Far-Infrared Laser Interferometry for Electron Density Measurements

Abstract
A far‐infrared laser interferometer suitable for use with thin, transient plasmas of densities from 1014 to greater than 1017 cm−3 has been developed. A HCN gas laser operating at 337 and 128 μm and a H2O gas laser operating at 119, 79, and 48 μm are used as sources for a Mach—Zehnder interferometer; fast InSb and Ge:Ga detectors are used to measure the phase shift produced by the plasma. The interferometer has been used to study the output of a plasma gun which produces a plasma 4 cm in diameter and average density greater than 1015 cm−3, a plasma not suitable for microwave or optical interferometry.