Plasma current start-up by outboard PF coils in JT-60U and TST-2

Plasma current (I_p) 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), I_p formation is possible without the existence of a PF null, with the initial vertical field (B_v) in the direction opposite to that required for equilibrium. Utilization of the extra flux swing provided by the PF coils before B_v switches sign is important in arriving at a high enough I _p that matches B_v 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 I_p of 4 kA was formed and sustained by RF power (P_RF) alone, without the use of induction. The plasma current centroid of this type of plasma is located on the outboard side. The dependences of I_p on P_RF and B_v 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.