High frequency axial oscillations in a partially magnetized direct-current magnetron plasma*

High frequency oscillations in discharge current are observed in a direct-current magnetron device operating on neon.  Two operating modes are found with distinct frequencies and discharge characteristics.  At low currents, the discharge has a positive voltage coefficient of resistance, and highly coherent 60 MHz fluctuations are measured.  Above 13 mA, the discharge voltage is relatively insensitive to current, and we observe secondary 5 - 10 MHz fluctuations in addition to turbulent fluctuations at 60 - 100 MHz.  A fluid model for lower-hybrid waves is invoked to explain the high frequency modes.  The low frequency modes concomitant with the onset of high frequency turbulence are attributed to a microturbulence-driven inverse cascade process.