Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TPS.2022.3201616

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.90.084501

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0039322

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0031256

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5133442

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/1.5132380

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/1.5116268

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

DOI： 10.1088/1741-4326/aafaac

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Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aafaac/pdf

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

DOI： 10.1585/pfr.13.3406107

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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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Begell House  

DOI： 10.1615/plasmamed.2018023695

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

When plasma is generated by applying high-voltage alternating current (AC), the driving force of the temporally and spatially varying electric field is applied to the plasma. The strength of the driving force of the plasma at each spatial position is different because the electrons constituting the atmospheric-pressure nonequilibrium (cold) plasma move at a high speed in space. If the force applied to the plasma is accelerated only by the driving force, the plasma will be accelerated infinitely. The equilibrium between the driving force and the restricting force due to the collision between the plasma and neutral particles determines the inertial force and the drift velocity of the plasma. Consequently, the drift velocity depends on the strength of the time-averaged AC electric field. The pressure applied by the AC electric field equilibrates with the plasma pressure. From the law of conservation of energy, the pressure equilibrium is maintained by varying the drift velocity of the plasma.

DOI： 10.7566/JPSJ.87.064501

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

When the working gas of an atmospheric-pressure non-equilibrium (cold) plasma flows into free space, the diameter of the resulting flow channel changes continuously. The shape of the channel is observed through the light emitted by the working gas of the atmospheric-pressure plasma. When the plasma jet forms a conical shape, the diameter of the cylindrical shape, which approximates the conical shape, defines the diameter of the flow channel. When the working gas flows into the atmosphere from the inside of a quartz tube, the gas mixes with air. The molar ratio of the working gas and air is estimated from the corresponding volume ratio through the relationship between the diameter of the cylindrical plasma channel and the inner diameter of the quartz tube. The Reynolds number is calculated from the kinematic viscosity of the mixed gas and the molar ratio. The gas flow rates for the upper limit of laminar flow and the lower limit of turbulent flow are determined by the corresponding Reynolds numbers estimated from the molar ratio. It is confirmed that the plasma jet length and the internal plasma length associated with strong light emission increase with the increasing gas flow rate until the rate for the upper limit of laminar flow and the lower limit of turbulent flow, respectively. Thus, we are able to explain the increasing trend in the plasma lengths with the diameter of the flow channel and the molar ratio by using the cylindrical approximation.

DOI： 10.7566/JPSJ.86.124502

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated the excitation temperature of atmospheric-pressure non-equilibrium (cold) plasma using a line-pair method. An atmospheric cold plasma was intermittently generated using a quartz tube, a rare gas, and a foil electrode by applying high-voltage alternating current. Because the plasma occurred intermittently, an interval appeared between each generated plasma bullet. We assessed the time-averaged effective intensity from the observed intensity at each wavelength measured using a spectrometer. When the exposure time of the spectrometer is longer than the observed period of the plasma emission light, the time-averaged effective intensity at each wavelength decreases because it includes the interval with no plasma emission light. The difference in intensity between wavelengths changes with frequency, because changing the frequency changes the interval between plasma bullets. Consequently, even if the plasma electron temperature does not depend on the frequency of the applied voltage, the excitation temperature estimated from the difference in intensity changes with the frequency. The plasma electron temperature can be estimated from the duty ratio of the observed period of plasma emission light, and we estimated the electron temperature in the helium and argon cold plasmas to be 1.0 eV. Published by AIP Publishing.

DOI： 10.1063/1.4985308

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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  

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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We measured the variation of magnetic and electrostatic fluctuations observed during the transition between the quasi-single helicity (QSH) and multi helicity (MH) states in the edge region of the Toroidal Pinch Experiment-Reversed eXperiment reversed-field pinch plasma [ Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. The high-frequency magnetic fluctuations largely decrease at the start of the QSH state. During the QSH state, the toroidal and radial magnetic fluctuations slightly increase again between 12 and 18 kHz but the decrease in the poloidal magnetic fluctuation is maintained. The confinement of fast electrons may be improved in the period of the QSH state although the radial magnetic fluctuation increases between 12 and 18 kHz. It is suggested that the improved confinement of fast electrons may be obtained by the decrease in the fast radial magnetic fluctuation having the frequency higher than 20 kHz in the period of the QSH state. Contrarily, the transition from QSH to MH states occurs with the increase in the slow toroidal magnetic fluctuation having the frequency lower than 10 kHz. The QSH state may bring a good confinement of fast electrons by the reduced high-frequency magnetic fluctuation compared with the MH state. Published by AIP Publishing.

