Ignition dynamics in microwave-generated microplasmas

Summary form only given. Microwave resonators are used to generate microplasmas in atmospheric-pressure argon. We investigate the ignition and filamentation dynamics in two systems with different techniques for modulating the microwave power: a diode-switched resonator in an array¹ with constant input power, and a single resonator with pulsed input power. The plasma evolution is examined using time-resolved photography and spectroscopic gas temperature measurements over time scales from tens of nanoseconds up to milliseconds.The two discharge systems exhibit markedly different behaviors at sub-microsecond time scales, corresponding to the different switching mechanisms. The diode-switched microplasma exhibits a relatively slow, monotonic rise in intensity due to a combination of electronic and ionization processes. In contrast, the pulsed microplasma experiences an initial spike in emission intensity due to the deposition of stored microwave energy into the plasma and electron overheating2. At time scales longer than a few tens of microseconds, the plasma dynamics in the two systems become quite similar. The microplasmas continue to grow in both intensity and size over several milliseconds. These dynamic behaviors are qualitatively explained by localized heating of the neutral gas, and are expected to appear in many types of microplasmas. We also discuss practical limits to plasma equilibration time in different microplasmas.