave from going into the beam diode, since intense microwaves will harmfully affect beam generation. A weakly relativistic oversized BWO is demonstrated using a Bragg reflector at the beam entrance of slow wave structure (SWS). The effect of the Bragg reflector on the BWO operation is examined, by changing the boundary condition at the beam entrance. The Bragg reflector improves the performance of the oversized BWO.

DOI： 10.1585/pfr.2.S1041

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

It has been confirmed that the normal mode of a surface wave on a corrugated metal plate can be excited by a sheet electron beam. Two types of instabilities are analyzed numerically. One is the Cherenkov instability between the surface normal mode and the slow-space-charge mode of the electron beam. This instability is possibly absolute instability. The oscillation frequency of this instability can be controlled by the amplitude of the corrugation depth. The electron beam energy for this instability is determined by the ratio of the period length and the amplitude of the corrugation. Another is the Smith-Purcell-type instability. This instability is always convective-type instability. The oscillation frequency is higher than that of the above-mentioned Cherenkov instability and is in a broad band. The temporal growth rate of this instability is strongly reduced as the beam energy is reduced.

DOI： 10.1143/JJAP.46.6051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INSTITUTION ENGINEERING TECHNOLOGY-IET  

The dispersion relation of a rectangularly periodic cylindrical slow-wave structure (SWS) with an annular electron beam of energy < 100 keV and current around 200 A has been derived and solved numerically. This type of system is typically used in backward wave high-power microwave devices and linear accelerators. The dispersion relation is analysed for both the cold structure (for zero beam current) and structure with an annular beam. The cold structure dispersion relation is characterised by the real value of frequency and wavenumber. When an electron beam propagates through the structure and the beam energy and current are sufficient to produce instability, microwave radiation results. The strength of this radiation can be qualitatively approximated from the temporal growth rate of the instability, which is defined by the imaginary values of the frequency corresponding to the complex conjugate roots of the dispersion relation. The temporal growth rate for various beam parameters has been calculated. The proposed SWS is very easy to be fabricated and can be used in the real experiments for generating high-power microwaves.

DOI： 10.1049/iet-map:20060279

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

(1) Four-time progress in ion-confining potentials phi(c) to 3.0 kV in comparison to phi(c) attained 1992-2002 is achieved in the hot-ion mode (T-i=several keV). A scaling of phi(c), which favorably increases with plug electron-cyclotron heating (ECH) powers (P-PECH), is obtained. (2) The advance in phi(c) leads to a finding of remarkable effects of radially sheared electric fields (dE(r)/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in phi(c) with increasing n(c) to similar to 10(19) m(-3) with the recovery of phi(c) with increasing P-PECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Phi(c), generates a strong E-r shear and peaked vorticity with the direction reversal of E(r)xB sheared flow near the Phi(c) peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in T-e and T-i rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW 20 ms) in a standard tandem-mirror operation raises T-e0 from 70 to 300 eV together with T-i perpendicular to 0 from 4.5 to 6.1 keV, and T-i//0 from 0.5 to 1.2 keV with tau(p0)=95 ms for phi(c) (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of tau(p0)/tau(E0) is observed to be 1.7 for phi(c) trapped ions (consistent with Pastukhov's theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (eta(ICH)similar to 0.3; nl(c)=4.5X10(17)m(-2)). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (T-e=600 eV and T-i=6.6 keV) with azimuthal E(r)xB sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.

DOI： 10.13182/FST07-A1305

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

We improve the performance of the K-band oversized backward wave oscillator. The beam is annular and weakly relativistic. The beam voltage is less than 100 kV and the beam current is less than 500 A. The operation frequencies are in the range of 23-27 GHz. The electromagnetic field of the K-band B WO concentrates in the vicinity of the SWS wall and the electron beam should be propagated within a few mm from the wall. The uniformity of annular beam is also very important for the efficient beam coupling. First, we modify the oversized SWS so that the beam interaction point is shifted to the pi point. Secondly, we improve uniformity of the annular electron beam and the output power increases up to about 500 kW level.

DOI： 10.13182/FST07-A1390

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

We analyze instabilities of oversized backward wave oscillator (BWO) designed for K-band operations in a weakly relativistic region (less than 100 kV). For the oversized BWO, the electromagnetic field is a surface wave localized near the waveguide wall. Instabilities due to the beam interactions with the surface wave are examined by considering three-dimensional beam perturbations. There exist the slow cyclotron and Cherenkov instabilities as in the case of non-oversized BWO. Nonaxisymmetric as well as axisymmetric instabilities are excited, even in the completely axisymmetric system. The growth rates of both modes are in the same order. The growth rates decrease exponentially if the beam is apart from the SWS wall.

DOI： 10.13182/FST07-A1392

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

An application of electron-cyclotron heating with an off-axis resonance location produces a cylindrical layer with energetic electrons and facilitates the formation for a profile of plasma rotation with a radially localized high-vorticity layer. These phenomena are detected by several types of end-loss ions- and electrons-analyser. Formation of radial transport barrier has been observed in the vicinity of the high-vorticity layer in GAMMA 10.

DOI： 10.13182/FST07-A1345

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

Generalized scaling laws for the formation of plasma confining potentials are investigated to find the physics essentials common to representative tandem-mirror operational modes in GAMMA 10. These modes are characterized in terms of (i) a high-potential mode and (ii) a hot-ion mode. The potential-formation scalings in these modes are consolidated and generalized on the basis of the consistency with finding of the wider validity of Cohen's strong electron-cyclotron heating (ECH) theory covering over both modes. A plateau-shaped electron distribution function is observed when a plug electron-confining potential is formed in the hot ion mode of GAMMA 10, as predicted in terms of the strong ECH theory.

