L-H Transition Dynamics in ITER-Similar Plasmas with Applied n=3 Magnetic Perturbations

摘要

In ITER-similar shape (ISS), low rotation plasmas [1] in DIII-D [=1.5-5×l0~(19)m~(-3), B_t=1.9-2T, I_p=1.5MA, q_(95)~3.6, balanced neutral beam injection (NBI)], the L-H threshold power P_(LH) with applied n=3 Resonant Magnetic Perturbations (RMP) [2,3] is found to increase strongly with decreasing collisionality. This is a concern for H-mode access in primarily ECH-heated ITER plasmas since RMP may be applied before the L-H transition in ITER to safely suppress the first ELM. The L-H power threshold increase with RMP is determined by initially accessing H-mode via applying balanced NBI and increasing steps in ECH power at high RMP field (applied via the DIII-D I-coil system consisting of six internal coils above and six coils below the outboard tokamak midplane [2]). NBI and ECH power are then removed to let the plasma return to L-mode. A second L-H transition is induced later in the same shot by applying NBI and stepping up ECH power again to obtain the threshold power with no applied RMP as a reference. Using this method to bracket P_(LH), a scan of P_(LH) vs. RMP perturbation strength is then obtained via stepping down the I-coil current (RMP perturbation strength) at constant beam and ECH power, as shown in Fig. 1.