Non-inductive plasma start-up and sustainment by wave heating at two frequencies in the TST-2 Spherical Tokamak

Non-inductive plasma start-up and sustainment experiments were performed in the TST-2 spherical tokamak (ST) device by injecting waves at 21 MHz (RF) and 2.45 GHz (EC). The fundamental electron cyclotron resonance layer exists in the vacuum vessel, and the EC heating (ECH) can generate and sustain the ST plasma. When the RF power was injected to the EC plasma, with a short overlapping of both powers, a current jump was induced and the current was sustained. The RF heating can sustain the plasma current at a lower magnetic field than the field necessary for the ECH-sustained plasma. Contrary to the EC sustainment, the RF sustainment requires fine adjustments of the RF injection timing and the vertical field strength. The plasma current disappears quickly, when the RF injection is too late or the vertical field is too weak. When the RF injection is too early or the vertical field is too strong, the plasma current decreases after the end of the ECH pulse.