Abstract
A helicity injection system for the Versatile Experiment Spherical Torus (VEST) has been successfully developed and commissioned. A high current electron gun utilizing hollow cathode and washer stacks has been designed and constructed with a single pulse power system that can provide voltages for both arc discharge and extraction sequentially. Tests for electron gun operation with the single pulse power system have been conducted under various toroidal and poloidal field strengths. The estimated plasma impedance, depending on the injection magnetic field structure, can be utilized for the optimal gun operation by impedance matching between the pulse power system and plasma. With the charging voltage of 1.2kV, injection current of 0.95kA has been obtained with the injection voltage of 410V for about 5ms. Initial helicity injection experiments have been conducted under various toroidal and poloidal field strengths and a toroidal plasma current of up to 3.8kA is observed with the current multiplication larger than the geometric stacking ratio, confirming the possibility of relaxation into tokamak-like plasma with closed flux formation.
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