Optical emission measurements in a pulsed, high power microwave discharge in low pressure air

The authors discuss their measurement of emission spectra over the 200- to 680-nm range and intensity histories at 337.1, 391.4, and 423.6 nm from microwave discharge plasmas, in low pressure air (0.01 to 3 torr). The discharges were formed at the constructive interference regions of an S-band microwave beam which was incident on an inclined metal plate. Spectra were recorded at --0.2-nm resolution using a scanning monochromator; the scans measured integrated light output averaged at any one wavelength over a few pulses, but gathered from a sequence of 10-Hz repetitive pulses lasting more than 10 minutes. Intensity-time histories of individual microwave pulses were measured using a pair of PMT's, each fitted with a bandpass filter of --2-nm bandpass at 337.1, 391.4, or 423.8 nm