永田 正義

Abstract Results from the successful solenoid-free plasma startup using the method of transient coaxial helicity injection (transient CHI) in the QUEST spherical tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first of its kind, floating single biased electrode configuration, which does not use such a vacuum break. Instead, the CHI electrode is simply insulated from the outer lower divertor plate support structure. This configuration is much more suitable for implementation in a fusion reactor than the previous configurations. Transient CHI generated toroidal currents of 135 kA were obtained. The toroidal current during the formation of a closed flux configuration was over 50 kA. These results bode well for the application of transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

Spherical tokamak (ST) research in Japan has produced many innovative results: (i) plasma start-up to I p > 70 kA was achieved by electron cyclotron wave (ECW) with N ∥ = 0.75, while electron heating to T e > 500 eV was achieved with N ∥ = 0.26 on QUEST. (ii) The radiofrequency (RF)-induced transport model was combined with the x-ray emission model, and extended magnetohydrodynamics equilibrium with kinetic electrons was developed to interpret fast-electron-dominated lower hybrid wave sustained plasmas on TST-2. (iii) Density as high as 30 times the cutoff density was achieved by electron Berstein wave current drive combined with electron beam injection on LATE. (iv) Multiple plasmoids formed by tearing instability in the elongated current sheet were observed, and flux closure and ion heating by plasmoid-mediated fast magnetic reconnection were observed on HIST. (v) Optimization of ECW-assisted inductive start-up with a vertical field with positive decay index was performed on TST-2. (vi) Stabilization of the vertical displacement event by a set of upper and lower helical field coils was demonstrated on TOKASTAR-2. (vii) A 6 h discharge was achieved by cool-down of the center stack cover on QUEST, where the plasma duration limit was consistent with the wall saturation time estimated by modeling. (viii) Extension of ion heating by plasma merging was achieved on TS-3U, TS-4U, UTST, MAST, and ST40.

Multiple plasmoid reconnection required for the flux closure in the transient-coaxial helicity injection (T-CHI) start-up process has been demonstrated in the Helicity Injected Spherical Torus (HIST) device. Two or three plasmoids are generated after the tearing instability of an elongated Sweet-Parker current sheet during the T-CHI. Here, we report that in the T-CHI start-up plasmas (H, D and He) with the strong toroidal (guide) field (ITF=140 kA), (i) the frequency of regular oscillations of reconnecting magnetic field decreases as the mass number increases, i.e., 250 kHz (H), 150 kHz (D) and 60 kHz (He). (ii) the oscillation propagates radially with 30 km/s (H), 20 km/s (D) and 12-18 km/s (He) from R= 0.25 m at the X-point toward the outboard side. It has been found that the propagation speed agrees with the Alfven speed. Consequently, the plasmoid reconnection could be related to the excitation of Alfven wave, leading to the ion heating in the T-CHI start-up plasmas.

Two-fluid (ions and electrons) flowing equilibrium configurations of spherical torus plasma are numerically determined by using the multi-grid method. The axisymmetric equilibrium equations consist of a pair of second-order partial differential equations for the magnetic (electron) and ion stream functions, and Bernoulli equations for the density. It is found from the numerical results that the multi-grid method with the damped Jacobi method in the smoothing step is effective for solving these equations with numerical stability, high accuracy, and high speed.

This paper presents the results of round-robin test (RRT) of repetitive partial discharge inception voltage (RPDIV) measurements with a stator coil winding of 4 kW randomwound motor. Nine independent laboratories in Japan participated in the RRT in order to validate evaluation procedures described in IEC 60034-18-41. The results provided new insights into RPDIV measurements with complete windings such as ambient humidity and pre-discharge effects.

The multichannel ion Doppler spectroscopic (M-IDS) system using a fast camera as detectors has been newly developed to measure the spatial profiles of impurity spectral line emissions from plasmas. The M-IDS system developed with eight viewing chords is characterized by the simplest optical systems, a fast sampling frequency, high number of shooting, and a low crosstalk of 3.5% on each chord. In order to verify the usefulness and reliability of the M-IDS system, we have applied it for the start-up plasma produced in the helicity injected spherical torus device. Resultantly, we have successfully measured the radial profiles of Doppler ion temperature T-i.D and Doppler ion velocity v(i.D) of the plasma with a good Gaussian fitting. However, we have found that the sampling frequency is upper limited by 25 kHz because the signal detected with a faster frequency from the CIII impurity line emission is too weak to be analyzed, and thus the sensitivity of the total system needs to be improved. (C) 2017 Wiley Periodicals, Inc. Electron Comm Jpn, 100(2): 68-75, 2017; Published online in Wiley Online Library (wileyonlinelibrary.com).

