Structures and regimes in topology of momentum space flow of runaway electron formation and avalanche in tokamak plasmas

The importance of understanding momentum-space flux topology to runaway electron formation and mitigation has been well established. Furthering the understanding of runaway electron formation and dynamics is crucial to many active physics problems, such as atmospheric lightning discharges, relativistic space plasmas, and, specifically to this work, tokamak discharges where runaways are expected to potentially cause serious damage in large tokamak fusion experiments such as ITER. In this study, we identify and further characterize distinct regimes of phase-space flows and correlate these momentum space flux regimes to runaway electron formation, distribution function properties, and the growth rate of the runaway avalanche mechanism. Between two regimes a clear divergence of the transport dynamics of the phase-space separatrix location with increasing parallel electric field is discussed. The correlation between the distinct regimes of phase-space circulation and existence of a knee-like structure in runaway avalanche growth rate is detailed, with enhancement of this structure due to toroidal geometry and ionized impurities highlighted.