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

Abstract

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.