Mode transitions and spoke structures in ExB Penning discharge

TL;DR

This study uses 2D particle-in-cell simulations to explore how increasing magnetic field strength causes transitions from spiral arm structures to m=1 spoke modes in ExB Penning discharges, revealing the role of plasma potential and boundary effects.

Contribution

It provides the first detailed simulation-based analysis of mode transitions and spoke structures in ExB Penning discharges, highlighting the effects of magnetic field strength and boundary conditions.

Findings

Azimuthal mode transitions occur with increasing magnetic field.

Spoke mode frequency is weakly affected by boundary geometry.

Ionization influences fluctuation characteristics but not basic structures.

Abstract

Two-dimensional particle-in-cell simulations in the (radial-azimuthal) plane perpendicular to the axial direction of a cylindrical ExB Penning discharge are presented. The low-pressure discharge is self-consistently supported by plasma ionization from the electron beam injected axially, along the direction of the external magnetic field. It is shown that with the increasing strength of the external magnetic field, the discharge undergoes a sequence of transitions between several azimuthal modes. Azimuthal m>1 spiral arm structures are excited at low magnetic field values as plasma confinement improves and the radial density profile becomes peaked. With a larger field, spiral arms with m>1 are replaced by the m=1 spoke mode, most clearly seen in plasma density. A transition from spiral arms to the spoke regime occurs when the plasma potential in the center changes from weakly positive (or zero) to negative. Further increase of the magnetic field results in a well-developed m=1 spoke mode with additional small-scale higher frequency m>1 structures inside and around the spoke. It is shown that while ionization and collisions affect some characteristics of the observed fluctuations, the basic features of the spoke and m>1 spiral structure remained similar without ionization. The role of energy conservation in small-scale high-frequency modes and spoke dynamics is discussed. It is demonstrated that in regimes with the m=1 spoke mode, additional m=4 harmonics of the ion and electron fluxes to the wall appear due to the square boundary. The frequency of the m=1 mode is weakly affected by the geometry of the boundary.