DOI： 10.1063/1.4967871

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We measured the variation of magnetic and electrostatic fluctuations observed during the gas puffing in the edge region of the toroidal pinch experiment-reversed experiment (TPE-RX) reversed-field pinch plasma. In the short period in which the electron density increased slowly just after the gas puffing, the confinement of fast electrons in the core region was maintained by the decrease in the fast radial magnetic fluctuation with the deepening of the reversal of the toroidal field. During the following period in which the electron density increased rapidly, the radial gradient of electron density decreased, and the loss of fast electrons from the core region increased owing to the increase in the toroidal and radial magnetic fluctuations in the high-frequency band, although the poloidal magnetic fluctuation decreased. Therefore, the confinement of fast electrons would be maintained by keeping the radial gradient of plasma thermal pressure with a moderate neutral particle supply of small quantity in a short time.

DOI： 10.7566/JPSJ.85.094501

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The atmospheric pressure plasma generated using a quartz tube, helium gas, and foil electrode by applying AC high voltage is investigated. A plasma plume is formed with a small bullet-like volume of plasma that travels at high speed inside of the quartz tube filled with a helium gas. The average movement velocity of the plasma bullet and the emitted light from helium plasma are observed inside the quartz tube. Both the average plasma bullet velocity and the power spectrum density (PSD) of the helium ion (He I) tend to decrease towards the tip of the plasma plume. The average plasma bullet velocity is linearly proportional to the PSD of He I. Therefore, the movement velocity of the plasma bullet is available to estimate from the PSD of He I at one measurement point. Published by AIP Publishing.

DOI： 10.1063/1.4963392

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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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Plasma plume is generated using a quartz tube, helium gas, and foil electrode by applying AC high voltage under the atmosphere. The plasma plume is released into the atmosphere from inside of the quartz tube and is seen as the continuous movement of the plasma bullet. The travel length of plasma bullet is defined from plasma energy and force due to electric field. The drift velocity of plasma bullet has the upper limit under atmospheric-pressure because the drift velocity is determined from the balance between electric field and resistive force due to collisions between plasma and air. The plasma plume charge depends on the drift velocity. Consequently, in the laminar flow of helium gas flow state, the travel length of the plasma plume logarithmically depends on the plasma plume charge which changes with both the electric field and the resistive force. Published by AIP Publishing.

DOI： 10.1063/1.4953907

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have studied the atmospheric pressure plasma generated using a quartz tube, helium gas, and foil electrode by applying AC high voltage. The plasma plume is released into the atmosphere from inside the quartz tube. The helium plasma temperature is investigated using a line pair method. The excitation temperature of the electrons in the helium plasma is estimated at about 700K along the plasma plume (column). The excitation temperature of the electrons decreases slightly towards the tip of the plasma plume. Because the emitted light of the helium ion (He I) excitation at a wavelength of 706 nm is weak at the tip of the plasma plume, the electron excitation temperature is overestimated. In the atmosphere region near the tip of the plasma plume, collisions between helium gas and air increase. Therefore, the electron excitation temperature determined using the observed emitted light from helium ion excitation inside of the quartz tube filled with a helium gas. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4942170

