Far-Infrared Interferometer for the Measurement of High Electron Densities*

The average electron concentration of a gas-discharge plasma can be inferred from the phase change suffered by an electromagnetic wave in its passage through the medium. With available microwave generators, the maximum electron density that can be measured is approximately 3×1014 cm−3. On the other hand, interferometers that operate at optical wavelengths become insensitive at electron densities less than approximately 1016 cm−3. An infrared interferometer is described here which operates at 0.03 cm wavelength and is able to detect densities above 5×1013 cm−3 in a plasma column 1 cm thick. In the design, incoherent radiation from a mercury arc lamp is split by a diffraction grating and the plasma placed in one of the diffracted beams. The beams are then recombined and fringe shifts measured by a Golay detector.