Automated Phase Counting Corrections for the C-2W Far-Infrared Interferometer

In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. Line-integrated electron density measurements from the multi-chord far-infrared (FIR) interferometer system provide critical information about plasma profiles and performance, and many derived quantities depend on reliable density measurements. Transient reductions in signal power due to refraction through strong plasma density gradients can cause phase counting errors, and system susceptibility to refraction is notably challenging at longer FIR wavelengths. The interferometer phase is calculated from digitized data using post-shot complex phase demodulation algorithms with high bandwidth. Phase counting errors can be identified on either a single channel basis or by combining profile information from all channels. Additionally, techniques successfully utilized by hardware based phase comparators can also be applied through software. The accuracy and robustness of these approaches are compared.

[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019)