Azimuthal and axial structures in 3D Particle-in-Cell simulations of Penning discharge

We report the results of 3D particle-in-cell simulations of cylindrical Penning dis-

charge in the so-called reflex configuration, where the cathode and anti-cathode are

biased to the same negative potential. The discharge is supported by thermal elec-

tron emission from the cathode. Electron and ion collisions, including ionization,

are fully accounted for. The emphasis is on a specific regime in which the plasma

potential at the center of the discharge is positive with respect to the chamber walls,

serving as an anode. Spatial and temporal scales of the observed azimuthal and

axial fluctuations and structures are characterized. It is suggested that azimuthal

structures are caused by the dissipative gradient-drift instability. We find that the

axial fluctuations related to the plasma-beam instabilities are weakly correlated with

the azimuthal perturbations of the density, so that the azimuthal modes rotate as

a whole and do not show any axial shear. The behavior of the electric potential is

more complex and involves intermittent structures in the r-z plane that modulate

the electron transport, producing the standing wave pattern (along the z-direction)

in the radial electron flux.