Gaps in the experimental data needed to reliably predict the performance of RF island stabilization in future devices

Many experiments on RF stabilization have produced valuable data. But experiments have largely used outside (low field side) launch ECCD with a narrow range of toroidal launch angles ≈ 20° relative to -R̂. It is likely that top launch ECCD will be preferred for fusion reactors because of its higher current drive efficiency, and the consequent reduction in recirculating power. Top launch optimizes at much larger toroidal launch angles. Theoretical calculations predict that there will be significant nonlinear effects in this regime. The theory needs to be validated experimentally. Experimental data is also lacking for the low collisionality regime, a regime of particular relevance for for tokamaks with a lithium wall because of their flat temperature profiles and peaked density profiles. This regime is predicted to have much larger electron temperature perturbations in the island than the more collisional regime, with implications for linear and nonlinear stabilization. Additionally, islands are predicted to lock at low width in ITER and in fusion reactors, because of the low torque relative to the inertia. A 2015 PRL by Volpe et al demonstrated ECCD stabilization of locked islands in DIII-D, but experiments are needed to investigate the control of the locked island phase using error field control coils. Finally, there is the question of stabilization via inside launch lower hybrid waves, of interest for ARC and its variants.