Electron cyclotron drift instability and anomalous transport: two-fluid moment theory and modeling

TL;DR

This paper demonstrates that the electron cyclotron drift instability and associated anomalous transport can be effectively modeled using a two-fluid approach, providing insights into space and laboratory plasma behaviors without full kinetic simulations.

Contribution

The study introduces a self-consistent two-fluid model for ECDI and anomalous transport, aligning well with kinetic theory at lower temperatures and capturing secondary instabilities.

Findings

Two-fluid models agree with kinetic theory at lower temperatures.

Inclusion of more moments reveals secondary unstable branches.

Numerical simulations confirm growth and transport predictions.

Abstract

In the presence of a strong electric field perpendicular to the magnetic field, the electron cross-field (E$\times$B) flow relative to the unmagnetized ions can cause the Electron Cyclotron Drift Instability (ECDI) due to resonances of the ion acoustic mode and the electron cyclotron harmonics. This occurs in collisionless shock ramps in space, and in $\rm{E \times B}$ discharge devices such as Hall thrusters. ECDI can induce an electron flow parallel to the background E field at a speed greatly exceeding predictions by classical collision theory. Such anomalous transport may lead to particle thermalization at space shocks, and may cause unfavorable plasma flows towards the walls of E$\times$B devices. The development of ECDI and anomalous transport is often considered fully-kinetic. In this work, however, we demonstrate that a reduced variant of this instability, and more importantly, the associated anomalous transport, can be treated self-consistently in a two-fluid framework without any collision. By treating electrons and ions on an equal footing, the free energy allows the growth of an anomalous electron flow parallel to the background E field. We first present linear analyses of the instability in the two-fluid 5- and 10-moment models, and compare them against the fully-kinetic theory. At lower temperatures, the two-fluid fastest-growing mode is in good agreement with the kinetic result. Also, by including more ($>=10$) moments, secondary (and possibly higher) unstable branches can be recovered. The dependence of the instability on various parameters is also explored. We then carry out direct numerical simulations of the cross-field setup using the 5-moment model. The growth of the instability and the anomalous transport is confirmed. Finally, 5-moment and Vlasov simulations using identical parameters in the lower-temperature regime are performed, showing reasonable agreement.