In this paper, we use hybrid simulation to elucidate the plasma heating mechanism due to waves excited in Field-Reversed Configuration (FRC) plasma. The plasma parameters are a separatrix radius of 0.16 m and a separatrix length of 1.16 m (x-point position is z = +/- 0.57 m). The wave excitation antenna consists of two loop antennas with a radius of 0.3 m and is placed at a position of z = +/- 0.5 m. The current waveform of the antenna is a sine wave with a maximum current value of 30 kA and a frequency of 160 kHz. The simulation results showed that the excited waves caused compression/expansion of the plasma, and at the same time, the temperature of the plasma increased or decreased at the compressed/expanded position. When waves are applied, a 23% increase in the volume-averaged ion temperature in the separatrix is observed compared to the case without waves applied. On the other hand, no increase in electron temperature is observed. For the electron fluid, the adiabatic condition is well established, and temperature changes are observed as the plasma compressed and expanded. On the other hand, for ions, kinetic energy perpendicular to the magnetic field lines increases during compression, and part of this energy is transferred to the energy of the parallel component by collisionless pitch angle scattering, resulting in heating due to the so-called magnetic pumping.

A hybrid simulation (a model that treats ions as particles and electrons as fluid) is performed to analyse the propagation of waves excited in the field-reversed configuration plasma and the resulting plasma response. The current of the wave excitation antenna changes in a sine wave, and its frequency is set so that it has an ion cyclotron resonance point inside the separatrix. When the antenna current is maximum, a magnetic field with a magnitude of 40% of the external magnetic field is created on the separatrix. A toroidal magnetic field is excited in the plasma by applying waves. The observed propagation velocity of the toroidal magnetic field is comparable with the shear Alfvén wave outside the separatrix, and is on the same order within the separatrix. This result has a tendency similar to the propagation velocity outside the separatrix reported in the wave experiment in the past FIX machine. The simulation results also show that when the excited magnetic field propagates in the axial direction, the separatrix is compressed or expanded, and the high-density region of the ions formed thereby moves in the axial direction. In addition, the excited magnetic energy is rapidly decreased near the position where the velocities of the shear Alfvén wave and the ion sound wave are equal (local beta value is 0.88). It is found that the decay of the excited magnetic energy occurred at a point outside the ion cyclotron resonance point. This suggests that the compression and expansion of the plasma is caused while maintaining the quasi-equilibrium state according to the change in the external magnetic pressure.

A super-Alfvenic/sonic collisional merging formation of field-reversed configurations (FRCs) with low-density and high-temperature initial-FRCs was attempted on the FAT-CM device at Nihon University. To vary the density and temperature of initial-FRCs, the low-density/high-temperature (LD/HT) FRC formation technique was applied to the initial-FRC formation. The electron density of initial-FRCs formed using the LD/HT FRC formation technique was reduced to about 50% of that in the standard cases. The ion temperature was increased as the electron density decreased because the plasma pressure completely balances with the external magnetic pressure in an ideal FRC. The ion mean-free-path also increased to the equivalent value of the diameter of the initial-FRCs. Therefore, the initial-FRCs will be collision-less. These collision-less initial-FRCs were successfully translated. The observation results of the collisional merging formation process of FRC from the internal magnetic probe array and two axially arranged interferometers indicate that the performance of the FRC formed after the collision and merging declined in cases with collision-less FRCs and it depends on the kinetic energy in the collision process. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research

Using a hybrid simulation code, we studied the application of low-frequency wave to a field-reversed configuration plasma. The wave application antenna was arranged to sandwich the separatrix, and the antenna current was varied sinusoidally with a maximum of 30 kA and a frequency of 80 kHz. The simulation revealed that when the ion pressure is regarded as a tensor, the parallel ion temperature estimated from the axial velocity distribution near the midplane exceeds the perpendicular temperature. That is, the plasma ions respond anisotropically to a low-frequency wave. (C) 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Wave excitation and propagation by applying a low frequency wave to a high-beta plasma are simulated by a 3-dimensional hybrid model, where ions are treated as particles while electrons are regarded as fluids. It is found that excitation of the toroidal magnetic field occurs outside the separatrix and the wave attenuated significantly near the separatrix. Although the attenuation due to an ion cyclotron resonance is conceivable, no temperature rise in the resonance region is observed. The attenuation condition is given here on the basis of a linear theory, and it shows a good agreement with hybrid simulation results in terms of the attenuation position. (C) 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

