Equilibrium Analysis of EC‐Sustained and RF‐Sustained ST Plasmas

Plasma current start‐up and formation of the ST configuration without the use of the central solenoid is a critical issue in ST research. In the TST‐2 spherical tokamak (R = 0.38 m, a = 0.25 m), sustainment of an ECRF (2.45 GHz) produced ST plasma by low frequency (21 MHz) RF power alone was demonstrated. Since direct RF current drive can be ruled out, this result implies that the ST configuration is sustained entirely by pressure‐driven currents. The Grad‐Shafranov equilibrium was generalized to take into account the open field line region with finite plasma current and pressure (truncated equilibrium). In addition to the precessional current of trapped particles, Pfirsch‐Schluter current flowing along the open field line (and partially returning through the vacuum vessel) contributes to the toroidal plasma current. Three phases of plasma start‐up are analyzed: (i) the current formation phase, (ii) the current jump phase, and (iii) the current sustainment phase. In the current formation phase, the plasma...