Millimeter-wave interferometer for measuring plasma electron density

A simple, low-cost millimeter-wave interferometer has been developed to measure the electron density of laboratory-type plasmas with densities n≤2×1012 cm−3 . Two 38.0-GHz Gunn oscillators, whose oscillation frequencies are offset by 40 MHz, are used in a heterodyne configuration. The high intermediate frequency allows the use of inexpensive, free-running mechanically tuned Gunn oscillators and permits this system to be utilized in the study of density fluctuations. A quadrature phase comparator is used to give simultaneously both the sine and cosine components of the plasma interference signal. The phase noise of the system is found to be <0.1°, with a settling time <100 ns. The system is to be used on a laboratory plasma to look at both the integrated electron density and the level of density fluctuations.