Non-inductive Plasma Current Start-up by EC and RF Power in the TST-2 Spherical Tokamak

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device. Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high βp spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed the detailed information on three discharge phases: (i) In the initial current formation phase, linearity between the plasma current and the stored energy was confirmed. (ii) In the current jump phase, the initial closed flux surfaces cause a change in the current increasing rate, but the stored energy does not show such a change. (iii) The current sustained plasma is characterised by the fraction of the current inside the last closed flux surface to the total current, and the fraction seems to determine the ratio of the plasma current to the external vertical field. MHD instabilities often terminate the RF sustained plasma, but no such phenomenon was observed in the EC sustained plasma.