Microprocessor-controlled phase analysis for a 2-mm microwave interferometer on the TCA tokamak

The method of phase analysis is presented for a 2-mm microwave interferometer built to measure the plasma density in a tokamak. Use is made of a fast phase shifter (FPS) constructed from a switchable four-port circulator to resolve the phase ambiguity inherent in a simple interferometer. The phase between 0–2π is calculated by a microprocessor accessing a look-up table and the total phase is calculated by taking the 0 to 2π crossings into consideration. It is software controlled and the complete sampling and analysis cycle takes 24 μs which is sufficiently rapid for the application considered. The phase information is available in analog and digital form.