Orbit-averaging and deposition accuracy for runaway electron beams in hybrid kinetic-MHD simulations of the runaway plateau

Abstract

We develop a new procedure that combines the Kinetic Orbit Runaway electrons Code (KORC) and the NIMROD extended-MHD code to simulate runaway electrons (REs) in the post-disruption plateau. KORC integrates guiding-center orbits using a barycentric-based binary search strategy to generate initial guesses for the Newton-Raphson logical-to-physical coordinate inversion, guaranteeing reliable particle-to-mesh mapping in NIMROD, whose fields remain static for the present study. Samples are drawn in accord with experimental parallel current profiles of RE beams during the plateau phase. Deposition in NIMROD is verified through comparison with a Python-based finite element code that ensures periodicity in the poloidal direction and continuity at the magnetic axis. Accurate representation of near-axis fields requires finer mesh resolution to prevent under- and overshoots in current density from orbit inaccuracies. Yet, at a fixed particle count, increasing mesh resolution amplifies statistical noise in the deposited fields. An orbit-averaging method accumulates partial current deposits over multiple kinetic steps and reduces the statistical noise with little added computational cost. By coupling kinetic routines from KORC directly into the NIMROD codebase, these developments lay essential groundwork for future self-consistent KORC-NIMROD coupling.