Detailed observation of electric potential and radial ion flow were conducted to investigate non-MHD effect during merging formation process of a field-reversed configuration (FRC) in weakly-magnetized region. In counter-helicity spheromak merging, reconnection of opposing toroidal magnetic fields induces the radial electric field whose direction is determined by the polarity of toroidal fields of two spheromaks. In weakly magnetized region, this radial electric field was found to modify the radial ion outflow pattern, affecting the density and current profiles to bifurcate the formed oblate FRC into two equilibria with different magnetic and pressure profiles.

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Burst of bremsstrahlung soft X-ray emission was observed during spherical tokamak merging process in the University of Tokyo Spherical Tokamak (UTST) experiment. The soft X-ray signal waveform coincided well with the enhancement of the reconnection electric field and its intensity showed nearly linear dependency on the strength of the toroidal magnetic field. These results suggest that the electrons near the X point are effectively accelerated in the troidal direction by the parallel electric field during plasma merging in the presence of the strong toroidal magnetic field.

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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.

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A magnetized coaxial plasma gun (MCPG) has been studied in the context of nuclear fusion research, for particle and magnetic helicity injection and spheromak formation through electromagnetically accelerated magnetized plasmoid. On the other hand, most of physical vapor deposition (PVD) techniques have been developed for surface modification and new substance composition in field of electric-appliance, material, semiconductor, etc. However, in the conventional methods, conditions of deposition are limited by productive efficiency and thermal load to the substrate. We proposed a novel PVD method by utilizing electromagnetic acceleration of MCPG. In this study, we deposited Al films on SiO₂ by the method, and evaluated the deposition rate and the adhesion strength. We confirmed the adhesion strength of about 2.1 times higher than the conventional method. This method can deposit a thin film with high adhesion strength by electromagnetically accelerating particles.

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Translation of a Field-Reversed Configuration (FRC) into a quasi-spherical confinement region with super-alfvénic translation speed has been successfully conducted. Translation speed can be controlled in the range from 80 to 150km/s by changing statically filled gas pressure. The translated FRC experiences radial decompression process rapidly when it enters into the confinement chamber. The separatrix shape of translated FRC is controlled through the translation process. Especially, in the case of FRC translation into deuterium gas atmosphere, elongation of the FRC is roughly unity. It indicates possible application of center solenoid onto the high-beta compact torus of FRC. Also, as a new finding, a rotational instability with an n=2 deformation has been observed in the translated FRC.

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We developed a novel slide-type two-dimensional Thomson scattering measurement system for quantitative analysis of electron heating during plasma merging. Axial and radial profiles of electron temperature and density were measured for the first time in UTST device. Local electron heating by magnetic reconnection was measured inside the current sheet.

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Rapid transition from the Grad-Shafranov equilibrium state in the open-field region of field-reversed configuration (FRC) plasma is found by the ion-particle and electron-fluid hybrid simulation. Although the internal equilibrium state is robust, the pressure near the mirror end decreases within the characteristic thermal transit time as edge ions move toward the axial end and are lost from the confinement region. Therefore, effects of end-loss ions should be considered to reproduce an equilibrium state of FRCs numerically.

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We found detailed energy transfer processes of magnetic reconnection: ion outflow energy through anisotropic ion thermal energy to isotropic one. The compact torus plasma merging can form high-β torus plasma using rapid reconnection heating of ions. Local ion temperatures and flows was measured using IDSP(Ion Dynamics Spectroscopy Probe). The relaxations of kinetic and anisotropic ion energy are caused by large plasma density pileup observed in the downstream of reconnection.

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High frequency magnetic fluctuations in the lower hybrid frequency range were observed in the upstream region of reconnection site during counter-helicity merging experiment in TS-4. In contrast, no high frequency fluctuation was observed during co-helicity merging which contains considerable toroidal guide field at the X point. Toroidal electric field measured in counter-helicity merging was larger than that measured in co-helicity merging. These observations indicate the fluctuations might contribute to the fast reconnection in counter-helicity merging.

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Axial scan of 2D Doppler tomography measurement made clear detailed 2D profiles of ion temperature during and after merging of two spherical tokamak (ST) plasmas. The magnetic reconnection heats plasma ions in the downstream, causing preferential ion heating around a specific flux surface. The merging STs were found to form the hollow ion temperature profile inside the produced ST.

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