Spot patterns and instabilities in a pulsed low-pressure rf glow discharge

During the development of a plasma imaging system, luminous streaks and instabilities occurred in a pulsed rf glow discharge operated in Ar at reduced pressure. Sputtering sources with a flat cathode having an area of 10-50 cm²were designed to reproduce the surface spatial structure of the cathode in the plasma emission, providing optical emission spectrometry with spatial resolution for the chemical analysis of the cathode material. Unfortunately, depending on the discharge parameters and design of the discharge cell, spot patterns appeared and disappeared, and at times were stable or unstable. The behavior and possible origin of these patterns are described in this article.