3d Distortion of the Plasma Boundary in the Presence of Saturated MHD Instabilities or Applied Resonant Magnetic Perturbations ITPA MHD Working Group 12 Final Report

The three dimensional plasma boundary displacements induced by either longlasting core MHD instabilities or by applied non-axisymmetric magnetic perturbations have been measured in ASDEX Upgrade, DIII-D, JET, MAST and NSTX. For the core instabilities, the displacements are usually small, although in extreme cases in MAST when the rotation braking is strong and global a significant displacement can be observed. The most concerning instability for ITER is the saturated internal kink, or helical core, which can be found in plasmas with a wide region of low magnetic shear such as the hybrid scenario, which can lead to non-negligible boundary displacements. Nonetheless, the boundary displacement resultant from core MHD instabilities in ITER is predicted to be less than ±1% of the minor radius. The displacements arising from applied resonant magnetic perturbations (RMPs) are measured up to ±5% of the minor radius in present day machines. Good agreement has been found between these experimental measurements and a range of models ­ from vacuum field line tracing, to ideal three dimensional MHD equilibrium reconstruction, to nonlinear plasma amplification. The measured displacement across a range of machines is found to correlate linearly with the applied resonant field predicted by vacuum modelling. The RMP-induced displacements foreseen in ITER are expected to lie within the range of those predicted by the different models, meaning less than ±1.75% (±3.5cm) of the minor radius in the H-mode baseline and less than ±2.5% (±5cm) in a 9MA plasma. Whilst a displacement of 7cm peak-to-peak in the baseline scenario is marginally acceptable from both a plasma control and heat loading perspective, it is important that ITER adopts a plasma control system which can account for a three dimensional boundary corrugation to avoid an n = 0 correction which would otherwise exacerbate the displacement caused by the applied fields.
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