Investigation of multi-periodic self-trigger plasma in a AC-driven Atmospheric Pressure Plasma Jet*

In this work, a helium plasma jet driven by 15~18kHz sinusoidal voltage ignites multi-periodic self-triggered mode or random mode depending on applied voltage, driven frequency and inter-electrode gap distance. Most of the observed multi-periodic bullets operate every 2 or 3 sinusoidal period. Such bullets show similarities with pulsed operating mode, having a jitter less than 100 ns. The presence of an outer grounded electrode ring is a key parameter permitting the ignition of multi-periodic bullets; it also enhances the propagation length up to 8 times. Fast imaging reveals that the negative bullets and positive bullets have different ignition mechanisms. Self-triggered pre-ionization waves are observed to propagate back and forth in the inter-electrode gap region before the formation of negative bullets; The negative bullets enables the ignition of positive bullets. Bullet velocities for different polarities and gap distance are compared using optical emission spectrum.