DOI： 10.13182/FST07-A1351

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOCIETY  

The effects of the plasma-confining potentials and the associated radially sheared electric fields on the central-cell electron energy confinement are theoretically and experimentally investigated in the GAMMA 10 tandem mirror. In particular, the scaling of the central-cell electron temperatures with electron-confining potentials is studied on the basis of the local energy-balance equation. The obtained theoretical scaling of electron temperatures with electron-confining potentials is then compared with the experimentally observed relation between these two parameters.
Recently, by the use of new 0.5-M'W level gyrotrons in the plug region, four-time progress in the formation of the ion-confining potential phi(c) including a new record of 3 W has been achieved in a hot-ion mode having bulk-ion temperature T-i = several keV In the hot-ion mode, intermittent vortex-like turbulent structures are observed in the case without the gyrotron injections; in this case, radially produced weak shear of electric fields dE(r)/dr and appreciable transverse losses are observed. However, during the application of electron-cyclotron heatings, the associated potential rise produces a stronger shear in the central cell (dE(r)/dr = several 10 kV/m2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement.
In order to investigate the effect of the radially sheared electric fields on the electron energy confinement, the radial profiles of the thermal diffusivity are derived from the local power-balance analysis by the use of the data from the following various diagnostics in the above-described hot-ion mode. The obtained radial profiles of radial electric field and thermal diffusivity imply that the reduction of the thermal diffusivity is associated with the radially produced strong shear of electric fields.

DOI： 10.13182/FST07-A1396

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Language：English   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

Mirror devices, having open-ended regions, provide intrinsic important advantages in terms of the control of radial-potential or E_r shear profiles on the basis of axial particle loss control for ambipolar potential formation. From the viewpoint of the mechanism investigation of radially localized transport-barrier formation, an idea of off-axis-resonant electron-cyclotron heating (ECH) is proposed and applied in the tandem mirror GAMMA 10, by the use of the aforementioned intrinsic advantage of mirror devices. The off-axis ECH produces a cylindrical layer with energetic electrons and facilitates the formation for a localized bumped ambipolar potential with a strong shear of radial electric fields E_r. Remarkable effects of dE_r/dr on the suppression of turbulent fluctuations with plasma confinement improvement are found by the use of signals of end-loss currents flowing from the central cell as well as the central-cell soft x ray. In association with the reduction of the fluctuations due to strong E_r shear formation, increases in ion and electron temperatures are found.

DOI： 10.1585/pfr.2.S1077

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

The Cherenkov instability used in slow-wave devices has been well studied in the literature. However, in previous analyses, the beam motion is restricted to the longitudinal direction assuming an infinitely strong magnetic field. For the finite strength magnetic field, the transverse beam perturbation cannot be ignored and leads to the slow cyclotron instability. Recently, a new version of self-consistent field theory considering three-dimensional perturbation has been developed based on a solid beam, in which the effect of the transverse perturbation appears as a surface charge at a fixed boundary. In the case of a thin annular beam, the boundary is modulated and is essentially different from the solid beam case. We propose a self-consistent field theory considering the moving modified boundary surface. The slow cyclotron instability due to the modulation of an infinitesimally thin annular electron beam is presented.

DOI： 10.1017/S0022377806005095

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

Backward wave oscillators (BWOs) have been studied as a candidate high-power microwave source. To increase the operation frequency, an oversized slow-wave Structure (SWS) is used. The operation at reduced voltage is preferable for practical applications. This work is aimed at numerically examining the operation mode of a weakly relativistic oversized BWO. We examine not only the axisymmetric transverse magnetic mode but also the non-axisymmetric hybrid modes of the oversized SWS. Both of them are candidates for the operation modes. These modes are surface waves whose fields are concentrated near the SWS wall. They overlap in frequency and are not separated by stop-bands. For an efficient beam interaction, the injected electron beam needs to be controlled more accurately than in the non-oversized SWS case.

DOI： 10.1017/S0022377806005290

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

For the purpose of end-loss-ion and -electron analyses in open-field plasmas, a compact-sized electrostatic end-loss-current detector is proposed on the basis of a self-collection principle for suppressing the effects of secondary-electron emission from a metal collector. For employing this specific method, it is worth noting that no further additional magnetic systems except the ambient open-ended magnetic fields are required in the detector operation. This characteristic property provides a compactness of the total detection system and availability for its use in plasma confinement devices without disturbing plasma-confining magnetic fields. The detector consists of a set of parallel metal plates with respect to lines of ambient magnetic forces of a plasma device for analyzing incident ion currents along with a grid for shielding the collector against strays due to the metal-plate biasing. The characterization experiments are carried out by the use of a test-ion-beam line along with an additional use of a Helmholtz coil system for the formation of open magnetic fields similar to those in the GAMMA 10 end region. The applications of the developed end-loss-current detector in the GAMMA 10 plasma experiments are demonstrated under the conditions with simultaneous incidence of energetic electrons produced by electron-cyclotron heatings for end-loss-plugging potential formation. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2338304

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Our proposed "matrix-type" semiconductor detectors are applied for studying the local energy balance of bulk electrons in the tandem-mirror GAMMA 10. The matrix-type detector array consists of compactly produced six "rows" having different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channel units ("columns") for measuring radiation profiles in the radial direction of plasmas. These various SiO2 layers are, thus, employed as "unbreakable ultrathin radiation-absorption filters" having various thicknesses to distinguish x rays from charge-exchange neutral particles and analyze the radial profiles of both plasma ion and electron temperatures simultaneously. The radial profiles of the energy confinement time and the thermal diffusivity obtained from the local energy balance analysis imply that the improvement of the plasma confinement is associated with the strong shear of radial electric fields due to a high plasma confining potentials. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2219404

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The quantum efficiency of an ultralow-energy-sensitive pure-Ge (ULE Ge) detector is investigated using synchrotron radiation from the storage ring at AIST especially for x-ray pulse-height analyses (PHAs), down to a few hundred eV. Several types of x-ray diagnostics such as x-ray PHA, x-ray absorption methods, and x-ray tomography using the ULE Ge detector, a NaI (Tl) detector, as well as a microchannel-plate tomography system are employed for investigating electron distribution functions and electron temperature profiles with preliminary central electron-cyclotron heating in the central cell of the GAMMA 10 tandem mirror. These measurements play an important role in studying an essential physics scaling of the electron temperature as a function of electron confining potential in tandem mirror plasmas. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2351922

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

The dispersion characteristics of surface wave propagated on a deeply corrugated metal surface are analyzed. Surface wave becomes slow wave, because surface wave is evanescent wave to the direction away from the corrugated metal surface. The surface wave is possible to be backward wave, due to the periodic nature of the corrugation of metal surface. Because the Rayleigh hypothesis has failed for the analysis of an electromagnetic field upon a deeply corrugated surface, the dispersion relations are solved by a direct numerical computation of wave equations.

