Fast Backward Diffusion of Runaway Electrons in Tokamak Facilitated by Slow-X Waves with Fully Kinetic Simulations

Nonlinear dynamics of runaway electron induced wave-particle interactions can significantly modify the runaway distributions critical to tokamak operations. Here we present the first-ever fully kinetic simulations of runaway-driven instabilities towards nonlinear saturation in a warm plasma as in tokamak start up. The uncovered physics differs greatly from previous quasilinear studies in the literature. We find that the slow-X modes grow an order of magnitude faster than the whistler modes, and they go through parametric decay to produce whistlers much faster than direct drive by runaways. The slow-X waves initiate a chain of wave-particle resonances that strongly diffuse runaways to the backward direction at moderate and high energy. This reduces almost half of the current carried by high-energy runaways, over a time scale (about one microsecond) orders of magnitude faster than experimental shot duration or collisional current decay.