Design of a phasemeter for real-time measurements of the average plasma density with the microwave interferometer of the tokamak T-15MD

Interferometry is one of the key diagnostics for fusion research. This diagnostic measures electron plasma density. Interferometers that work on an intermediate frequency are widespread nowadays. The phase shift between interferometer signals has to be measured. The use of a microwave interferometer on machines, such as a tokamak, leads to challenging requirements for the phasemeter operation. We present a method to provide real-time measurements of phase shifts with values much higher than 2π. The phasemeter has been designed for the microwave interferometer of the T-15MD tokamak. It is based on microcircuits AD8302. Two signals with frequencies of 5 and 10 MHz are used as interferometer outputs. The phasemeter output will be used to create a feedback loop for the working gas injection into the tokamak chamber to control the plasma density. The phasemeter has been successfully tested. The typical phase error and non-linearity of the device are 1° and 5 × 10-4, respectively.