High-power microwave energy coupling to nitrogen during breakdown

Microwave energy coupling to nitrogen for breakdown conditions has been investigated. The experiments were performed in 25-Torr nitrogen using a 35-GHz 1-μs microwave pulse focused to 33 kW/cm2. The plasma created during breakdown was observed to travel towards the source with a velocity of 4.6×106 cm/s. Spectroscopic measurements using a nitrogen-helium mixture indicate the average electron density over a 2-cm chord to be 4.4×1012 cm−3, and the electron temperature to be 3.8 eV. Microwave cutoff was observed photographically, and suggests the peak electron density to be the collisional critical density. Computer simulations were also performed, and their results are in accord with the experimental results.