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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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Authorship：Lead author,　Corresponding author   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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Authorship：Lead author,　Corresponding author   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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Authorship：Lead author,　Corresponding author   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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We found that spontaneous improved confinement was brought about depending on the operating region in the Toroidal Pinch Experiment-Reversed eXperiment (TPE-RX) reversed-field pinch plasma [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. Gradual decay of the toroidal magnetic field at plasma surface B-tw reversal makes it possible to realize a prolonged discharge, and the poloidal beta value and energy confinement time increase in the latter half of the discharge, where reversal and pinch parameters become shallow and low, respectively. In the latter half of the discharge, the plasma current and volume-averaged toroidal magnetic field < B-t > increase again, the electron density slowly decays, the electron temperature and soft X-ray radiation intensity increase, and the magnetic fluctuations are markedly reduced. In this period of improved confinement, the value of < B-t >-B-tw)/B-pw, where B-pw is the poloidal magnetic field at the plasma surface, stays almost constant, which indicates that the dynamo action occurs without large magnetohydrodynamic activities. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4901350

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

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. (c) 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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：KOREAN PHYSICAL SOC  

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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Authorship：Lead author,　Corresponding author   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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.1.015084

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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：Japan Society of Plasma Science and Nuclear Fusion Research  

DOI： 10.1585/pfr.8.2401085

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CiNii Books

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER NUCLEAR SOC  

We have measured detailed axial profiles of electron density, floating potential, and axial magnetic field in the field-reversed configuration (FRC) sustained by the rotating magnetic field. To study the influence on the equilibrium of two kinds of bias magnetic field configuration - straight (pure solenoidal) and mirror -, experiments have been carried out in the FRC Injection Experiment apparatus. The case of mirror configuration has longer quasi-steady state compared with the case of straight configuration. The steeper density gradient outside the separatrix is generated by the uniform gradient of magnetic field due to the mirror configuration. The axial parallel diffusion is suppressed due to the steeper density gradient outside the separatrix. Therefore, the mirror bias magnetic field configuration works to improve the plasma confinement.

DOI： 10.13182/FST13-A16892

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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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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)  

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  

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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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：AMER INST PHYSICS  

Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures. (C) 2011 American Institute of Physics. [doi:10.1063/1.3652848]

DOI： 10.1063/1.3652848

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The magnetic fluctuations and electrostatic probe potential have been measured in the Toroidal Pinch Experiment - RX (TPE-RX) reversed-field pinch plasma [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)] (at the plasma surface r/a = 1.00). Fast electrons with energy comparable to or slightly higher than the core electron temperature are observed as many spikes in the electrostatic probe signal. These electrons are diffused by a fluctuating magnetic field from the core region. During the period of mild deepening of the reversal of the edge toroidal field, a significant reduction in the spike signal, increases in electron density and soft x-ray radiation, and a decrease in the D alpha line radiation are observed, even though the reduction in magnetic fluctuations is not significant during the same period, which indicates that the mild deepening of the reversal of the toroidal field can improve the confinement of fast electrons. (C) 2011 American Institute of Physics. [doi:10.1063/1.3596540]

DOI： 10.1063/1.3596540

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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 (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

Two-dimensional (2D) turbulence measurement by using microwave imaging reflectometry (MIR) has been performed in the reversed field pinch (RFP) plasma in TPE-RX. In this work, the maximum entropy method (MEM) is used to estimate the 2D k-spectrum. The turbulences between the pulsed poloidal current drive (PPCD) and the high Θ plasmas are studied. The m = 1 modes are dominant in PPCD plasma, while in high Θ plasma the k spectrum expands to the high k range and the turbulence propagates in the electron drift direction. The preliminary result shows that PPCD plasma switches off the turbulence, and the high Θ plasma has strong turbulence.

DOI： 10.1585/pfr.5.S1019

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

Field-reversed configurations (FRCs) driven by rotating magnetic fields (RMFs) with spatial high-harmonic components have been studied in the metal flux conserver of the FRC injection experiment (FIX). The high-harmonic RMF method has some unique features; (1) field lines of the RMF do not penetrate or cross the vessel wall, (2) selective penetration/exclusion of the fundamental/high-harmonic RMF component will result in a generation of effective magnetic pressure near the separatrix, which helps to keep the separatrix away from the vessel wall, (3) strong azimuthal non-uniformity of the RMF will cause the n = 4 deformation of the core FRC plasma, which will eliminate the destructive modes caused by the rotation of the plasma column. The RMF method with high harmonics will provide quasi-steady current drive of high-beta FRC plasmas without destructive n = 2 rotational mode and will be helpful in reducing the particle loss and thermal load when applied to the fusion core plasma.

