Coherent high-frequency axial oscillations in a partially magnetized direct current magnetron discharge

High frequency oscillations are observed in a neon plasma of a direct current magnetron discharge. At low discharge currents, we see highly coherent 60 MHz fluctuations. Above a distinct current threshold, secondary 5 - 10 MHz fluctuations emerge in addition to turbulent fluctuations in the 60 - 100 MHz range. The oscillations in the total discharge current suggest axial wave propagation. A lower-hybrid wave theory is invoked to model the high frequency oscillations. We attribute the low frequency modes to a turbulence-driven inverse cascade process, as suggested by recent simulations.