Evolution of ionization waves in a multi-pulsed plasma jet: the role of memory charges

In this paper, we discuss the properties of ionization waves (IWs) in a multi-pulsed plasma jet while using the two-dimensional computational approach. The IWs are generated by application of three short negative pulses with a repetition frequency 12.5 MHz. The simulations are performed continuously during a single run while accounting for charges accumulated inside (surface charges) and outside (space charges) the tube. The plasma forming gas mixture (He/O₂=99.8%/0.2%) is injected through the discharge tube into the surrounding humid air. We show that an IW can emerge from the tube exit at a pulse rising edge (as a negative IW) and at a falling edge of the same pulse (as a positive IW). It is demonstrated that remnants of the negative and positive charges play an essential role in the discharge evolution. The first pulse travels the shortest distance as it propagates through the initially non-ionized environment. The IWs developing during the second pulse essentially enlarge the plasma plume length. At the same time, the IWs generated by the third pulse eventually decay due to the remnants of charges accumulated during the previous pulses. Accumulated memory charges can lead to the IW extinction.