Investigation of Instabilities in Microstrip-Sustained Microplasma

Plasmas at atmospheric pressure are vulnerable to ionization overheating instability followed by a destructive glow-to-arc transition (GAT) as power density increases. We report the characteristics of steady-state atmospheric pressure microplasmas in nonflowing argon and air driven by up to 40 W of microwave power. These microdischarges are supported by either 1) a quarter-wave microstrip resonator or 2) a microstrip transmission line. Modeling shows that the resonator configuration rejects excess power as the discharge resistance drops below 1 k(Omega ) and the microplasma remains stable. The transmission line configuration couples power efficiently to a 100-(Omega ) discharge resistance and produces a more intense microplasma. Electrode materials of copper, aluminum, and lead-based solder are investigated on both polymer-composite and alumina substrates. In both the configurations, copper electrodes are robust. Metal electrodes with lower melting points, however, may be evaporated by the highest power microdischarges and this induces arc-like behavior. Discharges supported on polymer-based substrates are damaged and show optical emission from C₂ as well as CN and CH, but alumina substrates are unaffected by the microplasma. With the correct materials selection, both microstrip transmission line and resonator microplasma generators effectively ballast the discharge against a GAT.