Plasma Initiation and Start-Up Studies in the DIII-D Tokamak with Second-Harmonic EC Assist

Second-harmonic X-mode (X2) electron cyclotron (EC) heating (ECH) has been used in DIII-D in conjunction with plasma initiation and current ramp-up. Although the toroidal inductive electric field E in DIII-D is high enough (0.9 to 1.0 V/m) to allow robust start-up without EC assist, start-up in fusion devices such as ITER will have lower fields (E = 0.3 V/m), and EC assist can provide a reproducible breakdown and 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 start-up and is defined as preionization. Preionization first occurs near the X2 resonance location and then expands in the vessel volume. Perpendicular launch (k_[parallel] = 0) is found to produce the strongest preionization. The power threshold for preionization can be reduced by optimizing the prefill and the vertical field, although the lowest power threshold is not at the optimum value for ohmic start-up alone. 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 heating in new tokamaks such as KSTAR and ITER to energies where preionization can occur. The ITER start-up 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.