Rotating spokes in a partially magnetized, direct-current magnetron plasma

Coherent fluctuations in discharge current are measured using a segmented anode on an obstructed direct-current planar magnetron discharge. The fluctuations are on the MHz scale and are phase shifted along the closed-drift ExB direction, indicating the presence of rotating spoke plasma structures. The frequency and wavenumber of rotating spoke disturbances are studied for neon, argon, krypton, and xenon plasmas across a range of electrical discharge and pressure conditions. In neon, the plasma disturbances all propagate in the -ExB direction (opposite the electron drift). In argon, krypton, and xenon, the spoke propagation direction is seen to reverse from -ExB to +ExB by increasing current density or ambient pressure. The propagation reversal is accompanied by a decrease in the oscillation frequency and an increase in the phase velocity. The spoke phase velocities are an order of magnitude smaller than the ExB electron drift velocity. The number of spokes present in the discharge depends non-linearly on the pressure and current density. Dimensional analysis is used to study the frequency-wavenumber relationship across the dataset, and the resulting similarity solution exhibits a scaling with the Alfvén critical ionization velocity, suggesting an ionization wave phenomenon underlying the spoke dynamics.