Two-Color Heterodyne Interferometry for Electron Density Measurements in CMFX: Design, Implementation, and Drift Compensation

Interferometric electron density measurements during pulsed and steady-state operation of the Centrifugal Mirror Fusion Experiment (CMFX) are presented. A single-chord, 10.6 μm twocolor heterodyne interferometer with density resolution of ∼ 2 × 1018 m−3 for long-duration (>3 ms) pulses, and higher resolution for shorter durations, is employed. Line integrated density measurements using two distinct power drive systems are investigated: 1) short (1-20 ms) capacitor pulses with voltages exceeding 30 kV and power exceeding 100 kW, and 2) a direct-drive DC supply capable of driving up to 100 kV with a maximum current of 1 A up to 10 s durations. The interferometer's design, implementation, and verification are detailed. Phase drift effects due to temperature-dependent refractive index fluctuations in ZnSe, attributed to atomic line radiation from the experiment, are identified and analyzed. A drift compensation method using a wide-angle, broad visible/NIR responsive photodiode as a proportional proxy for total radiated energy over time, and thus temperature increase on the ZnSe window is established.