DOI： 10.1007/s10894-008-9168-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Field-reversed configurations (FRCs) sustained by rotating magnetic fields (RMFs) with spatial high-harmonic components have been studied in a metal flux conserver of the FRC injection experiment apparatus. In the RMF-FRC equilibrium, significant azimuthal variation of the axial magnetic field, the electron density and the electron rotation velocity are observed. This deformation of the core FRC plasma is brought about by the strong azimuthal non-uniformity of the RMF and the configuration is consistent with the continuity of the electron flow flux and the magnetic field reversal conditions. The RMF with spatial high-harmonic components provides quasi-steady current drive of high-beta and singly-connected FRC plasmas without destructive modes and will be helpful in reducing the particle loss and thermal load when applied to the fusion core plasma.

DOI： 10.1088/0029-5515/49/5/055010

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

A high poloidal beta, β_p, was achieved using pulsed poloidal current drive (PPCD) in a toroidal pinch experiment-RX (TPE-RX). The plasma electron density and temperature increased, hence improving β_p from 5 to 30% during PPCD. β_p is almost equal to the total beta in the reversed-field pinch (RFP). D-alpha emission from recycling deuterium by the plasma-wall interaction decreased during the PPCD. An improved particle confinement time was indicated by a ten fold increase in the ratio of the total number of particles to the D-alpha emission. Single pellet injection into the good particle confinement plasma, achieved by the PPCD, produced high density plasma with high beta values. The plasma electron density rapidly increased to triple that of standard plasma, and high density was maintained till the end of the PPCD period. The electron temperature was lower than that of PPCD without ice pellet injection, but β_p remained almost the same because the plasma electron density was higher with pellet injection.

DOI： 10.1585/pfr.4.022

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We have measured fast magnetic fluctuations (> 10 kHz) using a complex edge probe in the edge region of TPE-RX reversed-field pinch plasma. We observed that the frequency of magnetic fluctuation at the peak power in the frequency spectrum varies with time, and this frequency variation is determined by the values of pinch and reversed parameters, although the amplitude of the fast magnetic fluctuations are almost independent of those parameter values.

DOI： 10.1585/pfr.3.060

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The effects of an internal structure on the equilibrium of a field-reversed configuration (FRC) plasma sustained by rotating magnetic field is investigated by using detailed electrostatic probe measurements in the FRC Injection Experiment apparatus [S. Okada, et al., Nucl. Fusion. 45, 1094 (2005)]. An internal structure installed axially on the geometrical axis, which simulates Ohmic transformer or external toroidal field coils on the FRC device, brings about substantial changes in plasma density profile. The internal structure generates steep density-gradients not only on the inner side but on the outer side of the torus. The radial electric field is observed to sustain the ion thermal pressure-gradient in the FRC without the internal structure; however, the radial electric field is not sufficient to sustain the increased ion thermal pressure-gradient in the FRC with the internal structure. Spontaneously driven azimuthal ion flow will be accountable for the imbalance of the radial pressure which is modified by the internal structure. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2982493

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

Microwave imaging reflectometry (MIR) was developed in TPE-RX, one of the world's largest reversed field pinch (RFP) devices. The system optics are made of aluminum mirrors, Teflon lenses, and Plexiglas plates in order to reduce size. In this system, frequencies are stabilized so that noise can be reduced using narrow bandpass filters. A 4×4 2-D mixer array and phase detection system have also been developed. With this system, density fluctuations in the high-Θ RFP plasma, pulsed poloidal current drive (PPCD) plasma, and quasi-single helicity (QSH) plasma are observed in TPE-RX. This is the first MIR experiment in an RFP device.

