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A moving atmospheric-pressure nonthermal-equilibrium argon plasma bullet was investigated by emission spectrum and plasma current. Moving plasma changes spatially and temporally. Therefore, it is necessary to consider the time variation of plasma in measurement. Thus, the validity of the electron density estimated by the two types of optical and electrical measurement methods was examined. The electron density and temperature are estimated from the analysis of continuum spectrum (CS). Moreover, the electron density was also estimated from the relationship between the current and the propagation velocity of the plasma luminescence. The plasma current was estimated from the power-line currents with and without plasmas. The propagation velocity of the plasma luminescence was measured by applying the time-of-flight (ToF) method on the observed emission intensity. The electron density estimated from the analysis of CS is comparable to that estimated from the effective peak plasma current. Consequently, the analysis of CS is applicable to a moving plasma bullet.

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.

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 $\pi $ -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 $\pi $ -point region for the long length of SWS.

The dependence of the charge of an atmospheric-pressure non-thermal-equilibrium argon plasma incident on a copper object under floating conditions was investigated. The plasma generation and propagation charges, which are the time integral of the current, were estimated from the power-line current and the current passing through a quartz tube from which the plasma was emitted, which was measured using a current monitor. The plasma generation and propagation charges increased with the surface area of the copper object. The phenomenon of space-charge limitation was confirmed from the dependence of the plasma charge on the surface area of the copper object. The drift velocity of the plasma charge was estimated based on the relationship between the difference in measurement times considering the positions of the current monitors. The charge flux was estimated from the plasma charge and its drift velocity. The charge flux was linearly proportional to the product of the square of the applied voltage and the surface area of the copper object. Consequently, from Gauss's law, it was clarified that space-charge limitation occurs when the sum of the charge flux and the product of the square of the applied voltage and the surface area of the copper object is excessive for the product of the constants of permittivity and electron mobility.

In many plasma applications, the plasma current, which is determined by the plasma density and drift velocity, is an important parameter when investigating the induced phenomenon and its effects. However, it is unclear which physical parameters are responsible for regulating the current. Plasma is generated by a balance between the driving and restricting forces. The driving force originates from the electric and magnetic fields, and the restricting force originates from collisions between different species (electrons, ions, and neutral particles). When the electric field fluctuates over time, the pressure of the driving force is generated. The pressure of the driving force depends on the square of the electric field and the reciprocal of the repetition frequency. The pressure of the restricting force depends on the gas flow velocity and the collision frequency. When electrons and/or ions flow with the flowing neutral gas, a plasma current is generated from flowing charged particles. The magnitude of the plasma current is linearly proportional to the pressure caused by the driving force, and the variation in the plasma current depends on the pressure caused by the restricting force. In addition, the plasma current varies with the applied period of the voltage. Consequently, the plasma charge, which is the time integral of the plasma current, shows a linear relationship with the driving force and is thus regulated by the force balance between the driving and restricting forces. Therefore, the plasma current value and its generation time are regulated by the strength, applied period, and repetition frequency of the applied voltage and the neutral gas flow velocity.

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.

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.

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.

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 ∼1018 m−3 by the method. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The main findings of this study are that the plasma propagation state changes with the force equilibrium relationship between the driving force due to the electromagnetic field and neutral gas flow. The plasma propagation transitions from a continuous state to a discontinuous state when plasma emission light intensity changes. The plasma emission light intensity changes suddenly as the applied voltage frequency varies. The frequency at which plasma emission light changes is inversely proportional to the dynamic pressure with the flow velocity of neutral gas. The plasma with strong light emission at high frequency propagates continuously, while that with weak light emission at low frequency propagates discontinuously. Because the plasma current with strong light emission is larger than that with weak light emission, the plasma charge quantity—the amount of plasma generated—increases. Consequently, when the plasma quantity is enough to exist continuously as a group, the plasma group propagates in space with time variations like a stationary wave. However, when the plasma quantity is inadequate to exist continuously, the plasma group propagates through the space discontinuously like a bullet state.

