Plasma start-up design and first plasma experiment in VEST

Abstract

First plasma start-up experiment in Versatile Experiment Spherical Torus (VEST) has been successfully carried out with conventional start-up scheme using a central solenoid. A start-up design code has been developed for the development of start-up scenarios in VEST, which can predict evolutions of vacuum field structure and plasma current with given operation parameters from the PF coil power supply circuit considering the eddy currents. Start-up scenarios are successfully developed with the code, generating field null region in the inboard side at the onset of loop voltage in order to maximize the connection length, and then providing the required poloidal magnetic field for stable equilibrium as the plasma current evolves. Hydrogen is used as a working gas and the toroidal field of 0.1 T with 0.2 s flat-top is applied on magnetic axis during the discharge. With the assist of ECH (Electron Cyclotron Heating) pre-ionization (6 kW, 2.45 GHz), a plasma current of ∼70 kA has been generated with the pulse duration of ∼10 ms. The elongation and edge safety factor are estimated to be 1.6 and 3.7 respectively.