DOI： 10.1585/pfr.3.053

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A denser plasma with a lower electron temperature is produced by radio frequency discharge with the use of multi hollow-cathode (HC). The effect of HC discharge and the diffusion of the plasma between HC and anode are depended on the gas pressure. A highly uniform plasma generated on the anode is modified by changing the effective width of grooves on the HC. The correction of the electron density distribution with the effect of HC discharge is compared with that of the electron temperature distribution.

DOI： 10.1088/1742-6596/106/1/012018

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

A very high beta value was obtained during pulsed poloidal current drive (PPCD) operation in a reversed-field pinch (RFP) plasma during a TPE-RX experiment. Poloidal beta is almost equal to total beta in the RFP, and is as high as 30% near the end of PPCD operation. During the PPCD operation, the magnetic fluctuation associated with the dynamo effect is reduced, and improved confinement is realized, which results in this high beta value.

DOI： 10.1585/pfr.2.050

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

Fast magnetic fluctuation (0) levels are measured using a newly developed complex edge probe (CEP) in the edge region of TPE-RX reversed-field pinch plasma. The CEP is installed inside a vacuum vessel and is sensitive to fast delta B. The delta B levels measured using the CEP are compared with those measured using an extensive magnetic measurement system (MMS), which is located outside the vacuum vessel and has little sensitivity to fast delta B. The delta B levels before and after the appearance of the locked mode (LM) are compared in both the CEP and MMS signals. It was found that the rapid increase in the delta B signal level obtained using the MMS just after the appearance of LM is mainly caused by the slowing of plasma rotation.

DOI： 10.1143/JJAP.46.6831

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A highly uniform plasma with the uniformity of +/- 1.7% for the ion saturation current was produced over the surface of flat grounded electrode. The approach is based on modifying the hollow-cathode so that the distribution of the ion saturation current over the electrode surface may be uniform by optimizing the distribution of the width of the grooves on the hollow cathode. The highly uniform plasma in the modifying hollow-cathode discharge also inherits the remarkable properties of the high electron density and the low electron temperature of the hollow-cathode discharge on the grounded electrode surface.

DOI： 10.1143/JJAP.46.6032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The confinement properties of a reversed-field pinch ( RFP) device are limited by the high level of magnetic stochasticity produced by the interaction of tearing modes in the plasma core. Away to reduce the core magnetic stochasticity is the application of the pulsed poloidal current drive ( PPCD) technique. By applying the PPCD, tearing modes are stabilized, magnetic stochasticity and turbulence are reduced and hence the confinement is enhanced. In the TPE-RX device, standard RFP plasmas are characterized by magnetic and electrostatic edge turbulence dominated by intermittent phenomena; on the contrary, the improved confinement regime generated by the PPCD is characterized by turbulence with a self-similar nature. We show that the increase in tau(p) is well correlated with the change in the nature of both the magnetic and the electrostatic turbulence. While plasmas with low confinement are dominated by intermittent events and by magnetic and electrostatic activity with a non-self-similar nature, plasmas with high confinement show turbulence with a self-similar character.

DOI： 10.1088/0741-3335/49/3/001

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The cold pulse propagation is a powerful tool for the study of heat transport and the determination of perturbative electron heat diffusivity chi(CP)(e). In this paper the experimental study of cold pulses generated by pellet injection in a reversed-field pinch device is described. The most common phenomenology is an inward cold pulse propagation, from the edge to the core; however, we will describe that outward propagation can occur if the pellet ablates up to the plasma centre. A model to simulate the dynamics of the pulse (i.e. the relation between the arrival time of the pulse and its radial position) has been developed. With this model it is possible to explain the cold pulse phenomenologies as well as to estimate chi(CP)(e) in the plasma core. The heat diffusivity in standard and enhanced confinement plasmas has been determined and compared; in standard regimes chi(CP)(e) is three times larger than in enhanced confinement regimes. Finally, it has been possible to confirm a scaling between diffusivity and magnetic stochasticity.

