Developing a Full Orbit Hybrid Kinetic-MHD Code for Runaway Electron Modeling

In tokamaks, disruptions cause rapidly changing magnetic fields that can drive large inductive electric fields. These fields can accelerate electrons to relativistic speeds and create runaway electron (RE) beams, which can cause significant damage to the tokamak walls. To understand how to mitigate RE beams in future devices, accurate simulations are needed. Prior work [1] has implemented guiding center (GC) routines from KORC into the NIMROD extended-MHD code, working toward a hybrid kinetic-MHD model. However, full orbit (FO) effects are important for accurately modeling phenomena such as particle deconfinement and orbits in fields varying on the scale of the Larmor radius. We implement a FO particle-pushing algorithm in NIMROD and analytically verify the trajectory for a constant magnetic field. In post-processing analysis, we compare the GC and FO models as well as their conservation properties across a range of timesteps. Furthermore, we profile the code and subsequently optimize it for GPU acceleration, facilitating the simulation of a large number of particles.

[1] O.E. López et al., Preprint at https://doi.org/10.48550/arXiv.2505.09801 (2025).