Electron density measurements in a pulse-repetitive microwave discharge in air

Abstract

We have developed a technique for absolute measurements of electron density in pulse-repetitive microwave discharges in air. The technique is based on the time-resolved absolute intensity of a nitrogen spectral band belonging to the Second Positive System, the kinetic model and the detailed particle balance of the N2C3Πu (ν = 0) state. This new approach bridges the gap between two existing electron density measurement methods (Langmuir probe and Stark broadening). The electron density is obtained from the time-dependent rate equation for the population of N2C3Πu (ν = 0) using recorded waveforms of the absolute C3Πu → B3Πg (0-0) band intensity, the forward and reflected microwave power density. Measured electron density waveforms using numerical and approximated analytical methods are presented for the case of pulse repetitive planar surface microwave discharge at the aperture of a horn antenna covered with alumina ceramic plate. The discharge was generated in air at 11.8 Torr with a X-band microwave generator using 3.5 μs microwave pulses at peak power of 210 kW. In this case, we were able to time resolve the electron density within a single 3.5 μs pulse. We obtained (9.0 ± 0.6) × 1013 cm–3 for the peak and (5.0 ± 0.6) × 1013 cm–3 for the pulse-average electron density. The technique presents a convenient, non-intrusive diagnostic method for local, time-defined measurements of electron density in short duration discharges near atmospheric pressures.