A quasi-optical 1 mm microwave heterodyne interferometer for plasma diagnostics using a frequency-tripled Gunn oscillator

A heterodyne interferometer for electron density measurements in low-density plasmas is described. It operates at 309 GHz. The available power amounts to 5 mW. A reference microwave field, shifted by 10 kHz, is generated by the Doppler shift connected with the back reflection from the grooves of a grating on the circumference of a rotating wheel. Collimation and focusing of the microwave radiation are performed with elliptic aluminum mirrors. Schottky barrier diodes operated in mixer configuration are used as detectors. The interferometer is applied to the line-of-sight integrated electron density measurement of low-density plasmas. Line densities down to several 10 14 m -2 can be measured; the spatial resolution is slightly better than 5 mm.