DOI： 10.1585/pfr.1.025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

Power-up of gyrotrons was carried out and corresponding launcher systems were designed for plug and central-cell ECRH systems in the GAMMA 10 tandem mirror. Then, a high-power ECRH experiment was started. For the plug ECRH, new 500-kW gyrotrons produced a new record value of the confining potential. For the central-cell ECRH, development of a new antenna system has increased the transmission rate of incident microwave power and focused it onto the machine axis in the resonance surface. As a result, a clear increase of the diamagnetism was observed during the pulse of ECRH.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INT ATOMIC ENERGY AGENCY  

Following the 19th IAEA Fusion Energy Conference (Lyon, 2002), (1) three-time progress in the formation of ion-confining potential heights phi(c) including a record of 2.1 W in comparison to those attained 1992-2002 is achieved for tandem-mirror plasmas in the hot-ion mode with ion temperatures of several kiloelectronvolts. (2) The advance in the potential formation gives the bases for finding the remarkable effects of radially produced shear of electric fields E,, or non-uniform sheared plasma rotation on the suppression of intermittent vortex-like turbulent fluctuations. (i) Such a shear effect is visually highlighted by x-ray tomography diagnostics; that is, spatially and temporally intermittent vortex-like fluctuated structures are clearly observed as two-dimensionally reconstructed visual structures for the first time in kiloelectronvolt order ion-cyclotron heated plasmas having a weak shear in GAMMA 10. (ii) However, during the application of plug electron-cyclotron heatings (ECH), the associated potential rise produces a stronger shear (dE(r)/dr = several 10kVm(-2)) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement. X-ray observations also show elongation of a vortex structure from a circular into an ellipsoidal shape, as depicted in H-mode theories, with an outward shift. (3) For the physics interpretations and control of such potential and the associated shear formation, the validity of our proposed theory of the potential formation is extensionally tested under the conditions with auxiliary heating. The data described above fit well to the extended surfaces calculated from our proposed consolidated theory of the strong ECH theory (plateau formation) with Pastukhov's theory on energy confinement.

DOI： 10.1088/0029-5515/45/12/021

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Language：English   Publishing type：Research paper (bulletin of university, research institution)  

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Performance of oversized backward wave oscillators (BWOs) is investigated. Due to the axial mode operation, there exists a critical value of beam energy for the oscillation, i.e., a starting energy. The output power depends on the beam radius and voltage. The maximum output powers are above 100 kW (K-band) and above 50 kW (Q-band). The corresponding quality factor Pf2 is about 6×104 [kW·GHz2]. By changing the condition at the SWS end, an enhanced performance of the K-band oversized BWO is observed in a low magnetic field region about 0.4 T. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.125.733

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Nonaxisymmetric Cherenkov and slow cyclotron instabilities in a backward wave oscillator are investigated by using a new version of self-consistent linear theory. The slow cyclotron instability as well as Cherenkov instability can be driven by an axial electron beam without any initial perpendicular velocity. In a low magnetic field region, the radial displacement of beam plays an important role in the beam interactions. The slow cyclotron instability becomes strong, when the perturbed cyclotron motion and the electromagnetic wave rotate in the same direction. The rotational direction of electromagnetic wave also affects the Cherenkov instability. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.125.739

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Cherenkov and slow cyclotron instabilities driven by an axially injected electron beam in periodically corrugated cylindrical waveguide are studied by using a new version of self-consistent linear theory considering three dimensional beam perturbations. For the bounded microwave systems, the self-consistent boundary conditions require all field components and the normal modes in the beam are hybrid of transverse magnetic (TM) and transverse electric (TE) modes, even in the axisymmetric case. The slow cyclotron instability as well as Cherenkov instability can be driven by the axially streaming electron beam without any initial perpendicular velocity. In the low magnetic field region, the charge caused by the radial displacement of the beam plays an important role in the beam interaction with the electromagnetic mode. The radial displacement become large by decreasing the magnetic field and the slow cyclotron instability becomes strong and compete with the Cherenkov instability. For the axisymmetric case, the Cherenkov instability is dominant and suppresses the slow cyclotron instability, On the other hand, the slow cyclotron instability becomes dominant and suppresses the Cherenkov instability, for the nonaxisymmetric mode with the left-hand circular polarization. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.124.477

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Operation mode of an oversized backward wave oscillator is studied experimentally, using an oversized slow-wave structure. The dispersion characteristics of the slow-wave structure are determined by the average radius, corrugation amplitude and pitch length of the periodic waveguide. The corrugation amplitude and the corrugation period mainly determine the upper cutoff frequency of the lowest mode. The lower cutoff frequency is determined by the average radius. In this work, a Q-band periodic slow-wave structure is fabricated from aluminum. The diameter of the slow-wave structure is about 4 times free space wavelength. The beam voltage is weakly relativistic, around 50 kV, and the beam current is in the range of 100 A. The output powers of several 10 kW are obtained in the Q-band, i.e., the estimated frequency is in the range of 34–41 GH. The output modes are examined by measuring the radiation patterns. Nonaxisymmetric as well as axisymmetric modes are observed. It is demonstrated for the first time that the nonaxisymmetric and axisymmetric operations of the oversized BWO can be controlled by changing the corrugation number of slow-wave structure. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.124.456

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

Cherenkov instability driven by an unmagnetized electron beam in a periodically corrugated waveguide is studied by developing a new version of self-consistent linear theory. The conventional space-charge and cyclotron modes of magnetized beam degenerate in this case. The normal electromagnetic modes are pure transverse magnetic (TM) and transverse electric (TE) modes in axisymmetric cases and are hybrid modes having both TM and TE components in nonaxisymmetric cases. Using the developed linear theory, the axisymmetric and nonaxisymmetric Cherenkov instabilities due to the unmagnetized beam in the periodic system are examined numerically. The unmagnetized beam can excite the axisymmetric TM and nonaxisymmetric hybrid modes. A comparison of the numerically obtained results is made with those of the recent C-band Pasotron experiment.

