福本 直之

© 2016 The Institute of Electrical Engineers of Japan. 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 ×1021m-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.

© 2016 The Institute of Electrical Engineers of Japan. 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.Dand Doppler ion velocity vi.Dof 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.

© 2014 The Institute of Electrical Engineers of Japan. 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.

The dynamo current drive with two-fluid effects has been investigated in the helicity-driven spherical torus (ST) plasmas on HIST. 2D internal magnetic field measurements of the ST configuration has verified the flux amplification and the generation of the closed flux during the coaxial helicity injection (CHI). We have found that not only MHD 〈δv×δB〉|| dynamo but also Hall 〈δj×δB〉||/en dynamo plays an important role in the helicity transport for the current sustainment. Hall-MHD induced-electric fields satisfy the parallel mean-field Ohm's law balance. We have also observed transverse propagation and damping of a low-frequency Alfven wave. The dynamo activities for the current drive may be caused by the Alfven waves.

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.

Flux amplification and sustainment of the spherical torus (ST) configurations by operating in Multi-pulsing Coaxial Helicity Injection (M-CHI) method have been demonstrated in the Helicity Injected Spherical Torus (HIST) device. We have studied the plasma flow structures and the dynamo electric field necessary to balance Ohm's law in the double pulsing CHI discharges. The experimental results indicate that two-fluid effects are necessary to understand the CHI current drive.