Drift-reduced fluid modeling of rapidly rotating plasmas

TL;DR

This study explores how rapid plasma rotation influences stability, particularly Kelvin-Helmholtz and interchange instabilities, using a drift-reduced fluid model that incorporates finite Larmor-radius effects and centrifugal forces.

Contribution

It introduces a modified drift-reduced fluid model including centrifugal force effects and identifies regimes of RDI behavior based on background profiles, advancing understanding of plasma stability under rapid rotation.

Findings

Global plasma stability depends on background profile characteristics.

Three regimes of RDI behavior are identified and predicted by a simple criterion.

KH modes reduce plasma resistance to RDI when both are unstable.

Abstract

In this paper, we investigate the effects of rapid rotation (Mach number M ~ 1) on plasma fluid stability, focusing specifically on Kelvin-Helmholtz (KH) and interchange instabilities - including both magnetic-curvature-driven (CDI) and rotation-driven (RDI) interchanges. Building on previous studies of shear flow stabilization, we utilize a drift- reduced fluid approach rather than standard magnetohydrodynamics to capture finite Larmor-radius effects. To achieve this, the drift-reduced equations were modified to include the centrifugal force and implemented in hermes-3 (Dudson et al. 2024), an extension to the BOUT++ (Dudson et al. 2009) framework. Because plasma rotation both drives the RDI and provides stabilizing shear flow, we find that the global plasma stability is sensitive to background profile characteristics. We identify three distinct regimes of RDI behavior and establish a simple criterion based on the density and velocity profiles to predict RDI susceptibility. This approach is similar to recent local gyrokinetic studies of shear flow that compared instability growth rates to shearing rates (Ivanov et al. 2025). Finally, by examining cases where the plasma is both interchange- and KH-unstable, we find that global KH modes make the plasma less resistant to RDI.