Spatial and temporal characteristics of atomic nitrogen in a pulsed microwave discharge

Abstract

The temporal and spatial characteristics of atomic nitrogen in a pulsed microwave discharge with 2.46 GHz field frequency with 20 Hz microwave pulse repetition rate, between 50 and 500 µs pulse duration and pressures from 2.5 to 20 mbar have been analysed experimentally by TALIF and explained by modelling. In the temporal profiles, in the middle of the plasma an unexpected increase in atomic nitrogen density within 20 µs after switching off microwave power supply has been observed. This could be explained by a kinetic plasma model taking into account the excitation of ground state nitrogen atoms N(4S) to the electronically excited states N(2D) and N(2P) by electron impact. As a second effect, the spatial and temporal density profiles of atomic nitrogen in the pulsed plasma showed a dominant diffusion behaviour in the phase of decreasing atomic nitrogen density between two microwave pulses. Fitting a model of diffusion led to diffusion coefficients of the order of 100 cm2 s−1, which correspond to translational temperatures of about 500 K. In this model, additional production of atomic nitrogen during the late afterglow proved to be important in weakening the density decrease.