DOI： 10.1088/0029-5515/47/2/007

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

By applying a small positive pulse (Delta B-ta) in toroidal magnetic field, the quasi-single helicity (QSH) state can be obtained with a controllable and reproducible manner in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi , Fusion Eng. Des. 45, 421 (1999)]. The QSH state in RFP is one of the states where the improved confinement can be observed, and is important for development toward the pure single helicity (SH) state. In the SH state, the dynamo-action for sustaining the RFP configuration will be driven by a single helical mode and its harmonics, and the anomalous plasma loss can be avoided which is caused by the multi-helicity dynamo action in ordinary RFPs. In the operating condition presented here, the reversal of toroidal magnetic field (B-ta) is maintained at a shallow value (similar to-1 mT) for a certain period (similar to 20 ms) after the setting up of the RFP configuration and then the positive Delta B-ta (less than or similar to 5 mT magnitude and similar to 2 ms width) is applied to the B-ta, which is usually negative during the sustaining phase of RFP. Just after applying the pulse, the m/n=1/6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) grows dominantly and the configuration goes into QSH state. This QSH state can be sustained for a long period (up to similar to 45 ms) almost until the end of discharge by applying a delayed reversal of B-ta with appropriate timing and magnitude. The setting up of the QSH states shows a reproducibility of almost 100% with the same timing corresponding to the applied positive pulse. This observation can confirm the interpretation in the former report [Y. Hirano , Phys. Plasmas 12, 112501 (2005)], in which it is claimed that the QSH state is obtained when a small positive pulse in toroidal magnetic field spontaneously appears. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2405343

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Deuterium ice pellet injection has been used to control the density profile and expand the operation region of a reversed-field pinch (RFP) experiment on a toroidal pinch experiment-RX (TPE-RX) device. With pellet injection during the pulsed poloidal current drive (PPCD) operation, which is one of the good confinement operation conditions in RFP, electron density increases to more than three times that of standard discharge. The pellet is ablated within only approximately 1 ms, nevertheless, this high density can be almost constantly sustained for ∼10ms until completion of the PPCD period. When compared with ordinary discharge without PPCD, particle confinement time increases by a factor of five in the latter part of the PPCD period when the pellet is injected at the start of PPCD discharge. © 2006 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.45.L1124

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

A reversed-field pinch (RFP) start-up assisted by a magnetized plasma flow injection was demonstrated for the first time on a TPE-RX machine. This sequence of experiments aimed to establish a new method of ionization, gas-fill and helicity injection in the start-up phase of an RFP. In this start-up method, magnetized and well-ionized plasma is formed by a magnetized coaxial plasma gun and injected into the torus chamber as an initial pre-ionized plasma for RFP formation. In the initial experiments, attenuated density pump-out and comparatively slow decay of the toroidal flux and plasma current were observed as evidence of its being an effective start-up method. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2006.07.089

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An experimental investigation on the characteristics of compound RF discharge plasmas with a tapered shape hollow-cathode (HQ compared with a plane cathode is presented for the development of processing plasma. Dense plasmas are shown to be generated in the wide range of working pressures, 3 < p < 90 Pa. It is clarified that the higher RF power yields the greater HC effect to increase the ratio of. electron density with the HC to that without it. The HC effect is shown to work well for decreasing the self-bias potential compared with the plane cathode. It is clarified that the dense plasma with relatively low electron temperature produced by the compound RF discharge results from the synergetic effect of the lowered input PF power density and the effective increased discharge area due to the HC effect.

