The effect of seed electrons on the repeatability of fast ionization wave

Abstract

The APPJs driven by pulsed direct current (DC) or alternative current (AC) voltages at frequencies in the kHz range are typically composed of bullet-like plasma volumes travelling with high velocities, which is similar to the traditional streamers1. APPJs are significantly different from the traditional streamers. Two specific features of the plasma bullets distinguish them from traditional streamers. The first one is that, because of the flowing of the working gas, plasma bullets guided by the gas jet move along a predetermined path — the gas jet axis, rather than the traditional streamers in free space with many branches. Another important feature is that, in contrast to the traditional streamers, which propagate in a stochastic manner, plasma bullets have been found to exhibit high repeatability, i.e. propagating the same distance after the same delay time. The propagation of the fast ionization wave is in either repeatable mode or random mode2. Seed electrons between the nearby pulses play decisive role in determining the repeatability. The mode changes depending on the frequency of the applied voltage and the mixture percentage of the working gas. By measuring the propagation velocities and the ignition delay time, it is found that the propagation velocities of the plasma plume are always repeatable when the plasma propagating in both repeatable mode and random mode. But the jitter of the ignition delay time strongly depends on the frequency of the applied voltage and the mixture percentage of the working gas. It decreases to less than about 2 ns when the frequency is increased to higher than the critical frequency. It is found that the seed electron density for repeatable propagation mode should be higher than 108 cm−3 at 2×104 Pa and 106 cm−3 for 4×103 Pa by numerical calculation. And so on, the seed electron density for repeatability should be about 1010 cm−3 at atmospheric pressure.