Energy transfer processes from ELM-like pulsed helium (He) plasmas with a pulse duration of similar to 0.1 ms to aluminum (Al) and tungsten (W) surfaces were experimentally investigated by the use of a magnetized coaxial plasma gun device. The surface absorbed energy density of the He pulsed plasma on the W surface measured with a calorimeter was similar to 0.44 MJ m(-2), whereas it was similar to 0.15 MJ m(-2) on the Al surface. A vapor layer in front of the Al surface exposed to the He pulsed plasma was clearly identified by Al neutral emission line (Al I) measured with a high time resolution spectrometer, and fast imaging with a high-speed visible camera filtered around the Al I emission line. On the other hand, no clear evaporation in front of the W surface exposed to the He pulsed plasma was observed in the present condition. Discussions on the reduction in the surface absorbed energy density on the Al surface are provided by considering the latent heat of vaporization and radiation cooling due to the Al vapor cloud.

A magnetized coaxial plasma gun (MCPG) device at Univ. of Hyogo was used as a simulation facility for pulse heat loads such as ELMs (Edge Localized Modes) onto plasma-facing materials in magnetically confined fusion devices. In the present study, characterization of the compact toroid (CT) plasmas produced by the MCPG device was performed. Dependences of the CT plasma parameters of line-averaged electron density, ion temperature and flow velocity on the gun discharge voltage and the bias magnetic flux were measured. As a result, it was observed that the pulse length, the line-averaged electron density, and the ion temperature and flow velocity were approximately 200 μs, approximately 6 ×1021 m-3, approximately 30 eV, and approximately 50 km s-1, respectively. The surface energy density measured with a tungsten calorimeter was approximately 0.4 MJ m-2. Thus, it was shown that the ELM-like pulsed plasma heat loads with the energy density and the pulsed length predicted in the ITER divertor can be produced using the MCPG device.

The multi-channel ion Doppler spectroscopic (M-IDS) system using a fast camera as detectors has been newly developed to measure the spatial profiles of impurity spectral line emissions from plasmas. The M-IDS system developed with 8 viewing chords is characterized by the simplest optical systems, a fast sampling frequency, high number of shooting and a low crosstalk of 3.5% on each chord. In order to verify the usefulness and reliability of the M-IDS system, we have applied it for the start-up plasma produced in the Helicity Injected Spherical Torus (HIST) device. Resultantly, we have successfully measured the radial profiles of Doppler ion temperature Ti.D and Doppler ion velocity vi.D of the plasma with a good Gaussian fitting. However, we have found that the sampling frequency is upper limited by 25 kHz because the signal detected with a faster frequency from the CIII impurity line emission is too weak to be analyzed, so that the sensitivity of the total system needs to be improved.

<p>A fast two-color pyrometer system was developed to measure the back-surface temperature of thin tungsten materials during plasma-gun generated edge localized mode-like pulsed plasma irradiation. The developed pyrometer system had a time resolution of ∼5 µs and the lowest measureable temperature was ∼1600 K. We observed that the back-surface temperature of the thin tungsten material during the pulsed plasma irradiation reached ∼3280 K. The absorbed energy density and the pulse width of the pulsed heat load estimated by the measured time evolution of the back-surface temperature and 3D heat analyses using ANSYS code were ∼0.52 MJm⁻² and ∼1.6 ms, respectively.</p>

In this study, surface damaged W monoblocks (melting and cracking) by a pulsed plasma gun and an e-beam devices were exposed to cyclic heat loads (simulating normal heat loads and slow transients) and pulsed heat loads (simulating ELMs) to observe the effects of surface damage on surface erosion and heat removal capability. Heat load tests simulating the normal heat load (10 MW/m(2), 10 sec, 300 cycles) and the slow transient (similar to 20 MW/m(2), 10 sec, 300 cycles) were performed by the e-beam. The surface morphology changes after the heat load tests were observed using laser sccaning microscopy and FE-SEM After e-beam irradiation of similar to 20 MW/m(2), the longitudinal cracks crossing over entire mono blocks appeared on the surfaces of all monoblocks. Recrystallization and additional crack formation were also observed on the surface. However, there was no significant change of heat removal capability. In the additional pulsed heat load test, the energy fluence of 0.042-0.30MJ/m(2) was applied with pulse numbers of 10(3) and 10(4).The surface morphology changes after laser irradiation were observed using laser scanning microscope. After laser irradiation, the grain ejection occurred above a certain energy fluence (similar to 25 % of melting threshold).

