How the plasma bullet stops propagating

Summary form only given. The plumes generated by low temperature plasma jets were found to consist of plasma packets/bullets traveling at high velocity. For the case of pulsed devices and when helium is used as the operating gas, two main factors influence the plasma bullet lifetime and therefore the length of the plume. These factors are the helium mole fraction in the gas channel and the width of the applied voltage pulse. It is found that, in general, the helium mole fraction should be more than a critical limit to sustain the plasma bullet propagation. However, the width of the applied high voltage pulse also plays a crucial role. For long pulses (greater than 700 ns) it is the diffusion of oxygen into the helium channel that extinguishes the bullet. But for pulse widths less than 700 ns, the plasma bullet stops propagating after the secondary discharge ignition, which occurs at the end of the voltage pulse. The secondary discharge is ignited due to the presence of charges accumulated on the dielectric-covered electrodes. The plasma thus created provides electrons that get accelerated by the field towards the bullet. These electrons cancel out the effect of the positive charges present on the bullet head, which is to attract electrons ahead of the bullet (created by photoionization) causing avalanche and in this way sustaining the bullet propagation. So, in effect, for short pulses, the ignition of a secondary discharge brings about the interruption of the events that are responsible for the bullet propagation. In this paper, experimental evidence supporting the mechanisms explained above will be presented.