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 N 2C 3u ( 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 N 2C 3u ( 0) using recorded waveforms of the absolute C 3u → B 3g (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 s 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 s pulse. We obtained (9.0 0.6) 10 13 cm -3 for the peak and (5.0 0.6) 10 13 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. © 2011 American Institute of Physics.