We have developed a unique experimental device of so-called double plasma gun, which consists of two magnetized coaxial plasma gun (MCPG) devices, in order to clarify effects of vapor shielding on material erosion due to transient events in magnetically confined fusion devices. Two ELM-like pulsed plasmas produced by the two MCPG devices were injected into a target chamber with a variable time difference. For generating ablated plasmas in front of a target material, an aluminum foil sample in the target chamber was exposed to a pulsed plasma produced by the 1st MCPG device. The 2nd pulsed plasma was produced with a time delay of 70 mu s. It was found that a surface absorbed energy measured by a calorimeter was reduced to similar to 66% of that without the Al foil sample. Thus, the reduction of the incoming plasma energy by the vapor shielding effect was successfully demonstrated in the present experiment. (C) 2014 Elsevier B.V. All rights reserved.

Surface modifications of toughened, fine-grained, recrystallized tungsten (TFGR W) materials with 1.1 wt.% TiC and 3.3 wt.% TaC dispersoids due to repetitive ELM-like pulsed (similar to 0.15 ms) helium plasma irradiation have been investigated by using a magnetized coaxial plasma gun. No surface cracking at the center part of the TFGR W samples exposed to 20 plasma pulses of similar to 0.3 MJ m(-2) was observed. The suppression of surface crack formation due to the increase of the grain boundary strength by addition of TiC and TaC dispersoids was confirmed in comparison with a pure W material. On the other hand, surface cracks and small pits appeared at the edge part of the TFGR W sample after the pulsed plasma irradiation. Erosion of the TiC and TaC dispersoids due to the pulsed plasma irradiation could cause the small pits on the surface, resulting in the surface crack formation. (C) 2014 Elsevier B.V. All rights reserved.

Effects of humidity and pre-discharges on repetitive partial discharge inception voltage (RPDIV) with twisted enameled wires under bipolar and unipolar impulse voltages have been investigated. The RPDIV under the bipolar impulse voltages have become relatively scattered during the early stage of the voltage sequence in the dry condition. After sufficient number of the voltage sequence, it has reached to a stable state. On the other hand, the RPDIV have reached to a stable state from the beginning of measurement in the humid condition. In the case of the unipolar impulse voltages, no scattered state of the RPDIV has been observed. Surface potential on the enameled wire after the discharge has been measured with a surface potential meter in order to clarify effects of pre-discharges on the RPDIV. As the results, the fluctuation in the RPDIV is associated with the accumulation of the surface charges generated by the discharges.

Coaxial Helicity Injection (CHI) is one of useful methods for start-up to spherical torus (ST) and spheromak plasmas. The new approach of CHI so called the Multi-pulsing CHI operation has been proposed for the purpose of achieving the quasi-steady-state plasma and demonstrated in the sustained spheromak physics experiment (SSPX) gun-spheromak device for the first time. We conducted the initial experiment with the first application of double-pulsing CHI for the ST configurations. The experimental results showed flux/current amplification and sustainment of the plasmas. We observed repeating the driven phase and the decay phase from change of profile of force-free parameter λ. By the double-pulsing CHI operation, the sustainment time had increased up to about 8 ms which was longer than that in the single discharge. We investigated dependency of plasma current, density and the current density profile on the external toroidal field strength.

Characterization of a pulsed plasma formed with a plasma gun device was performed. The averaged electron density was measured with an interferometer system using a Zeeman laser at 633 nm. It was found that the plasma with the density of similar to 10(22) m(-3) was successfully formed. The temporal evolution of the plasma was observed with a fast framing camera. It was found that the shape of the plasma was significantly varied temporally. From the intensity profile, the vertical emission profile was obtained using Abel inversion. The emission profile had a flat top shape with the radius of similar to 20 mm, and gradually decreased with the radius. The penetration behavior of the gun plasma across the magnetic field was observed.

The feasibility of compact torus (CT) neutralization fuel injection method is studied by a simulation model using particle and MHD hybrid techniques. The neutralization process is simulated by using rate-coefficients. The magnetic and electric fields are found to respond sluggishly to the neutralization process. Slow ions generated by charge-exchange have been added to the model, although the CT neutralization process is not significantly affected by this.

