Unequal Path Length Interferometry for ZaP-HD Flow Z-Pinch Plasma Experiment

Laser interferometry is a common diagnostic used to measure electron density in plasma experiments. Traditionally, laser interferometers have been employed under the assumption that the scene and reference beams must be equal in length. While this practice maximizes the signal to noise ratio, it provides challenges to experiments requiring multiple laser beams in laboratories with space constraints. Allowing beam paths to be unequal in length would permit increased flexibility in optical setups. In pursuit of this flexibility, some researchers have shown that gas tube laser interferometers with unequal path lengths can produce accurate measurements, provided that the difference in path lengths is equal to some integer multiple of double the cavity length of the gas tube laser. These investigations, however, assumed that the spatial periodicity seen in a homodyne Michelson interferometer configuration will remain constant when employing the same path length differences on a heterodyne Mach-Zehnder interferometer configuration, with which actual plasma density measurements were collected. This work aims to close the gap between proofs of concept and experimental implementations by investigating the signal quality of a Mach-Zehnder heterodyne quadrature helium-neon (HeNe) interferometer over a range of path length differences. Experimental methods and results are given for the benchtop investigation of signal quality. Application of the setup is discussed for measuring plasma density in ZaP-HD, an experimental device at the University of Washington used to demonstrate a sheared-flow-stabilized Z-pinch nuclear fusion space thruster concept.