A multi-chord, two-color interferometer using Hilbert transform phase detection for measuring electron density in...

A new, four-chord, CO₂/He-Ne heterodyne interferometer has been designed and built for measuring line-averaged plasma density in the HIT-SI3 and subsequent HIT-SIU sustained spheromak devices. The two-color system successfully eliminates vibration-induced errors caused by mirrors that are secured to the vacuum chamber and is able to resolve electron densities n_e in the full operating range of 10¹⁸-10²⁰ m^-3 in both experiments with an integrated error of 4.7 × 10¹⁷ m^-2. Data are presented from high toroidal current plasma discharges, showing the time evolution of electron densities n_e and j_ϕ/n_e along multiple chords.