Synergistic effects of steady state plasma irradiation and transient heat/particle loads to plasma facing materials are an important plasma surface interaction issue in future fusion devices. To investigate the effects systematically, a new plasma gun device was developed and was combined to the linear plasma device NAGDIS-I. Fundamental characteristics of the plasma gun device are revealed in this paper. The amount of tungsten released from the electrode was measured with a material probe method. Also, the emissions from the impurities were investigated spectroscopically. To measure the plasma density, an interferometer measurement was developed the ion temperature was measured spectroscopically. Moreover, a preliminary irradiation test of the plasma pulse to a tungsten sample was done, and the surface morphology change was shown with the analysis by a scanning electron microscope. © 2013 Elsevier B.V. All rights reserved.

This paper discusses the lifetime evaluation of twisted pairs which was constructed by a surge proof magnet wire. Two kinds of twisted pairs were constructed; one is a conventional twisted pairs and the other is a special twisted pairs which has varnish treatment at both edge sides of it. The both lifetime characteristics were evaluated and the breakdown positions were also investigated. As a result, it was found that the lifetime of the edge parts of usual twisted pairs was about 50% shorter than that of the other positions. It was estimated that the partial discharge activity at the edge part is stronger than that at the other positions, by an ultraviolet light measurement or others. This paper also describes a concept and quality control method for the inverter fed random wound motor which was manufactured by using a surge proof magnet wire.

The feasibility of compact torus (CT) neutralization fuel injection method is studied by a simulation model using particle andMHD hybrid techniques. The neutralization process is simulated by using rate-coefficients. The magnetic and electric fields are found to respond sluggishly to the neutralization process. Slow ions generated by charge-exchange have been added to the model, although the CT neutralization process was not significantly affected by this. Finally, the minimum length for the CT neutralizer is proposed as 2m from the simulation run time 10 μs at a CT injection speed of 200km/s. © 2012 The Japan Society of Plasma Science and Nuclear Fusion Research.

Surface damage of carbon fiber composite (CFC) and tungsten (W) due to repetitive ELM-like pulsed plasma irradiation has been investigated by using a magnetized coaxial plasma gun. CX2002U CFC and stress-relieved W samples were exposed to repetitive pulsed deuterium plasmas with duration of similar to 0.5 ms, incident ion energy of similar to 30 eV, and surface absorbed energy density of similar to 0.3-0.7 MJ/m(2). Bright spots on a CFC surface during pulsed plasma exposures were clearly observed with a high-speed camera, indicating a local surface heating. No melting of a W surface was observed under a single plasma pulse exposure at energy density of similar to 0.7 MJ/m2, although cracks were formed. Cracking of the W surface grew with repetitive pulsed plasma exposures. Subsequently, the surface melted due to localized heat absorption. (C) 2010 Elsevier B.V. All rights reserved.

The magnetized coaxial plasma gun (MCPG) device at University of Hyogo for simulation experiments of type-I ELM heat loads has been upgraded to improve the performance. The characteristics of the upgraded MCPG device have been investigated and compared with the previous one. The capacitor bank energy for the power supply of the MCPG was increased from 24.5 kJ to 144 kJ. In the preliminary experiments, the plasmoid with duration of similar to 0.6 ms, the incident ion energy of similar to 40 eV, and the surface absorbed energy density of similar to 2 MJ/m(2) was successfully produced by the present MCPG device at the gun voltage of 6 kV.

We have investigated the mechanism to rebuild the magnetic fields of the HD-ST plasmas during the M-CHI by using 3D MHD simulation. During the driven phase, the poloidal flux and the toroidal current are amplified by merging of the pre-existing plasma with ejected one which involves magnetic reconnections. During the decay phase, the HD-ST approaches the axisymmetric MHD equilibrium state without flow due to the dissipation of magnetic fluctuations to increase the closed flux surfaces. It is expected that by effectively controlling the M-CHI, during the decay phase, the magnetic fluctuations induced during the driven phase dissipate and the axisymmetric MHD equilibrium with the closed flux surfaces is achieved, resulting in the effective current and flux amplification during the driven phase.

