3D PIC/MCC modeling of the dynamics of rotating spokes in a Penning discharge

Penning discharges are plasma sources generated inside a cylinder with a large geometrical aspect ratio (length over radius) and with a magnetic field aligned with the longitudinal axis [1]. The magnetic field strength is sufficiently large to magnetize the electrons but not necessarily the ions (the latter are at most somewhat magnetized). In such plasma configurations, the boundary conditions can greatly modify the plasma properties. Electrons, being strongly magnetized, are attached to a field line which is short-circuited at both ends of the cylinder by either a biased metal plate or a dielectric surface. Biasing negatively the end-plates for instance will increase the residence time of the electrons inside the plasma discharge to a point where their mobility might be lower than the one of their ion counterpart. The plasma potential will then eventually become a well to confine electrostatically the ions and hence keep the whole plasma quasi-neutral. In such a situation, the plasma will become unstable and lead to the formation of large-scale rotating structures in the non-linear saturated regime. These structures, so-called spokes, rotate typically at a frequency of the order of 10’s of kHz depending on the mass of the ions [2,3]. In this work, we will simulate a Penning discharge powered by an electron beam using an explicit Particle-In-Cell (PIC) model with Monte-Carlo-Collisions (MCC) [4]. We will describe in detail the dynamics of the rotating spoke in 3D for Helium as a background gas and discuss the effect of the boundary conditions on the plasma behavior.

[1] E. Rodriguez et al., Phys. Plasmas 26, 053503 (2019).

[2] T. Powis et al., Phys. Plasmas 25, 072110 (2018).

[3] M. Tyushev et al., Phys. Plasmas 30, 033506 (2023).

[4] Fubiani et al., New J. Phys. 19, 015002 (2017).