DOI： 10.1143/JJAP.45.8883

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The characteristics of HeH+ beams, which convert into He 0 beam used for the alpha particle diagnostics, are described. In order to extract HeH+ beams as a primary beam, the strongly focused high current density hydrogen neutral beam system, which has the multi aperture concave type electrodes and the bucket type ion source, is used [5]. It has been shown that the neutral hydrogen beam is strongly focused into a diameter of ∼36 mm at the focal point, with the divergence angle of about ±0.8 deg, which is almost the same as for HeH+ beam. In the case of low energy beam of about 300 eV, it is measured by the mass spectrometer with energy analyzer that the number of HeH+ particles corresponds to more than ∼15 % of the total counts for H+, He+ and HeH + particles in the case of PHe-ratio= 90 %. In the case of 25 kV acceleration, if 15 % of the total current (which includes H+, H2+, H3+, He+ and HeH+ components) is assumed to be HeH+ component, the current density of HeH+ is estimated as ∼13 mA/cm2, which value is much larger than necessary value (∼2 mA/cm2) in ITER. However, to confirm it, we must directly measure the beam current of the high energy HeH+ component, separated among the other components (H+, H2+, H3+, He +) by using the magnetic field system. This procedure will be performed in near future.

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High-density operation during the improved confinement is achieved by the combination of the PPCD and the pellet injection. The nel increases three times higher than the standard discharge. The particle confinement is well improved at least five times by the PPCD. The pellet injection does not disturb the improved confinement by the PPCD. The Te increases by the PPCD, and the Te in the combination of the PPCD and the pellet injection also increases.

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A plasma biasing experiment using an insertable electrode has been carried out in the STP-3(M) reversed field pinch. The profiles of electron density n(c), electron temperature T, and plasma potential V, were measured with an electrostatic probe array in the edge region of the plasma. The effect of biasing is observed as an increase of edge electron density and suppression of fluctuation amplitudes in n(c) T-c and V-s. As a result, both the electrostatic particle and energy fluxes are reduced and confinement improvement is observed, which is caused by the formation of E x B velocity shear layer. The reduction in both fluctuation amplitudes and coherence between fluctuating quantities contributes to this improvement. The formation of velocity shear layer is dependent on the insertion depth of the electrode, which is possibly related to the location where additional power is deposited through biasing. The extension of plasma lifetime is also observed in the biased discharges.

DOI： 10.1143/JPSJ.74.605

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

Peripheral plasma electrostatic fluctuations are studied with an inserted electrostatic probe array in the STP-3(M) Reversed Field Pinch, and radial profiles of electron density, electron temperature, and plasma space potential are measured. The particle flux driven by electrostatic fluctuation is calculated with the correlation between the fluctuation of electron density and fluctuation of plasma space potential, yielding the result that the increase of electrostatic fluctuation at the edge causes particle diffusions. The agreement between the E-r x B drift velocity of the radial electric field by the gradient of plasma space potential and the ion thermal velocity at the edge indicates ion motion dominates the particle diffusion. The decrease of the toroidal phase velocity at the edge and the confinement degradation shows the radial outside flow of the E-t x B-p drift, where E-t is generated by the toroidal flow fluctuationof the E-r X B drift. Therefore, the electrostatic fluctuation caused by the radial propagation of plasma space potential is thought of as the dominance of the radial particle flux. (C) 2004 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20019

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Pellet injection has been operated in TPE-RX. The pellet is injected into the TPE-RX from a horizontal port. It has been shown that the pellet is deflected in the plasma current direction at plasma edge and goes towards plasma center. It is considered that the fast electrons ablate the pellet and the pellet trajectory is deflected as expected. Pellet injection was performed in standard and PPCD discharges at Ip=300 kA. The density increases due to the pellet injection is more than two times that of standard plasma. The significant density increase in PPCD discharges is maintained for a long period. Copyright © (2005) by the European Physical Society (EPS).

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

The energy distribution of neutral particles emitted from STP-3(M) reversed field pinch (RFP) plasma has been investigated by a neutral particle analyzer (NPA) using a two-channel energy detector with the time resolution of 0.5μs over the whole duration of the discharge. NPA data show a high energy tail as well as the bulk ion distribution. It is found that the ion temperatures of bulk and high energy tail component have reached to the maximum before reversed magnetic field configuration, and that the tail temperature increases in the quiet period (QP). Furthermore, we found that the high energy ion density increases in both the relaxation period and the QP. The ion energy confinement time evaluated from magnetic reconnection model and the obtained ion energy spectrum is found to be about 720μs much longer than the electron energy confinement time 60-100μs, when ion heating mechanisms are discussed.