DOI： 10.1143/JJAP.42.7095

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Under the international fusion cooperating research, He neutron detectors in the GAMMA 10 tandem mirror are calibrated by the use of a Cf-252 spontaneous fission neutron source (8.96 x 10(4) n/s). The calibration experiments are carried out with a "rail system" placed along the, magnetic axis of the GAMMA 10 central-cell region, where hot ions in the plasma experiments with the bulk temperatures of similar to10 keV are produced. As compared to a previous neutron monitoring system with a BF3 detector in GAMMA 10, the present He-3 systems are designed with about two orders-of-magnitude higher neutron-counting efficiency for analyzing a neutron emissivity from the plasmas in a single plasma discharge alone. Two He-3 systems. are installed near the middle and the end of the central cell so as to identify the central-cell hot-ion axial profile. The filling pressure of He-3, the effective length, and the diameter of the detector are designed as 5 bar, 300 mm, and 50 mm, respectively. The detector output spectra are carefully analyzed by the use of a preamplifier, a shaping amplifier, as well as a multichannel analyzer for each He-3 detector. In the present article, the neutron-counting data from the two He-3 detectors due to the on-axis Cf-252 scan are interpreted in terms of the d(-2) intensity dependence (d being the distance between the detector and the neutron source) as well as the effects of the central-cell magnetic coils and the other machine structural components. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1534402

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A new method for a simultaneous observation of both plasma ion and electron temperatures is proposed using one semiconductor-detector array alone. This method will provide a new application of semiconductor-detector arrays for monitoring the key parameter set of nuclear-fusion triple product (i.e., ion temperatures, densities, and confinement time) as well as for clarifying physics mechanisms of energy transport between plasma ions and electrons under various plasma confining conditions. This method is developed on the basis of an alternative "positive" use of a semiconductor "dead layer"
that is, an SiO2 layer is employed as a reliable ultra-thin energy analysis filter for low-energy charge-exchanged neutral particles from plasmas ranging in ion temperatures from 0.1 to several tens of kilo-electron-volts. Using recent fabrication techniques for the thin and uniform SiO2 layers of the order of tens to hundreds of angstrom, our computer simulation and its experimental verification show the availability of such semiconductors for distinguishing neutral particles (for ion temperatures) from X-rays (for electron temperatures). These are simultaneously emitted from the plasmas into semiconductor detectors
however, we employ their quite different penetration lengths and the resultant different deposition depths and profiles in semiconductor materials. As a result, their output signals are distinguishable for these two different and fundamental species of plasmas. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-9002(01)01899-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physreve.63.056503

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevE.63.056503/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Nuclear Society  

DOI： 10.13182/fst01-a11963470

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Interaction between an axially streaming electron beam and an axisymmetric TM mode in a periodically corrugated waveguide is analyzed using a linear fluid model. Effect of the perpendic ular perturbation of the beam by a finite magnetic field is considered. There are two mechanisms of Cherenkov interaction between a beam and a TM mode. One is the conventional mechanism attributed to a longitudinal perturbation. The other is attributed to the vertical perturbation of the beam surface and is dominant in the low magnetic field region. The slow cyclotron mode on an axially streaming beam is able to interact with an axisymmetric TM mode only if beam surface modulation is taken into account. The Cherenkov and cyclotron interactions degenerate for zero magnetic fields. This interaction has higher growth rate compared to the conventional Cherenkov interaction in the region of high current and weakly relativistic energy.

DOI： 10.1143/JPSJ.67.3462

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

A slow-wave electron cyclotron maser consisting of an oversized corrugated waveguide and an axially injected electron beam has been demonstrated experimentally. At resonance, the output rf power is strongly enhanced. Maximum output power of 200 kW with an electronic efficiency of 4% at 19.5 GHz is obtained for an electron beam with voltage of 35 kV and current of 150 A. The oscillation is observed to be a periodic TM(01) mode. The enhancement of microwave radiation is estimated to be caused by the combined resonance of Cherenkov interactions and an anomalous Doppler shifted electron cyclotron interaction.

DOI： 10.1103/PhysRevE.53.2726

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present an experimental study of the electromagnetic properties of transverse magnetic modes in a corrugated-wall cavity filled with a radially inhomogeneous plasma. The shifts of the resonant frequencies of a finite-length, corrugated cavity were measured as a function of the background plasma density and the dispersion diagram was reconstructed up to a peak plasma density of 10(12) cm(-3). Good agreement with a calculated dispersion diagram is obtained for plasma densities below 5 X 10(11) cm(-3).

DOI： 10.1103/PhysRevE.53.R2045

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The electromagnetic field properties of axisymmetric transverse magnetic (TM) modes in a specific slow wave structure (SWS) typically used in high power X-band backward wave oscillators are studied numerically. The dispersion relations, electric field distributions, and quality factors of the axial resonant modes are calculated. It is found that some of the axial modes can be switched from the volume modes to the surface ones by changing only the axial boundary of the SWS while the dispersion relations do not alter. This causes a drastic change in quality factors and can be of practical interest for controlling the coupling between beam and TM modes in high-power microwave devices. The calculated dispersion relations are verified experimentally for the TM(01) mode.

DOI： 10.1143/JPSJ.65.627

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An experiment on a backward wave oscillator (BWO) utilizing a large diameter corrugated metal wall slow wave structure (SWS) has been presented. Measurements are made above and below the starting beam energy for conventional BWO operation. For the latter case, an excited noise-level backward wave at 20 GHz (TM(01) mode) is coupled at the ends of the SWS to a forward anomalous Doppler shifted electron cyclotron (EC) beam mode which amplifies the original oscillation by an amount of 50 dB. Resultant microwave output powers of 200 kW with 4.5% electronic efficiency are observed near the condition of resonant magnetic field for an electron beam with energy 35 keV. Unlike the well-known fast wave devices, our slow wave EC device is simple to operate utilizing an electron beam with dominant parallel velocity in a uniform magnetic field.

