A high speed compact microwave interferometer for density fluctuation measurements in Sino-UNIted Spherical Tokamak

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.

Implementation and data processing of a five-channel microwave interferometer with high temporal resolution and low noise on Sino-UNIted Spherical Tokamak

A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94 GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 10¹⁵ m^-2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.