DOI： 10.1585/jspf.80.516

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The analysis of particle and energy fluxes was made by using electrostatic probes in the edge region of STP-3(M) reversed field pinch plasma. One of the new findings is that the plasma potential profile in the case of high current plasma indicates an increase in radial electric field E-r (= -delV(s)) in the region where 0.92 < r/a < 1.00, which implies the generation of E-r x B-p velocity shear. The electrostatic part of the particle flux was observed to decrease in the shear region, while the electron density and temperature profiles remained unchanged. Thus, spontaneous generation of velocity shear suppresses the electrostatic particle flux locally, but transition to global confinement improvement is not realized. The contribution of electrostatic fluctuations to energy flux in the edge is estimated as about 50%, and of stochastic diffusion is estimated as about 50%.

DOI： 10.1143/JPSJ.73.907

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Peripheral plasma electrostatic fluctuations are studied with inserted electrostatic probe array in the STP-3(M) Reversed Field Pinch, and radial profiles of electron density, electron temperature, and plasma space potential are measured. The particle flux driven by electrostatic fluctuation is calculated with the correlation between the fluctuation of electron density and fluctuation of plasma space potential, bringing us the results that the increase of electrostatic fluctuation at the edge causes particle diffusions. The agreement between the Er × B drift velocity of the radial electric field by the gradient of plasma space potential and the ion thermal velocity at the edge indicates ion motion dominates the particle diffusion. The decrease of the toroidal phase velocity at the edge and the confinement degradation shows the radial outside flow of the Et × Bp drift, where Et is generated by the toroidal flow fluctuation of the Er × B drift. Therefore, the electrostatic fluctuation caused by the radial propagation of plasma space potential is thought as the dominance of the radial particle flux. © 2003, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.123.1094

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In reversed field pinches (RFPs), edge fast electrons play some important roles in the energy transport and RFP dynamo. There have been here detailed measurements of electron currents and floating potential with electrostatic probes at the edge region, showing that field-aligned Current is carried by superthermal electrons, which results ill reversed toroidal current Outside the reversal surface. The time evolution of radial electric field, which is estimated from that of floating potential measured with insertable probes with some assumptions, revealed that radially Outward electric field is formed in about 100 mus. Given that the radial electric field is a result of superthermal electron diffusion in a stochastic magnetic field title, then the estimated diffusion coefficient agrees well with the numerically calculated values, which were obtained by using Monte-Carlo particle orbit calculation in combination with a 3-dimentional nonlinear MHD simulation. It is concluded that the radial electric field is formed by stochastic diffusion of superthermal electrons, which also play important roles in RFP dynamo.

DOI： 10.1143/JPSJ.71.1680

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Language：English   Publishing type：Article, review, commentary, editorial, etc. (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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Authorship：Last author,　Corresponding author   Language：Japanese   Publishing type：Internal/External technical report, pre-print, etc.  

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Authorship：Corresponding author   Language：Japanese   Publishing type：Internal/External technical report, pre-print, etc.  

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Language：English   Publishing type：Article, review, commentary, editorial, etc. (international conference proceedings)  

We have studied atmospheric pressure plasma generated using a quartz tube, helium gas and foil electrode by applying radio frequency and high voltage. The atmospheric pressure plasma with form of bullet is released like as a plume into the atmosphere. To study the characteristics of the atmospheric pressure plasma, the plasma current is measured using a current probe and the electron drift velocity is measured using a photo detector. The plasma circulation and plume currents are estimated from the circuit currents. The plasma plume density is examined from the plasma plume current and the drift velocity. The plasma plume charge is estimated as about 23 nAs at around 5.0 slm. The plasma density on the plasma released into the atmosphere is estimated as ∼ 1018 m-3. © 2013 IEEE.

DOI： 10.1109/PPC.2013.6627417

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Article, review, commentary, editorial, etc. (international conference proceedings)  

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