Demonstration of Multipulsed Current Drive Scenario using Coaxial Helicity Injection in the HIST Spherical Torus Plasmas

Full text: The Helicity Injected Spherical Torus (HIST) device has been developed towards high-current start up and sustainment by Mutipulsed Coaxial Helicity Injection (M-CHI) method. Multiple pulses operation of the coaxial plasma gun can build the magnetic field of spherical torus (ST) and spheromak plasmas in a stepwise manner. Successive double gun pulses have been demonstrated to amplify the magnetic field and the plasma current against resistive decay. The resistive 3D-MHD numerical simulation has reproduced the current amplification by the M-CHI method and confirmed that stochastic magnetic field is reduced so that closed flux surfaces are created during the current drive. These experimental and computational results from STs have provided, for the first time, availability of a quasi-steady-state 'refluxing' mode in which the magnetic field is allowed to decay partially before being rebuilt. Our goal is to achieve simultaneously the good energy confinement and the current sustainment by the M-CHI method. (author)