A field-reversed configuration (FRC) is a high-beta compact toroid, ideally with an exclusive poloidal magnetic field. As an FRC has a simply-connected geometry, it can be translated along an external guide magnetic field and trapped in a confinement region with a quasi-static external magnetic field. In the FAT (FRC amplification via translation)-ICD (inductive current drive) experiment, a center solenoid is installed in a quasi-spherical confinement region to create an inductive toroidal current drive. An FRC formed by a field-reversed theta-pinch is translated at a velocity of 100 - 200 km/s into a confinement region comprising a center structure. Such an FRC then changes from a simply-connected structure to a torus. The magnetic field structures of FRCs have been directly observed using an internal magnetic probe array installed at the mid-plane of the confinement region. We observed that an FRC can be translated successfully without any disruptive perturbation to its structure and that the magnetic flux can be quadrupled via the inductive current drive, as compared with the cases without an inductive current drive. Herein, we report in detail the evidence for the inductive drive and relaxation process of the translated FRC. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research

Macroscopic two-dimensional (2D) images of flowing weakly magnetized lithium (Li+) ion beams are systematically presented. When the beam density approaches the Brillouin density limit, the 2D shape of the flowing Li+ ion beam deforms gradually. Also, the images clearly show not only the m = 1 shift mode but also higher modes of the flowing Li+ ion beams. These observed modes are analyzed by using a Fourier transform method.

A computed multiple tomography system for the investigation of translated field reversed configuration (FRC) plasmas was established. With this system, we have found that translated FRC plasmas were categorized into three types.

Effects of a rotating magnetic field (RMF) on the electron energy distribution function (EEDF) and on the electron density are investigated with the aim of controlling the radical composition of inductively coupled plasmas. By adjusting the RMF frequency and generation power, the desired electron density and electron energy shift are obtained. Consequently, the amount and fraction of high-energy electrons, which are mostly responsible for direct dissociation processes of raw molecules, will be controlled externally. This controllability, with no electrode exposed to plasma, will enable us to control radical components and their flux during plasma processing. (C) 2013 AIP Publishing LLC.

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.

The effects of a rotating magnetic field (RMF) on the electron energy distribution function (EEDF) are studied with the aim of controlling the EEDF of radio-frequency (rf) inductively coupled plasmas. In order to obtain the EEDF correctly, the external filter and the reference electrode of the probe system are used to compensate for the effect of the rf (13.56MHz) and the RMF (1.7–2.9MHz) fluctuations of the plasma potential. With increasing RMF frequency, the high energy component of the EEDF increases. Space potential and effective electron temperature also tend to increase with increasing RMF frequency.

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.

We have measured detailed radial profiles of electron density and electron temperature in the field-reversed configuration (FRC) sustained by the rotating magnetic field (RMF). To study the influence on the equilibrium of two kinds of bias magnetic field configuration straight (purely solenoidal) and mirror -, experiments have been carried out in the FRC Injection Experiment apparatus with metal vacuum vessel. The electron thermal pressure is well expressed as a function of magnetic flux during quasi-steady state in both configurations. In the straight bias field configuration, electron density increased with time only around the magnetic axis. On the other hand, in the mirror configuration, electron density increased with time in the entire plasma region. This is most appropriately ascribed to the improvement of confinement by strong mirror field of maximum mirror ratio of 4.0.

Radial displacement of field reversed configuration (FRC) plasma was observed in translation experiments using computer tomography (CT) at two different cross sections in the FIX machine. Two sets of CT devices were installed at the upstream and downstream sides of the confinement chamber. Each CT device has three arrays of detectors sensitive to the near-infrared radiation. The Fourier-Bessel inversion technique was employed to reconstruct the two-dimensional distributions of light emitted from the FRC plasma. In some cases, the peak of the reconstructed emission profile at both upstream and downstream sides was displaced from the center in the same direction, suggesting that the FRC plasma was displaced as a rigid body in the radial direction without tilting.