Dynamics of repetitive plasma bullets in He plasma jets into air

Atmospheric-pressure plasma jets formed by dielectric barrier discharges and injected into ambient air are effective sources for production of chemically active non-thermal plasmas [1]. The jets are repetitively pulsed and so are composed of a sequence of ionization waves with speeds up to 10⁸ cm/s. The luminous plume of the plasma jets can be longer than 10 cm. With typical applied voltages of a few kV, the mean electric field in the plume can be less than 1 kV/cm which is less than the avalanche field in air or in He. As a result, a conventional ionization wave (the bullet) cannot be sustained for such long distances. However, for repetition frequencies of a few to tens of kHz, each new plasma bullet propagates in a gas excited and preionized by the previous plasma bullet. The pulse-periodic plasma jet then must develop from the initially unionized (and unexcited gas) having a short extent to a preionized channel having a longer extent.