Multimachine Extrapolation of Neoclassical Tearing Mode Physics to ITER

New data and inter-machine comparisons are presented exploring the role of rotation, r* and error fields in governing tearing mode b limits. It is found that conventional aspect ratio tokamak bN limits due to 2/1 tearing modes fall from values in excess of 3 with the usual strong neutral beam torque injection, to ~2 in low torque plasmas for ITER-like baseline scenarios. The fractional rates of fall in bN limits with rotation Mach number seem broadly consistent between tokamaks (including spherical tokamaks), and indeed also with onset thresholds for 3/2 NTMs in the conventional tokamak (all extrapolate to fall by about 1 unit in bN for complete withdrawal of co-injected torque). Analysis of the detailed behaviour suggests an action through changes to the rotation shear impacting the intrinsic stability of the tearing mode, and further, that this is not through so called NTM 'small island' effects, but is more likely due to modifications of the classical tearing stability parameter, D¢. In addition, an enhanced error field effect has been observed at and below the ITER baseline bN~1.9 (well below the ideal b limit, where such effects usually manifest), whereby modest levels of error field can assist 2/1 mode formation, particularly at low rotation or when the natural tearing instability b threshold is approached. Nevertheless, ITER baseline-like scenarios are found to be just stable, provided there is good error field correction. Turning to r* dependence, previous databases for the metastable threshold for the 3/2 NTM have been extended, with new data from JT-60U at low r* confirming that ITER will operate well above this threshold and so be susceptible to NTM triggering events. A new database constructed for the 2/1 NTM bN limit in hybrid scenarios indicates r* effects can be dominated by variations from other parameters, such as q profile shape or fast particle content. These results are of high significance for the extrapolation to ITER, indicating that the expected trends of NTM theory do indeed manifest towards increased tearing mode susceptibility, but that ITER-like scenarios remain stable at the necessary operating points (e.g. in bN). However they also show that the mechanisms by which the key parameters act to change the stability (e.g. through changes to D¢) leave the door open for further control techniques through the manipulation of plasma profiles.
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