Measuring the Line Density of Plasma Electrons by Three-Probe Microwave Interferometry

In this paper, a three-probe microwave interference method for measuring the line density of plasma electrons is presented. The problem of phase ambiguity is solved by measuring changes in the line electron density and starting the measurement process before plasma discharge initiation. It is shown that ignoring that the interprobe distances are not necessarily equal to their setting values may be responsible for a marked error, and a simple technique for electrical measurements of the actual interprobe distances is presented.