Scaling of Spoke Rotation Frequency within a Penning Discharge

TL;DR

This paper demonstrates that plasma spokes in a Penning discharge develop due to a non-linear phase of the collisionless Simon-Hoh instability, with their frequency scaling predictably with key plasma parameters.

Contribution

It reveals that spoke formation does not require ionization and establishes a scaling law for spoke frequency based on plasma parameters.

Findings

Spokes develop in both collisional and collisionless simulations.

Spoke frequency scales with 5-40 words)

Spoke formation is linked to the collisionless Simon-Hoh instability.

Abstract

A rotating plasma spoke is shown to develop in two-dimensional full-sized kinetic simulations of a Penning discharge cross-section. Electron cross-field transport within the discharge is highly anomalous and correlates strongly with the spoke phase. Similarity between collisional and collisionless simulations demonstrates that ionization is not necessary for spoke formation. Parameter scans with discharge current $I_d$, applied magnetic field strength $B$ and ion mass $m_i$ show that spoke frequency scales with $\sqrt{e E_r L_n / m_i}$, where $E_r$ is the radial electric field, $L_n$ is the gradient length scale and $e$ is the fundamental charge. This scaling suggests that the spoke may develop as a non-linear phase of the collisionless Simon-Hoh instability.