He–Ne interferometer for density measurements in plasma opening switch experiments

A He–Ne interferometer is used to measure the line-integrated electron density in the plasma of an operating plasma opening switch (POS). A heterodyne technique is used, where an acousto-optic modulator generates a 40-MHz frequency shift between the reference and scene beams. The phase shift is calculated numerically from the zero-crossing times of the beat signal. The precision of the phase measurement is ±0.5°, however, several nonplasma sources of phase shift are present just after the current through the POS is interrupted. In particular, charged particle flow into the vacuum window results in a negative phase shift. This anomalous phase shift is effectively eliminated by using magnets to deflect the particles away from the window. Measurements during POS shots show a localized decrease in line density prior to current interruption.