DOI： 10.1143/JPSJ.64.4473

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We study the starting conditions for a large diameter (diameter/wavelength = 4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM(01) mode. This geometry is very promising for high power handling capability. We analyze two separate threshold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for the fundamental mode can be much larger than those of other higher order modes thus leading to coherent operation of large diameter sources free from mode competition.

DOI： 10.1109/27.376576

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The electromagnetic fields of the higher order axial resonant modes in a slow wave structure are analyzed and found to have considerably different characteristics from those of the conventional fundamental mode. Here, the reflections at both ends produce axial resonant modes corresponding to axisymmetric transverse magnetic (TM) modes. The period of field modulation of some of the higher order axial modes is shorter than that of the usual mode in a cylindrical waveguide, which could be of practical interest for higher power, higher frequency operation of backward wave oscillators, A perturbation technique is used to ascertain the field distribution inside the resonant cavity, and the numerical results thus obtained are compared to some experimental data.

DOI： 10.1109/22.375229

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA LAUSANNE  

A large-diameter plasma-filled backward-wave oscillator (BWO) is investigated experimentally. The parameters of slow wave structure are chosen so that the oscillation frequency is about 20 GHz at 60 keV beam energy. Plasma is produced by the beam and has favorable effects for beam propagation and Cerenkov oscillations. The output power of the BWO with plasma is observed to be three to six times that of vacuum BWO. The power level is several kilowatts and the efficiency is about 0.01%. For Cerenkov oscillations of a large-diameter BWO, the beam energy mainly determines the starting conditions for oscillation. The output power is strongly enhanced when the guiding magnetic field approaches the fundamental electron cyclotron resonance. This mechanism is closely related to the anomalous Doppler cyclotron resonance. The maximum power of 480 kW with an efficiency of 5% is achieved even for a relatively low beam energy of 60 keV.

DOI： 10.1016/0920-3796(94)00203-J

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Specific slow wave structures are needed in order to produce coherent Cherenkov radiation in overmoded relativistic generators. The electromagnetic characteristics of such slow wave, resonant, finite length structures commonly used in relativistic backward wave oscillators have been studied both experimentally and theoretically. In experiments, perturbation techniques were used to study both the fundamental and higher order symmetric transverse magnetic (TM) modes. Finite length effects lead to end reflections and quantization of the wave number. The effects of end reflections in open slow wave structures were found from the spectral broadening of the discrete resonances of the different axial modes. The measured axial and radial field distributions are in excellent agreement with the results of a 2-D code developed for the calculation of the fields in these structures.

DOI： 10.1109/27.338269

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A new theoretical and computational method, which can analyze uniformly the working of slow wave oscillators with various shapes of the waveguide, is developed. Time evolution of nonlinear boundary value problems in 2D space are reduced to 1D space boundary value problems by the power expansion method, and the reduced equations can be solved numerically by the new numerical scheme HIDM (higher order implicit difference method) with high accuracy. Dynamics of the backward wave oscillator driven by an intense relativistic electron beam are analyzed by this method. It is found that a throttled shape waveguide is preferable to obtain uniform output power of microwave.

DOI： 10.1016/0168-9002(93)90030-L

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The effect of thermal-diffusion charge on the x-ray energy response of silicon surface-barrier (SSB) detectors have generally been ignored; consequently, the SSB response has been believed to be analyzed using the thickness of the depletion layer alone. Our new theory on the SSB x-ray response [J. Appl. Phys. 72, 3363 (1992)] was prepared for addressing recent confusion on plasma x-ray analyses using SSB detectors [Rev. Sci. Instrum. 59, 1380 (1988); 61, 693 (1990); 63, 4850 (1992)]. This approach was made under the assumption of a dominant contribution of the diffusion-charge signal in the vicinity of the x-ray incident location because of the strong reduction of the x-ray produced charge within the thermal-diffusion length. In this report, the comparison between this approximation (having an approximated solution) and the exact numerical calculation (using an integral form) is carried out. Necessity and importance of such three-dimensional treatments for the data analyses as well as the design of multichannel semiconductor-array detectors developed for plasma x-ray tomography diagnostics are highlighted. Furthermore, for the total diffusing-charge amount, the calculated results from our theory and the values using the comment from Donolato agree well within the accuracy of 1%.

DOI： 10.1063/1.354864

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The linear dispersion relation of low frequency electromagnetic plasma waves in a corrugated plasma waveguide is analyzed successfully for a particular model in which difficulties for satisfying Floquet theorem rigorously are overcome. The resultant absolute instabilities are investigated numerically in some detail.

DOI： 10.1143/JPSJ.61.4022

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Field patterns and Poynting vectors of TM01 mode in sinusoidally corrupted waveguides are analyzed numerically. It is shown that a plasma column new the axis excludes the rf electric field lines towards the peripheral region, resulting in Poynting vectors in the axial direction there. The excitation of TM01 mode in corrugated waveguides is shown to be classified into two cases; surface and the volume waves. A possible physical explanation for the enhanced radiation observed previously from plasma filled backward wave oscillators is tried from the present results.

DOI： 10.1143/JPSJ.61.3966

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An analytical method based on a new theoretical model for the x-ray energy responses of silicon surface-barrier (SSB) detectors has been proposed. This method may address a recent confusing issue in the x-ray detection characteristics of SSB semiconductor detectors; that is, the x-ray responses of SSB detectors as well as p-i-n diodes used in underbiased operations were recently found to be contrary to the commonly held belief that the x-ray sensitivity of an SSB detector is determined by the thickness of the depletion layer. The model presented includes a signal contribution from thermally diffusing charge that is created in the field-free substrate region within a diffusion length from the depletion layer along with a signal contribution from charge created in the depletion layer. This model predicts a large signal contribution from the charge-diffusion effect on the SSB responses to high-energy x rays. Formulas and calculated results supporting SSB calibration data have been represented. These analytical methods might be developed to apply the analyses and predictions of energy responses of various types of silicon detectors including p-i-n diodes as well as charge-coupled devices.

