Plasma Current Start-up by Outboard PF Coils in JT-60U and TST-2

Abstract

Plasma current (Ip) initiation and ramp‐up utilizing induction by poloidal field (PF) coils located on the outboard side of the torus were demonstrated in both conventional aspect ratio tokamak (100 kA in JT‐60U) and spherical tokamak (10 kA in TST‐2). In the presence of sufficient source of plasma by RF ionization (1 MW in JT‐60U, 100 kW in TST‐2), Ip formation is possible without the existence of a PF null, with the initial vertical field (Bv) in the direction opposite to that required for equilibrium. Utilization of the extra flux swing provided by the PF coils before Bv switches sign is important in arriving at a high enough Ip that matches Bv at the end of PF coil current ramp. In JT‐60U, recharging of the OH coil was observed with only perpendicular and counter NB injection, suggesting overdrive by bootstrap current. In TST‐2, a quasi‐steady‐state Ip of 4 kA was formed and sustained by RF power (PRF) alone, without the use of induction. The plasma current centroid of this type of plasma is located on the outboard side. The dependences of Ip on PRF and Bv were found to be weak. The results presented here offer encouragement to CS‐less operation of a tokamak fusion reactor, which should have a large impact on the economic competitiveness.