Runaway current reconstitution after a 3D MHD flush of the runaway plateau

Benign termination of a mega-Ampere level runaway (RE) current has been convincingly demonstrated in JET [1] and DIII-D [2], establishing it as a leading candidate for runaway mitigation on ITER. In particular, these experiments have demonstrated that a global MHD instability triggered by low-Z (deuterium) injection is able to expel the majority of REs and distribute them over a broad area of the vessel wall, thus terminating the RE plateau while avoiding localized damage. Extrapolation of this approach to reactor scale devices hinges on the near complete expulsion of the existing REs by the MHD instability. In this work, we show that the number of REs that survive a global MHD instability depends sensitively on the electron phase space distribution during the current plateau, which in turn is a sensitive function of the impurity content of the plasma. In particular, the strong pitch-angle scattering that coincides with plasmas containing significant amounts of high-Z material leads to a substantial number of magnetically trapped energetic electrons. These trapped electrons remain well confined in the presence of a stochastic magnetic field, and are shown to be sufficient to initiate the partial reformation of the RE plateau via the avalanche mechanism for plasmas carrying current in excess of a few mega-Amperes across a range of plasma conditions. The present work will elucidate strategies through which the phase space distribution of REs during the current plateau can be tailored to minimize this trapped population of electrons, along with determining critical parameters for expediting the decay of the remnant electron population before the magnetic flux surfaces are able to reform.

[1] Reux et al., Phys. Rev. Lett. (2021), [2] Paz-Soldan et al. Nucl. Fusion (2021)