DOI： 10.1063/1.351458

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Starting current conditions for initiating radiation from a backward oscillator with finite structure length and with non-zero end reflections are analyzed numerically. Because of the leak of radiation at both ends, the beam starting current for initiating oscillation cannot be Zero. Various models for computing the condition are compared and discussed from the stand point of actual experiments.

DOI： 10.1143/JPSJ.61.3566

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Using synchrotron radiation, the detection characteristics of microchannel plates, silicon surface barrier detectors and gold photocathodes are investigated; in particular, the detailed structures of the detection efficiency including an extended X-ray absorption fine structure (EXAFS), an X-ray absorption near-edge structure (XANES) and a chemical shift is reported along the with discussion on the physical mechanisms. Also, the applications of these detectors to X-ray tomography diagnostics for the observation of the cross-sections of fusion-oriented tandem-mirror plasmas are described. These data give information on the spatial distribution of the hot electrons which play an important role in the formation of thermal-barrier potentials phi(b). Also, the tomographic reconstructed X-ray data in the central cell show the effect of phi(b) formation on the improvement in the central-cell electron confinement for the first time.

DOI： 10.1016/0168-583X(92)95423-O

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Numerical analysis for radiation level in a high-power backward wave oscillator with a finite-length slow-wave structure with end reflections has been made using an algorithm called the higher-order implicit difference method (HIDM). Spatial and temporal evolutions of a small localized disturbance in an RF field are followed by relativistic fluid equations and Maxwell equations. The radiation intensity of the oscillation decreases substantially with increasing energy spread beyond 10% of the beam.

DOI： 10.1143/JPSJ.61.1136

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Absolute instability in a plasma-filled backward wave oscillator with sinusoidally corrugated slow wave structure driven by an intense relativistic electron beam has been analyzed numerically. The maximum spatial growth rate of the plasma-filled waveguide is found to be larger than that of an optimally designed vacuum structure. The excitation of a finite length structure is investigated. Although the spatial growth rates obtained in the linear theory increase with plasma density in the finite length plasma-filled structure for the fundamental TM01 mode of oscillation, negative global spatial growth rates prevent the coherent oscillation for intermediate ranges of plasma density, beam current, and even for structure length.

DOI： 10.1063/1.860225

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Several types of x-ray diagnostics, such as x-ray energy spectrum analyses, x-ray absorption methods, and x-ray tomographic reconstructions using various types of x-ray detectors (i.e., a Si(Li), a pure Ge, a NaI(T1), Si surface-barrier detectors, as well as microchannel plates) have been employed for obtaining various shapes of electron-velocity distribution functions as well as their spatial profiles: (i) a plateau-shaped electron-velocity distribution function in the plug region, supporting a scaling theory between thermal-barrier potentials and ion-confining potentials (Cohen's strong electron-cyclotron heating theory); (ii) mirror-trapped 60-keV relativistic Maxwellian electrons in the thermal-barrier region; and (iii) two-component Maxwellian electrons in the central cell have been observed in the tandem-mirror GAMMA 10. During experiments with thermal barriers, it has been observed that the bulk-electron temperature and its evolution with time in the central cell are quite different from the electron energy and its temporal evolution in the plug region. These differences as well as the different shape of the electron-velocity distribution function in each region have clearly demonstrated the existence of a thermal-isolation effect due to the thermal barriers between the central cell and the plug region. X-ray tomography data in these three regions have shown good axisymmetric radial profiles peaked on the magnetic axis; this axisymmetric shape is useful for reducing nonambipolar radial particle losses. Furthermore, the validity of the Pastukhov theory for electron-energy confinement that is enhanced due to the formation of thermal-barrier potentials has been demonstrated using the electron-energy balance analyses. These data have represented a good electron-energy confinement capability due to the formation of thermal barriers.

DOI： 10.1103/PhysRevA.45.2532

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Equilibria of r.f. Tokamak plasmas during lower hybrid (LH) wave-driven current start-up experiments in the WT-2 and WT-3 Tokamaks are studied by comparing spatial profiles of tail and bulk electrons with simple model equilibria for the electron tail. When the plasma current I(p) is smaller than the Alfven critical current I(A), the confinement areas of the tail and bulk electrons are not coincident. The cross-section of the outermost flux surface is always found to be larger than the tail cross-section. Particularly in the case of the small Tokamak WT-2 (a(L) = 9 cm) is the outermost flux surface coincident with the limiter circle and much larger than the tail cross-section, indicating that good confinement of bulk electrons which carry LH waves and provide electrons to the tail is crucial for r.f. current start-up.

DOI： 10.1088/0741-3335/33/14/002

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A plasma current of up to 70 kA has been sustained in WT-3 discharges by second harmonic (2-OMEGA-e) electron cyclotron (EC) waves alone, with zero loop voltage, after shutting off the Ohmic heating power. Further, in the case of high power EC wave injection, ramp-up discharges have been obtained. Pulse height analysis of hard X-rays in the line of sight at various angles to the toroidal field shows that the velocity distribution function of the high energy tail electrons is asymmetric in the toroidal direction. The 2-OMEGA-e EC wave is mainly absorbed by the tail electrons, and a 2-OMEGA-e EC driven current is generated by enhancing the asymmetry of the distribution. The figure of merit of 2-OMEGA-e EC current drive (ECCD) is eta-EC(2) = (3.2-6.4) x 10(-2) (10(19) A/W.m2), which is one order of magnitude smaller than that of lower hybrid current drive in WT-3. This low value of eta-EC(2) can be attributed to low confinement of the current carrying, high energy tail electrons produced by 2-OMEGA-e ECCD.

DOI： 10.1088/0029-5515/31/9/006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN KIKAI-SHINKO BUILDING  

The linear theory of electromagnetic radiation from a localized plasma-loaded backward wave oscillator with sinusoidally corrugated slow wave structure driven by an annular intense relativistic electron beam has been derived and analyzed numerically. The enhanced radiation from a plasma-loaded structure is attributed to the increase in the spatial growth rate of the absolute instability. The backward wave oscillator instability without plasma is found to be most effective in a case where the beam travels near the wall of the slow wave structure. For the given number of plasma particles per unit length of the structure, the decrease in the plasma column radius enhances the output radiation.