The inactivation of Bacillus atrophaeus spores by a dielectric barrier discharge (DBD) plasma in atmospheric humid air was investigated in order to develop a low-temperature, low-cost, and high-speed plasma sterilization technique. The biological indicators covered with a Tyvek sheet were set just outside the DBD plasma region, where air temperature and humidity as a discharge gas were precisely controlled by an environmental test chamber. The results show that the inactivation of B. atrophaeus spores was found to be dependent strongly on humidity, and was completed within 15 min at a relative humidity of 90% and a temperature of 30 °C. The treatment time for sterilization is shorter than those of conventional sterilization methods using ethylene oxide gas and dry heat treatment. The inactivation rates depend on not only relative humidity but also temperature, so that water content in air could determine the generation of reactive species such as hydroxyl radicals that are effective for the inactivation of B. atrophaeus spores.

A new concept of particle fueling method is proposed, and its feasibility is studied by a Monte Carlo simulation; by using a compact torus (CT) injection technique and subsequent neutralization of an accelerated CT plasma, ultrahigh-speed neutral-particle flow can be generated. The charge-exchange reaction between ions in a CT plasma and neutral particles in the neutralizer is found to work efficiently to neutralize the CT plasma within a 1-m long neutralizer in a case that the neutral gas exceeds 5 x 10(-2) torr.

A compact-sized magnetized coaxial plasma gun (MCPG) with a single discharge stage is developed in order to simulate transient heat loads such as type I edge-localized modes (ELMs) in ITER. The MCPG produces hydrogen plasmas with an electron density of 1-4 x 10(21) m(-3), an ion energy of 15 eV and a pulsewidth of 0.5 ms. It is found that the absorbed energy density measured by a calorimeter reaches 0.9 MJ/m(2) which is comparable with a transient heat load of type I ELMs expected in ITER. Erosion of tungsten due to the plasma irradiation produced by the MCPG is confirmed only at the edge parts of the specimen in this experiment.

Kink instability and the subsequent plasma flow during the sustainment of a coaxial gun spheromak are investigated by three-dimensional nonlinear magnetohydrodynamic simulations. Analysis of the parallel current density λ profile in the central open column revealed that the n = 1 mode structure plays an important role in the relaxation and current drive. The toroidal flow (v_t ≈ 37 km/s) is driven by magnetic reconnection occurring as a result of the helical kink distortion of the central open column during repetitive plasmoid ejection and merging.

We investigated the characteristics of the partial discharge (PD) with twisted enameled wires by optical emission spectroscopy (OES) in atmospheric humid air. The electric field strength in the gap of the twisted enameled wires during the discharges was estimated from intensity ratio of the spectrum bands of nitrogen molecules obtained by the OES. It was found that the electric field strength was increased when increasing the relative humidity, and the increasing rate was different depending on the temperature. The increase of the electric field strength in the gap of the twisted pair could be related to a reduction of PD inception voltage (PDIV) observed in the experiment. The reduction of the PDIV could be determined by not only dielectric properties of enameled wires such as permittivity and surface conductivity but also changes of the discharge phenomena in the humid air. In this study, effects of the increase of the permittivity of the enameled wire due to the moisture absorption into the insulating layer were quantitatively discussed as one of the candidates to explain the reduction of the PDIV in the humid conditions. On the other hand, the discharge current amplitude was decreased with the increase of relative humidity at high temperature. It was considered that the discharge could be quenched by dissociative electron attachment to the large number of water molecules. The observed PD phenomena with respect to the change of the humidity are very important for quality management of low voltage inverter-fed random wound motors.

Application of an externally applied rotating magnetic field (RMF) for control of MHD relaxation phenomena driven by a coaxial helicity injection has been proposed in the HIST spherical torus device. In this letter, the plasma responses to the RMF evaluated by magnetic fields inside the plasma in HIST are shown. © 2009 The Institute of Electrical Engineers of Japan.

A compact-sized MCPG with a single discharge stage is developed so as to simulate transient heat loads such as type I ELMs in ITER. The MCPG produces H(2) plasma with the electron density of 1-4x10(21) m(-3), the ion energy of 15 eV and the pulse width of 0.5 ms. It is found that the absorbed energy density measured by a calorimeter reaches 0.9 MJ/m(2) which is comparable to transient heat load of type I ELMs expected in ITER. Erosion of tungsten due to the plasma irradiation produced by the MCPG was confirmed only at the edge parts of the specimen.

