Disruptive neoclassical tearing mode seeding in DIII-D with implications for ITER
R. J. La Haye, C. Chrystal, E. J. Strait, J.D. Callen, C. C. Hegna, E. C. Howell, M. Okabayashi, R.S. Wilcox
Abstract
Abstract New studies identify the critical parameters and physics governing disruptive neoclassical tearing mode (NTM) onset. An m / n = 2/1 mode in DIII-D that begins to grow robustly after a seeding event (edge localized mode ELM or sawtooth precursor and crash) causes the mode rotation to drop close to the plasma’s E r = 0 rest frame; this condition opens the stabilizing ion-polarization current ‘gate’ and destabilizes an otherwise marginally stable NTM. Our new experimental and theoretical insights and novel toroidal theory-based modeling are benchmarked and scalable to ITER and other future experiments. The nominal ITER rotation at q = 2 is found to be stabilizing (‘gate closed’) except for MHD-induced transients that could ‘open the gate’. Extrapolating from the DIII-D ITER baseline scenario (IBS) discharges, MHD transients are much more likely to destabilize problematic robustly growing 2/1 NTMs in ITER; this makes predictions of seeding and control of both ELMs and sawteeth imperative for more than just minimizing divertor pulsed-heat loading.