Enhancement of hydrogen radical density in atmospheric pressure plasma jet by a burst of nanosecond pulses at 1 MHz

The generation and enhancement of active species in non-thermal plasmas are always decisive issues referring to their successful applications. In this work, atmospheric pressure plasma jet (APPJ) is generated in Ar + 1% CH4 gas flow by a bipolar nanosecond high-voltage (HV) source with a maximum pulse repetition rate up to 1 MHz (i.e., minimum pulse interval ΔT = 1 µs) in burst mode. The absolute density of hydrogen atom at ground state is measured by the two-photon absorption laser induced fluorescence (TALIF) method. It is observed that with ΔT = 1 µs, the H atom density keeps increasing during the first eight HV pulses and later on the H atom density maintains at a quasi-stable value while more HV pulses are applied. When decreasing ΔT from 10 to 1 µs while keeping the total number of HV pulses the same (with similar coupled energy), the peak H atom density increases by a factor of more than four times, but the decay of H atom density after the pulse burst with ΔT = 1 µs is faster. Another effect of short ΔT is to extend the axial distribution of H atom outside the APPJ’s nozzle and the ΔT = 2 μs case has the highest averaged H atom density when taking its temporal evolution and axial distribution into consideration. This work proposes that the intensive nanosecond HV burst is an efficient approach to enhance the active species density in non-thermal plasmas when a rapid response is required.