The magnetized coaxial plasma gun (MCPG) was investigated to seek possibilities of its application for a transient high heat testing to the ITER divertor surfaces. The experimental results showed that the MCPG could generate the hydrogen plasma stream with the ion energy of 50 eV, the peak energy density of 0.7 MJ/m(2) and the discharge duration of similar to 0.05 ms. This capability is enough powerful to melt tungsten materials. Vaporization of a thin surface layer and a cloud of dense vapor plasma in front of the carbon calorimeter were observed by spectroscopic measurements. (C) 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Effects of ambient humidity and temperature on partial discharge (PD) characteristics in twisted pairs with conventional and nanocomposite enameled magnet wires have been investigated. Measurements of the PD inception voltage (PDIV) and the apparent PD charge were performed by applying an ac sinusoidal waveform voltage of 60 Hz to the twisted pair. The observed PDIV in this study can be classified into the following three behaviors. First, the PDIV in both wires decreases with increasing relative humidity (RH) at low temperature. Secondly, the PDIV in both wires slightly increases when increasing the RH at high temperature. Thus, there is a certain value of the absolute humidity to determine the lowest PDIV in both cases with the conventional and nanocomposite enameled wires. Thirdly, the PDIV in the conventional enameled wire significantly decreases at the R.H of 95 %, while the decrease rate of that in the nanocomposite enameled wire is relatively small. The observed PD phenomena are discussed in terms of material characterization of the enameled wires such as changes in permittivity due to moisture absorption and wettability under voltage application.

Two-dimensional two-fluid flowing equilibria of helicity-injected spherical torus with non-uniform density and both toroidal and poloidal flows for each species have been numerically determined by the nearby-fluids procedure. It is found from the numerical results that the equilibrium for the driven λ (≡ μ₀ j·B/B² ) profile exhibits a diamagnetic toroidal field, high-β (toroidal beta value, β_t = 32%), hollow current profile, and centrally broad density. In contrast, the decaying equilibrium exhibits a paramagnetic toroidal field, low-β (β_t = 10%), centrally peaked current profile, and density with a steep gradient in the outer edge region. In the driven case, the toroidal ion and electron flows are in the same direction, and two-fluid effects are less important since the E × B drift is dominant. In the decaying case, the toroidal ion and electron flows are opposite in the outer edge region, and two-fluid effects are significant locally in the edge due to the ion diamagnetic drift.

In the National Spherical Torus Experiment (NSTX), a peak plasma current up to 390 kA has been successfully generated by the Coaxial Helicity Injection (CHI) current drive method. The plasma rotation (∼ 20 km/s) driven in the E × B toroidal direction by CHI has been clearly identified by an ion Doppler spectroscopic measurement. The n = 1 mode has been also observed to rotate in the same direction. This rotating kink behavior observed for the first time in NSTX is consistent with the electron locking model developed in the Helicity Injected Torus-II (HIT-II) experiments to explain the mechanism of CHI current drive.

A two-fluid flowing equilibrium of a helicity-injected spherical torus (HI-ST) plasma in the more realistic region, including a flux conserver and a coaxial helicity source, is numerically determined by using the combination of the finite difference and the boundary element methods. It is found from the numerical results that magnetic configurations change from the high-q HI-ST (safety factor q &gt; 1) with paramagnetic toroidal field and low-beta (volume average beta value, &lt; beta &gt; approximate to 2%) through the helicity-injected spheromak and reversed-field pinch to the ultralow-q HI-ST (0 &lt; q &lt; 1) with diamagnetic toroidal field and high-beta(&lt; beta &gt; approximate to 18 %) as the external toroidal field at the inner edge regions decreases and reverses the sign. The effects of the ion flow on the MHD equilibrium configurations are discussed.

The compact toroid (CT) injection system for JFT-2M is composed of a CT injector, a gas delivery and vacuum system, a power supply system, and a diagnostics system. In particular, the power supply system delivers high performance for CT formation and acceleration. The CT formation capacitor bank unit achieved a formation current of 350 kA with a rise time less than 10 μs. Although the CT acceleration bank units are equipped with 14 ignitron switches instead of gap switches to attenuate the discharge noise level, an acceleration current of 400 kA with a short rise time of 9 μs is controlled within a jitter of much less than 1 μs. The resulting CT velocity and mass density satisfy the requirements for CT penetration into the tokamak plasma core at a toroidal field of 1 T. This CT injection system is thus suitable for CT injection in a middle-sized tokamak plasma such as the JFT-2M tokamak. © 2006 Elsevier B.V. All rights reserved.