In this work, we extend the three-dimensional, nonlinear resistive/two-fluid MHD code CLT to simulate runaway electron physics by implementing a widely adopted fluid runaway electron MHD (RE-MHD) model. To accurately handle the convection of runaway electrons—which travel at near-light speed along magnetic field lines—a third-order upwind scheme is applied. This scheme is systematically validated to confirm its conservation properties and numerical accuracy. Then, the influence of runaway electrons on the linear and nonlinear evolution of tearing modes and resistive-kink modes is investigated. The presence of runaway electrons introduces finite rotation frequencies to both types of modes and significantly increases their linear growth rates at high resistivity. Owing to a positive feedback process between the runaway current distribution and the magnetic island width, runaway electrons can increase the saturation level of tearing modes by several times. When the ratio of the runaway current to total current exceeds 50%, the system can only experience one crash and form a steady state with axisymmetric magnetic field due to lack of current source term to restore the current distribution.
Influence of resistive internal kink on runaway current profile