# Boundary-induced effect on the spoke-like activity in ExB plasma

## TL;DR

This study investigates how boundary conditions influence spoke-like activity and anomalous transport in an $E\times B$ plasma, revealing that dielectric walls enhance instability and cross-field currents, while conducting walls suppress them.

## Contribution

It demonstrates experimentally that boundary type significantly affects spoke activity and electron transport in $E\times B$ plasmas, linking boundary conditions to instability mechanisms.

## Key findings

- Dielectric walls lead to stronger, more coherent spoke activity.
- Conducting walls reduce anomalous cross-field currents.
- Large-scale $E\times B$ transport reaches 40-100% of discharge current.

## Abstract

The spoke instability in an $E\times B$ Penning discharge is shown to be strongly affected by the boundary that is perpendicular to $B$ field lines. The instability is the strongest when bounded by dielectric walls. With a conducting wall, biased to collect electron current from the plasma, the spoke becomes faster, less coherent and localised closer to the axis. The corresponding anomalous cross-field transport is assessed via simultaneous time-resolved measurements of plasma potential and density. This shows a dominant large-scale $E\times B$ anomalous character of the electron cross-field current for dielectric walls reaching $40-100$% of the discharge current, with an effective Hall parameter $\beta_\mathrm{eff}\sim10$. The anomalous current is greatly reduced with the conducting boundary (characterised by $\beta_\mathrm{eff}\sim10^2$). These experimental measurements are shown to be qualitatively consistent with the decrease of the $E$ field that triggers the collisionless Simon-Hoh instability.