RF current condensation with self-consistent ray-tracing and application to ITER

While mitigation will play a critical role in reducing the impact of disruptions in ITER, it should be used only as a "rarely-used last resort" [Strait et al. 2019], i.e. reliable disruption avoidance strategies are necessary, the most important of which is the stabilisation of magnetic islands. Indeed, in the JET tokamak equipped with an ITER-like wall, 95% of disruptions are preceded by magnetic islands [Gerasimov et al. 2018]. These can be stabilised by radio-frequency (RF) current drive at the island O-point, which generates a stabilising resonant component of the magnetic field [Reiman 1983]. RF waves damping on fast superthermal electrons, such as electron-cyclotron (EC) or lower-hybrid (LH) waves, display a strong sensitivity of damping to temperature. This can lead to the nonlinear current condensation effect [Reiman and Fisch 2018], a positive feedback loop where the power deposition raises the temperature, which in turn increases the damping, and so forth. In this manner, the power deposition, and thus also the current, can be increased and focused at the island O-point, thereby lending further help in stabilising magnetic islands.