Mechanism behind the recombination requirement for benign termination of relativistic electron beams

Abstract We present a first-principles explanation of the recombination requirement for benign termination of relativistic electron (RE) beams in tokamaks. Kinetic modeling including neutrals shows that a specific range in injected neutral quantity results in recombination of the background plasma, leading to a strong peak in its resistivity. Nonlinear magnetohydrodynamic (MHD) simulations using the JOREK code demonstrate that this preferentially amplifies edge tearing modes, producing a more stochastic edge magnetic field during RE deconfinement, resulting in a larger RE wetted area. We identify resistivity, not the free electron density, to govern access to benign termination. This provides the first broadly applicable and experimentally consistent picture of the MHD mechanisms behind the benign scenario, critical to its extrapolation to next-step devices.