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.

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.

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.

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.

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 stu...

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 SWS 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.

Two different directions of scale-to-scale energy transfer that corresponds to forward and inverse energy cascades, respectively, were clarified in toroidal plasma turbulence by means of experimental analysis. In the present work we first report the evidence that the turbulence dominated by broad wavenumber spectrum was developed under the action of forward (direct) energy cascade in a reversed field pinch (RFP) plasma, while the generation of GAM zonal flow (ZF) was related with the energy concentrating into low frequency region from higher frequency region due to inverse energy cascade in a tokamak plasma. The two evidences are compared. The results indicate a modulation mechanism in turbulence which is relative to the generation or development of specific turbulent flows. This hints a correlation between energy transfer and the development of toroidal plasma turbulence.

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.

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. A way 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 τp 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.

The cold pulse propagation is a powerful tool for the study of heat transport and the determination of perturbative electron heat diffusivity . 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 in the plasma core. The heat diffusivity in standard and enhanced confinement plasmas has been determined and compared; in standard regimes is three times larger than in enhanced confinement regimes. Finally, it has been possible to confirm a scaling between diffusivity and magnetic stochasticity.

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.

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 e , electron temperature T e and plasma potential V s 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 e , T e 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 × 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.

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 (=-∇ V s ) in the region where 0.92< r / a

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 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 Er × 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 of as the dominance of the radial particle flux. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(1): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.20019

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.

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 Er×Bp drift, where Et is generated by the toroidal flow fluctuation of the Et×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.

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 obs...

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 excitati...

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. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Backward wave oscillators (BWOs) have a slow-wave structure (SWS) with periodically corrugated walls. To increase the operation frequency and the power handling capability, the SWS is oversized with the diameter larger than the free-space wavelength of output electromagnetic wave. Generally, the slow wave becomes a surface wave concentrated on the corrugation of oversized SWS. Excited surface waves are converted into radiating waves at the ends of SWS, leading to BWO radiations1. There exists an alternative mechanism to radiate electromagnetic waves in periodic structures, i.e., Smith-Purcell (SP) mechanism which directly excites radiating waves2. Original SP radiation was a spontaneous emission. SP radiation more intense than the original one was demonstrated based on the stimulated emission, in which self-bunching of electron beam due to the backward-wave interaction plays an essential role3.

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.

Fast magnetic fluctuation (δB) 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 δB. The δB 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 δB. The δB 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 δB signal level obtained using the MMS just after the appearance of LM is mainly caused by the slowing of plasma rotation.

Summary form given only. Recently, plasma techniques under atmospheric pressure circumstances have been adapted for industrial, medical and biological applications. Atmospheric pressure plasmas, such as a plasma jet are typically produced using an arc discharge and a dielectric barrier discharge (DBD). The arc discharge produces a high-temperature plasma, whereas DBD produces a low-temperature plasma, which results in less thermal damage to materials. Atmospheric pressure plasma of DBD is generated intermittently using dielectric, rate gas and metal electrode by applying radio frequency and high voltage1. When it is used a quartz tube as a dielectric, plasma is released to the atmosphere. A plasma plume is a small bulletlike volume of plasma traveling at unusually high velocities2. We have studied a generation of high density plasma jet using focused plasma jets3. However, the detailed properties of the plasma current and density were still not estimated. In this work, we use helium plasma jet produced using a quartz tube and copper foil electrodes where a low frequency (~13 kHz) and a high voltage (-6 ~ +9 kV) are applied. The measurement of plasma current is used by a current probe. The plasma drift velocity is measured indirectly by the drift velocity of luminous bullet on the time of flight method. The plasma cross-section is estimated on the photographs. The plasma density is evaluated from these measurements values. The dependences of plasma current and density on the helium gas flow rate are examined. Detail experimental results and discussion will be presented at the conference.