Multi-chord fiber-coupled interferometry of supersonic plasma jets and comparisons with synthetic data

TL;DR

This paper demonstrates the use of a multi-chord fiber-coupled interferometer with long coherence length to measure and analyze the density and expansion of supersonic plasma jets, supported by numerical modeling.

Contribution

It introduces a simplified optical setup for interferometry and combines experimental data with modeling to characterize plasma jet properties.

Findings

Measured jet densities and velocities (~15-50 km/s)

Observed phase shifts indicating ionization dynamics

Derived 3D expansion and length of plasma jets

Abstract

A multi-chord fiber-coupled interferometer [Merritt et al., Rev. Sci. Instrum. 83, 033506 (2012)] is being used to make time-resolved density measurements of supersonic argon plasma jets on the Plasma Liner Experiment [Hsu et al., Bull. Amer. Phys. Soc. 56, 307 (2011)]. The long coherence length of the laser (>10 m) allows signal and reference path lengths to be mismatched by many meters without signal degradation, making for a greatly simplified optical layout. Measured interferometry phase shifts are consistent with a partially ionized plasma in which an initially positive phase shift becomes negative when the ionization fraction drops below a certain threshold. In this case, both free electrons and bound electrons in ions and neutral atoms contribute to the index of refraction. This paper illustrates how the interferometry data, aided by numerical modeling, are used to derive total jet density, jet propagation velocity (~15-50 km/s), jet length (~20-100 cm), and 3D expansion.