PLASMA INITIATION AND STARTUP STUDIES IN THE DIII-D TOKAMAK WITH SECOND HARMONIC ELECTRON CYCLOTRON ASSIST

Second harmonic X-mode (X2) electron cyclotron (EC) heating has been used in DIII-D to examine plasma initiation and burnthrough of low Z impurities. Although the toroidal inductive electric field (E_ɸ) in DIII-D is high enough (0.9-1.0 V/m) to allow robust startup without EC assist, startup in fusion devices such as ITER will have lower fields (E_ɸ = 0.3 V/m) and EC assist can provide an increased margin for burnthrough of low Z impurities. ECH, applied before the inductive electric field, is used to separate the various phases of plasma breakdown and startup. Ionization first occurs near the X2 resonance location and then expands in the vessel volume. Perpendicular launch (k_∥ = 0) is required for X2 pre-breakdown, and the power threshold can be reduced by optimizing prefill and vertical field, although the lowest power threshold is not at the optimum value for Ohmic startup. An orbit following code confirms that cold electrons (0.03 eV) can be sufficiently heated by ECH to energies above the threshold of ionization of hydrogen. This code predicts successful ionization in future tokamaks such as KSTAR and ITER. The ITER startup scenario has been simulated in DIII-D experiments and X2 ECH assist has been applied at reduced toroidal loop voltage to assist burnthrough and plasma current ramp up.