Hybrid Kinetic-MHD Simulations of Drift-Orbit Effects on the Stability and Non-Linear Dynamics of Runaway Electron Beams
During tokamak disruptions, the Ohmic current may be replaced by a non-inductive runaway electron (RE) current, affecting resistive stability. Previous studies suggest that, in the linear phase, the presence of REs acts destabilizing for tearing modes (TM) compared to a scenario with Ohmic current. In the non-linear regime, this translates to larger saturation amplitudes. These results are based on the assumption of zero drift-orbit deviation from the magnetic flux surfaces corresponding to the low-energy limit. This work investigates the importance of this kinetic effect by studying the linear and non-linear TM dynamics in RE beams with different RE energies, providing a clear picture of finite-orbit-width (FOW) effects. We use a hybrid fluid-kinetic model in the 3D non-linear magnetohydrodynamic (MHD) code JOREK, treating REs kinetically with a full-f Monte Carlo approach in self-consistent interaction with the MHD mode dynamics. The study shows that the presence of REs modifies the characteristics of the instability in several ways. First, we find that the major-radial displacement of drift orbits from flux surfaces induces an $m=1$ perturbation to the equilibrium current, introducing additional mode coupling between $(m,n)$ instabilities and the $(m\pm1,n)$ sidebands. Second, we find that increasing RE energy has a stabilizing effect on the MHD modes because REs cannot support narrow current sheets on rational flux surfaces owing to the drift-orbit displacement. This counteracts the destabilizing effect that REs have on TMs in the low-energy limit. For the scenario investigated, the stabilizing effect dominates over the additional mode coupling, reducing the development of stochastic magnetic regions with increasing RE energy and thereby lowering radial particle transport. Overall, we find that FOW effects can substantially alter the MHD stability and non-linear dynamics of RE beams.