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=0.38m, a=0.25m), 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-Schlüter 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 current is formed and increases slowly, roughly proportional to the stored energy. Closed flux surfaces do not exist and the current density profile is peaked on the outboard side. Once the plasma current increases beyond a critical value (approximately equal to the level necessary to satisfy the major radial force balance), the plasma current increases rapidly (current jump). Closed flux surfaces appear not at the beginning, but at the end of the current jump. In the sustainment phase, plasma has a high poloidal beta, β_p=O(1). The equilibrium is characterized by the hollowness of the current density profile, which also determines the fraction of the plasma current inside the last closed flux surface. The plasma equilibrium does not differ greatly between EC-sustained and RF-sustained plasmas. However, RF-sustained plasmas are more turbulent, with larger fluctuation levels over a wide frequency band (0-20 kHz), and in many cases MHD activity in the 5-10 kHz frequency band increases rapidly and terminates the discharge.