Thermomagnetic instability of plasma composition gradients

TL;DR

This paper demonstrates a new plasma instability driven by ion charge and temperature gradients, leading to magnetic field growth, with implications for inertial confinement fusion environments.

Contribution

It introduces a novel thermomagnetic instability mechanism driven by ion composition gradients, distinct from the traditional Biermann battery effect.

Findings

The instability can grow rapidly under typical fusion conditions.

Nonlinear Nernst advection stabilizes the instability in simulations.

Growth rate depends on ion gradients and heat flux.

Abstract

We show that, under Braginskii magneto-hydrodynamics, anti-parallel gradients in average ion charge state and electron temperature can be unstable to the growth of self-generated magnetic fields. The instability is analogous to the field-generating thermomagnetic instability, although it is driven by the collisional thermal force magnetic source term rather than the Biermann battery term. The gradient in ion charge state causes a gradient in collisionality, which couples with temperature perturbations to create a self-generated magnetic field. This magnetic field deflects the electron heat flux in a way that reinforces the temperature perturbation. The derived linearized growth rate, typically on hydrodynamic timescales, includes the resistive and thermal smoothing. It increases with large ion composition gradients and electron heat flux, conditions typical of the hohlraum walls or contaminant mix jets in inertial confinement fusion implosions. However, extended magneto-hydrodynamic simulations indicate that the instability is usually dominated and stabilized by the nonlinear Nernst advection, in a similar manner to the standard thermomagnetic instability.