Multi-chord fiber-coupled interferometer with a long coherence length laser

TL;DR

This paper presents a fiber-coupled heterodyne interferometer with a long coherence length laser for precise, flexible plasma density measurements in high-energy plasma experiments, enabling simplified setup and high sensitivity.

Contribution

The work introduces a fiber-optic, long coherence length laser interferometer that simplifies alignment and enhances measurement flexibility for plasma density diagnostics.

Findings

Operates effectively at densities as low as 10^15 cm^(-2)

Allows single reference path for multiple probe chords

Enables easy reconfiguration of measurement regions

Abstract

This paper describes a 561 nm laser heterodyne interferometer that provides time-resolved measurements of line-integrated plasma electron density within the range of 10^15-10^18 cm^(-2). Such plasmas are produced by railguns on the Plasma Liner Experiment (PLX), which aims to produce \mu s-, cm-, and Mbar-scale plasmas through the merging of thirty plasma jets in a spherically convergent geometry. A long coherence length, 320 mW laser allows for a strong, sub-fringe phase-shift signal without the need for closely-matched probe and reference path lengths. Thus only one reference path is required for all eight probe paths, and an individual probe chord can be altered without altering the reference or other probe path lengths. Fiber-optic decoupling of the probe chord optics on the vacuum chamber from the rest of the system allows the probe paths to be easily altered to focus on different spatial regions of the plasma. We demonstrate that sub-fringe resolution capability allows the interferometer to operate down to line-integrated densities of order 10^15 cm^(-2).