Impact of the radial electric field on plasma instabilities in a penning discharge: Insights from a 3D particle-in-cell model

This Letter describes the main features of the azimuthal and axial structures in a numerical model of a 3D low-pressure magnetized plasma column (a so-called Penning discharge). The simulations are performed with a 3D/3V explicit momentum conserving Particle-In-Cell (PIC) algorithm with Monte Carlo Collisions (MCC). The discharge is seeded by an electron beam injected on one end of the cylinder, ionizing helium atoms for gas pressures of mTorr-scale. We study how the magnetic field strength and a bias voltage on the column's end plates affect the radial electric field profile and the formation of rotating plasma instabilities. We observe kHz-scale rigid rotating structures localized azimuthally (so-called spokes) in the non-linear saturated regime when E·∇n>0 and large m-modes azimuthal waves in the ∼1 MHz range when E·∇n<0. Axial modes in the direction parallel to the magnetic field lines are also observed, which are weakly correlated with azimuthal modes but related to the non-linear plasma-beam instabilities.