Forecasting electron cyclotron current drive stabilization of neoclassical tearing modes in ITER
1 General Atomics San Diego 92186 California USA
Published online: 23 October 2017
The ITER baseline design relies on ECCD to stabilize confinementdegrading disruptive NTMs . However, the EC power required will take a toll on the fusion gain Q. The MDC-8 group (in existence since 2005) has the goal to provide a range of data to benchmark the Rutherford tearing stability equation for NTM evolution, allowing predictions for ITER ECCD requirements to be validated. Experimental contributors have included ASDEX UPGRADE, DIII-D, EAST, FTU, HL-2A, JT-60U, KSTAR, TEXTOR and TCV. While any m/n tearing mode island can reduce confinement, the m=2, n=1 mode at q=2 is particularly damaging. This mode is at a relatively large minor radius in the low q95∼3 safety factor of ITER and thus close to the resistive wall; with the relatively low rotation in ITER (large inertia, small torque), an uncontrolled mode will rapidly lock at low tearing mode amplitude with subsequent disruption . While progress is being made in modeling of the stability space and control  and experiments are promising, implementation still needs to be successfully demonstrated experimentally.
The ITPA consensus is that ITER's 24 1-MW gyrotrons will provide more than sufficient EC power from the upper launch mirrors to drive narrow (but not too narrow) ECCD at q=2 for stabilization, with good alignment. Broadening of the ECCD, by edge turbulence for example, is a concern that would demand more EC power but also make alignment easier. Pre-emption at lower CW power or active stabilization by early mode onset detection and higher peak (possibly lower average) pulsed power are issues still under continuing investigation.
Most EC-NTM experimental studies so far are at relatively high q95 with smaller radius at q=2 and thus higher Te for better current drive efficiency, higher rotation and weaker wall coupling. DIII-D, for example, is now well poised to pursue ECCD NTM stabilization at both low q95 and at low rotation in the 2017 campaign. The MDC-8 as a whole is proceeding to narrow the experimental focus for a comparison of observations from different devices. This will establish the physics basis for successful stabilization in ITER.
© The authors, published by EDP Sciences, 2017
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