Study on Similarity of Microdischarge Based on Unified Fluid Model

Small-scale plasmas (with a size of several hundred micrometres) generated by microdischarges have received growing attention during the past two decades [1]. In this work, we established a one-dimensional unified fluid model for argon microdischarge with a gap length of $400 \mu \mathrm{m}$. The model includes the fluid equations for the particle and energy conservation of the electrons and the heavy species in the plasma, and the heat conduction from the high temperature cathode [2, 3]. We investigated the validity of discharge similarity from Townsend, subnormal glow, normal glow, abnormal glow, glow-arc transition, to arc regimes at pressures of 200 Torr and 400 Torr. The $V-J$ characteristic curve shows that the similarity holds rigorously in the Townsend regime. By tuning the ballast resistor, the gas heating effect becomes important toward high current discharge regime which can violate the similarity law. In the arc discharge regime, when the thermionic emission current dominates the discharge current, the $V-J$ characteristic curves can accidentally overlap even though the similarity law does not apply. The simulation results extend the application of the similarity law to microdischarge which are beneficial for understanding the characteristics of different discharge modes in microgaps.