Far-infrared diagnostics of electron concentration in combustion MHD plasmas using interferometry and Faraday rotation

The first experiment used an HCOOH laser at 393.6 microns feeding a Michelson interferometer mounted around a combustor duct with open ports. Simultaneous measurements of positive-ion density and plasma temperature made with a Langmuir probe and line reversal apparatus verified the operation of the interferometer. Some phosphorus compound was added to the KOH seeded ethanol fuel to simulate the maximum levels found in coal. The negative ion production was much less than predicted, corresponding to a conductivity loss of 10-15% at the exit of an MHD generator. With a magnetic field present, measurement of the polarization rotation and induced ellipticity in a wave traveling along the field provides information on the plasma conductivity. Compared to interferometry, diagnostic apparatus based on Faraday rotation offers simpler optics and requires far less stringent mechanical stability at a cost of lower sensitivity (factor of 5 to 10). The concept was proven on a simple experiment using a manually rotated polarizer. These far infrared diagnostics work well with combustion MHD plasmas. They are sensitive to changes in electron concentration down to a few percent, better than the stability and reproducibility of most combustors. Interferometry is the preferred technique for small scale, high accuracy measurements, with Faraday rotation reserved for large systems or measurements within a working generator