Operational Limitation and Instability of a Microwave-Induced Microplasma Enclosed in a Microcavity at Low Gas Pressures

In this paper, operational limitations (the extinguishing of the stable glow discharge) and instabilities [glow-to-arc transition (GAT)] of microwave-induced microplasmas enclosed in microcavities operated at low gas pressures were investigated by experiments, in comparison with unenclosed microplasmas. For enclosed microplasmas, when gas pressure decreased, GAT occasionally occurred, whereas GAT was never detected for unenclosed microplasmas, because the gas temperatures of enclosed microplasmas were higher than those of unenclosed ones. For enclosed microplasmas operated at low gas pressures, an increase in the microcavity dimension is a valid method to avoid GAT. Extinguishing pressure of stable glow-discharge microplasma (
$p_{\mathrm { {ext}}})$
for microwave-induced microplasma enclosed in a microcavity microplasma was lower than that for microwave-induced microplasma generator without the PDMS cavity (UEC) microplasmas. The increase in input power decreased
$p_{\mathrm { {ext}}}$
for UEC microplasmas but showed a slight influence on
$p_{\mathrm { {ext}}}$
for EC microplasmas. This paper shed some light on understanding of the enclosed microplasmas operated at low gas pressures.