Development of a highly integrated millimeter-wave interferometer for plasma density diagnostics on the Helimak device

Low-density Helimak plasmas require non-intrusive density diagnostics with high phase sensitivity and high temporal resolution. Here we report the development of a compact 37.5 GHz millimeter-wave interferometer for real-time electron-density measurements on the Helimak device at Shenzhen University. The system combines a reflective double-pass optical path with heterodyne detection, so that the probing beam traverses the plasma twice and the line-integrated phase response is enhanced. A field-programmable gate array implements an all-phase fast Fourier transform (apFFT) algorithm for real-time phase extraction under intermediate-frequency drift. No-plasma measurements in the single-channel differential mode show an equivalent line-integrated density noise below 6×1014m−2. The interferometer was applied to Helimak discharges with different toroidal-field currents, and the measured line-averaged densities were consistent with the range obtained from Langmuir-probe diagnostics. The higher-current condition, which is closer to the ECR heating resonance, produced higher density and stronger density fluctuations than the lower-current condition. Time-frequency analysis also resolved broadband microscopic density fluctuations, demonstrating that the interferometer can measure both the slow evolution of line-averaged density and fast turbulence-related density fluctuations in Helimak plasmas.