DOI： 10.1143/JPSJ.60.2655

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The distribution function f(--> p) of fast electrons produced by lower hybrid current drive (LHCD) is investigated in the WT-3 tokamak, using a combination of measurements of the hard X-ray (HXR) angular distribution with respect to the toroidal magnetic field and observations of the HXR radial profile. The data obtained indicate the formation of a plateau-like region in f(-->p) which corresponds to a region of resonant interaction between the lower hybrid (LH) wave and the electrons. The energy of the fast electrons in the peripheral plasma region is observed to be higher than that in the central plasma region under operational conditions with a high plasma current (I(p) greater-than-or-equal-to 80 kA). At low current (I(p) less-than-or-similar-to 50 kA), however, the energy of fast electrons is constant along the plasma radius. In the current ramp-up phase, fast electrons are generated in the directions normal to and opposite to the LH wave propagation. The latter case is ascribed to a negatively biased toroidal electric field induced by the current ramp-up. To study the characteristic change of f(-->p) for various current drive mechanisms, HXR measurements are performed in electron cyclotron current driven (ECCD) plasma and in Ohmic heating (OH) plasma. In ECCD plasma, the perpendicular energy of fast electrons increases, which indicates that fast electrons are accelerated perpendicularly by electron cyclotron heating. In both LHCD and ECCD plasmas, fast electrons flow in the direction opposite to the wave propagation, while no such fast electrons are formed in OH plasma.

DOI： 10.1088/0029-5515/31/6/002

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An increase with time of the central-cell electron temperature during a period with thermal barrier potentials has been observed. The increase is explained by an improvement of the electron energy confinement due to the presence of thermal barriers. Different evolutions of the electron energy in the plug region have been observed; these are closely related with the variation in time of the confining potential of the plug electrons. The observed behaviour of the potential confined electrons provides the first data set for the evolution of the effect of thermal isolation caused by the formation of a thermal barrier in the kilovolt range.

DOI： 10.1088/0029-5515/31/4/012

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Soft X-rays (SXRs) emitted from plasma in the WT-3 Tokamak were measured by three SXR detector arrays, viewing the plasma from three different angles. From these experimental data, we reconstructed the two dimensional (2-D) contour of constant SXR emissivity as a function of time, using the technique of computer tomography (CT). Computer simulation results show that data from three or more sets of detector arrays are required to reconstruct the 2-D emissivity structure with sufficient accuracy to distinguish Kadomtsev's model from the quasi-interchange (Wesson's) model of sawtooth crash. Further, it is shown that if the noise amplitude is more than 1.5%, reconstruction of the real 2-D SXR emissivity structure is difficult.

DOI： 10.1088/0741-3335/32/14/003

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DOI： 10.1103/PhysRevLett.65.855

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DOI： 10.1109/27.55925

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DOI： 10.1088/0029-5515/30/4/003

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DOI： 10.1103/PhysRevLett.64.1373

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DOI： 10.1088/0029-5515/29/8/009

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The toroidal plasma current is started up by rf power alone and is ramped up to above 20 kA in a low density plasma. The fraction ε<SUB><I>R</I></SUB> of injected LHW power is converted into poloidal magnetic field energy and its maximum value is about 5 %. For high density cases, a quasi-steady state discharge is obtained and the figure of merit η<SUB><I>CD</I></SUB> for steady current drive by LHW is about 0.04 × 10<SUP>19</SUP> (A·m<SUP>-2</SUP>/W). The direct loss of fast electrons from the plasma is very large and results in the low ε<SUB><I>R</I></SUB> and η<SUB><I>CD</I></SUB> in our experiments.

DOI： 10.1585/jspf1958.61.106

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The lower-hybrid current drive (LHCD) experiments with a high frequency range such as ω= (2-3) × (Ω<SUB>e</SUB>Ω<SUB>i</SUB>) <SUP>1/2</SUP> on the WT-3 tokamak are reported. Experimental results on current sustainment and ramp-up in the low density region are basically consistent with the predictions based on the quasi-linear theory with the finite fast electron confinement time. But the density limit for LHCD is lower than that expected from ω<SUP>2</SUP>-scaling. It seems that the density limit is given by ω<SUB>pe</SUB><SUP>2</SUP>/Ω<SUB>e</SUB><SUP>2</SUP>=1 for the high frequency range. It is concluded that increase of ω is not effective to increase the density limit for LHCD in the high frequency range.

DOI： 10.1585/jspf1958.61.241

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Sawtooth oscillations appearing in Ohmically heated (OH) plasmas are suppressed by the lower hybrid current drive (LHCD) in the WT-3 tokamak ($R{=}65$ cm, $a{=}20$ cm, $I_{\text{p } }{\simeq}100$ kA, $\bar{n}_{\text{e } }{=}1-2\times 10^{13}$ cm-3, $q_{\text{a } }{\simeq}4.5$, and $P_{\text{LH } }{\gtrsim}100$ kW). After the suppression, sawteeth reappear following $m{=}1$ oscillations in the LHCD plasmas. The latter sawteeth have a large amplitude and long period ($T_{\text{s } }{=}2\text{--}5$ msec), compared with the former one ($T_{\text{s } }{=}0.5\text{--}1.5$ msec). Furthermore, they do not accompany a precursor and crash very fast.

DOI： 10.1143/JPSJ.57.3661

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DOI： 10.1103/PhysRevLett.60.1033

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Electron cyclotron current drive (EC-CD) experiments have been carried out atthe fundamental and the second harmonics onthe WT-III tokamak using extraordinary moderadiation at 56 GHz launched from the low field side. The EC driven current of 24 kAisattained at the power level of 100 kW and the plasma with the density of 2 ×1012cm−3 and the electron temperature of several hundredseV can be sustained during the EC pulse. The efficiency of EC-CD is 1·0 × 10−2 (1019m−2 A W−1) atthe fundamental and 3·5 × 10−2 at the second harmonic. Theunidirectional high energy electrons in theinitial target plasma is necessary for EC-CD. © 1988 Taylor &amp
Francis Group, LLC.

