Improvements Prepared for the ECCD Stabilization of m/n=2/1 NTMs in the DIII-D Low-Torque ITER Baseline Scenario*

Experiments in DIII-D are continuing to study how to minimize the average Electron Cyclotron Current Drive power directed at q=2 for stabilization of the m/n=2/1 Neoclassical Tearing Mode in the ITER baseline scenario. Standby “lie-in-wait” ECCD to “catch” a growing mode, turn on the ECCD, “subdue” the mode with return to standby EC off is under development. ITER will need to keep the average EC power for stabilization minimal to maximize Q.  The low-torque makes early mode detection imperative for prompt subduing before wall-locking. Previous DIII-D experiments identified problems: 1) gyrotrons could not fully be off in standby and only for a limited time and 2) the n=1 N1RMS for the catch had to be set too high to avoid false positives. Improvements to gyrotron modulation now allow fully-off without limit on off-time; ODDEVENSPEC is now deployed to distinguish n=1 m=2 from n=1 m=1 so as to lower the bar for catching the mode onset.  More gyrotons and EC power will facilitate promptly subduing the mode.