Non-diffusive pitch-angle scattering of runaway electrons in the presence of whistler waves

Wave–particle interactions between high-energy electrons and whistler waves lead to pitch-angle scattering of electrons in space, astrophysical, and fusion plasmas. In this work, we present a first-principles-informed computational framework to study runaway electron (RE)–whistler interactions in realistic three-dimensional tokamak geometry, enabling investigation of RE pitch-angle transport beyond the assumptions of quasilinear theory. This framework is realized by coupling all-orders spectral algorithm, which solves the full-wave Maxwell–Vlasov system for whistler eigenmodes in a given tokamak equilibrium, with kinetic orbit runaway electrons code, a kinetic orbit code that follows full-orbit RE trajectories in prescribed wave fields. Statistical analysis of the pitch-angle distribution moments and displacement variance is used to characterize the RE transport behavior. We find that REs undergo significant pitch-angle scattering and exhibit energy-dependent, non-diffusive transport behavior. In particular, we observe transitions between diffusive, subdiffusive, and superdiffusive transport regimes depending on the initial RE kinetic energy, a behavior not captured within the standard quasilinear diffusion framework. These results provide new physical insights into RE transport driven by wave–particle interactions and are relevant to the development of wave-based control schemes for REs in fusion plasmas.