DOI： 10.1080/00207218808945266

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Electron cyclotron emission (ECE) from the lower hybrid current drive (LHCD) as well as the ohmic heating (OH) plasmas in WT-3 were measured along the horizontal chord in the equatorial plane and the vertical chord where the magnitude of the toroidal field is constant, respectively. Measurements show that the high energy tail electrons carrying the plasma current build up within 10 msec after the injection of the lower hybrid wave. The ECE spectrum from the LHCD plasma in a quasi-steady state can be fitted with a numerically calculated spectrum assuming that the electron velocity distribution function is Maxwellian with the temperature T=50 keV. © 1988, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.57.2605

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DOI： 10.1088/0029-5515/27/9/006

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DOI： 10.1088/0029-5515/27/5/007

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DOI： 10.1143/JPSJ.55.4259

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DOI： 10.1088/0029-5515/26/3/008

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DOI： 10.1143/JPSJ.55.13

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DOI： 10.1088/0029-5515/26/1/010

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By injection of microwave power PEC near the electron cyclotron (EC) frequency into an Ohmically heated (OH) plasma in the WT-2 tokamak after OH power is shut off, the plasma current is sustained and ramped up by the EC wave only, without OH power. Here, EC-driven current is generated by EC heating of the suprathermal electron beam in OH plasma. Further, when PEC is injected into plasma sustained by lower-hybrid- (LH-) driven current, the plasma current and its rampup rate increase. Here, EC-driven current is generated by EC heating of the mildly relativistic electrons in LH-driven plasma. © 1986 The American Physical Society.

DOI： 10.1103/PhysRevLett.56.2180

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DOI： 10.1143/JJAP.24.L644

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When RF power PLHH near the lower hybrid frequency is injected into the ohmically heated plasma in the WT-2 tokamak, the RF-driven current is generated (Irf~-12 kA) and the plasma (ne~-3 x 1012 cm-3)is sustained by this current without ohmic current during the injection of PLhh- The efficiency ofRF current drive is obtained as ITf=0.06PWH/ne R (kA, kW, 1013 cm-3, m),where R is the major radius. The current 7rf is carried by suprathermal electrons which interact resonantly with lower hybrid waves. A high frequency instability and associated relaxation oscillation are observed, which can be suppressed by a simultaneous injection of the electron cyclotron heating power. © 1984, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.53.3399

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When rf power in the range of lower hybrid frequency was injected into the microwave discharge plasma at the electron cyclotron resonance in the WT-2 toroidal device, the toroidal plasma current is generated, started up to Ip~5KA, and sustained by rf power alone, without the Ohmic heating power (so-called rf tokamak). This lower-hybrid-wave driven current is produced only when high-energy tail electrons, which interact resonantly with the lower hybrid wave, are present in the initial electron cyclotron resonance plasma. © 1984, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.53.1047

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By application of electron cyclotron heating (ECH) to the WT-2 tokamak, the high-frequency instability (relaxation oscillation) excited by the lower-hybrid-wave-driven current is suppressed and the loop voltage decreases without voltage spikes, implying that the driven current increases. This suppression can be ascribed to the isotropization of the current-carrying beam electrons by ECH. © 1983 IOP Publishing Ltd.

DOI： 10.1088/0029-5515/23/2/012

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DOI： 10.1585/jspf1958.49.133

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A detailed design study is carried out on high power large diameter backward wave oscillator operating at 24 GHz for a beam energy of 100 keV. The ratio of the mean diameter of the slow wave structure to the wavelength of output microwaves is increased to be 4.8. Analysis is made within the scope of linear theory of absolute instability. The output power with efficiency 0.17% and oscillation frequency 20 GHz for 65 keV beam energy are obtained experimentally. The large diameter backward wave oscillator may be an attractive alternative source of high power millimeter microwaves which has simpler structure than that of conventional gyrotrons.

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10452347

Language：Japanese  

A Large diameter plasma-filled backward wave oscillator (BWO) utilizing sinusoidally corrugated slow wave structure (SWS) has been studied experimentally. The size parameters of the SWS are so selected that the device oscillates around 24GHz when it is driven by the electron beam of energy 100keV. The enhancement of 20GHz microwave radiation with maximum efficiency of 0.17% for the beam energy of 65keV from our designed large diameter BWO has been observed when plasma was introduced in the devices.

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10452151

Language：Japanese  

Axial density gratings with local rises in a plasma filled corrugated wall waveguide are experimentally found to be created by incident high-power microwaves. The density rises and the maxima of standing waves of the axial RF electric field are observed at the identical axial positions where the radius of the corrugated wall is minimum. A nonlinear plasma model to explain the physical meaning of the observed results is proposed.

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10451599

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Language：Japanese   Publisher：Japan Society for Simmulation Technology  

New computational treatment is shown to study the analysis of backward wave oscillator, which is one of a high-power microwave generator excited by an intensive relativistic electron beam. The frequency range of the generated microwaves is of the order of the 10 GHz. Dynamics of backward wave oscillator is solved by coupled systems of equations composed of relativistic fluid equations for electron beam and Maxwell's equation for microwave in a slow-wave structure. Newly developed analytical method, power expansion method reduce the original coupled system of partial differential equations in (r, z, t ) space into a coupled system of those in (z, t ) space, in which the spatial variation of waveguide radius is included. The reduced time evolutionary partial differential equations for nonlinear boundary value problems are solved numerically by newly developed numerical scheme, HIDM (Higher order Implicit Difference Method). It is shown that throttled type slow-wave structure is favorable for the production of high power and high quality microwaves.

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Language：English   Presentation type：Poster presentation  

Venue：Toki, Japan  

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Venue：Kumamoto, Japan  

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Venue：Kumamoto, Japan  

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Venue：Washington, DC, USA  

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Venue：Toki, Japan  

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Venue：Karlsruhe, Germany  

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Venue：Toki, Japan  

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