Kinetic simulation of the transition from a pulse-modulation microwave discharge to a continuous plasma

The generation of a very strong peak current in the first period (PCFP) in a pulse-modulated microwave discharge has been discussed in previous studies. In this paper we focus on the transition process from a pulsed discharge to a fully continuous one driven by the pulsed microwave power source by means of a kinetic model. The computational results show that by increasing the duty cycle or voltage modulation rate (VMR), the discharge eventually becomes fully continuous and PCFP can no longer be observed. In the transition process, the distributions of the electric field, electron energy probability function (EEPF) and plasma density are discussed according to the simulation data, showing different discharge structures. The simulations indicate that many high-energy electrons with electron energy larger than 20 eV and low-energy electrons with electron energy less than 3 eV could be generated in a pulsed microwave discharge, together with a reversal electric field formed in the anode sheath when PCFP occurs. However, only medium-energy electrons could be observed in a fully continuous discharge. Therefore, by investigating the transition process the pulse-modulated microwave discharges can be further optimized for plasma applications at atmospheric pressure.