The effect of the length to diameter ratio on capillary discharge plasmas

Abstract

The capillary geometry of discharge plasma sources determines the performance of capillary plasma generators. In this research, 16 types of generators with different geometric sizes and aspect ratios (the length divided by the diameter of the capillary, ranging from 0.67 to 22) were designed. Plasma parameters, including the temperature, static pressure, and velocity, were calculated and analyzed. The results show that the maximum temperature of the plasma decreased with increasing capillary aspect ratios. However, there was a difference between the temperature distributions above and below an aspect ratio threshold of 8.67. The simulations suggest that the aspect ratio threshold of the static pressure for generating shock waves is 15.33. For aspect ratios lower than this value, the maximum static pressure increased with increases in the aspect ratio. Otherwise, a shock wave would be generated, and the distribution of the static pressure would be consistent with distribution 2 (with a shock wave near the nozzle). The maximum velocity decreased as the capillary aspect ratios increased, with a threshold of 8.67. Since the plasma parameters were exponential functions of the aspect ratios, distribution and variation trends of the plasma parameters for different aspect ratios are proposed. These can be used to adjust the electrothermal plasma parameters and to design plasma generators.