Behavior of Ar plasmas formed in a mirror field electron cyclotron resonance microwave ion source

Abstract

Langmuir probe measurements, photography, and monitoring of downstream microwave propagation were used to characterize argon plasmas obtained with a mirror field electron cyclotron resonance source. For certain system operating parameters, abrupt changes in plasma characteristics were observed. These were consistent with a transition in the mirror region plasma density from overdense (plasma density>cutoff density) to underdense. At higher powers and pressures, the visible light emission from the central region of the plasma diminished. Ionization of a large fraction of injected atoms before they reach the center is a likely contributor to this phenomenon. The high ionization efficiency also contributes to substantial differences between mirror and downstream pressure measurements. As a result, downstream pressure is the preferred reference for system pressure control. Finally, particular operating conditions were found to result in source chamber sputtering due to plasma potentials in the mirror region in excess of 30 V.