A two-chord, 532-nm, heterodyne laser interferometric system has been developed for measuring the density of a low-temperature, partially ionized hypersonic plasma jets (with Mach number in the range 10–50) generated by the STG1 v1.2 coaxial plasma gun. Since the plasma jets are only weakly ionized, accurate determination of the degree of ionization is required in order to determine the total particle density ntot (ions and neutral atoms). While the issues have been addressed by previous authors, the present work reexamines these issues in a somewhat different plasma regime and resolves them in a different way. In the present work, a Boltzmann line-ratio spectroscopic method is used to determine the electron temperature and thus the degree of ionization of the plasma, assuming local thermal equilibrium. We also review the latest and most accurate measurement of the electrical polarizability of the argon atoms. At a distance of 280 mm from the muzzle of the gun, the temperature of the plasma jet was measured to be 1.20 ± 0.08 eV. Within the error bar of the measured electron temperature, the peak values of the total particle density (ions and neutrals) measured are ntot=(2.86 ± 1.51)×1022 m−3 according to Merritt's model, ntot=(2.5 ± 1.24)×1022 m−3 according to Lan's model. At the peak of the particle density, both models give a similar degree of ionization at about 30%. A high bandwidth is needed to accommodate the high velocity of the plasma jet, typically of about 100 km/s. To provide plenty of headroom for future upgrade of the system in terms of its bandwidth, a 350 MHz acousto-optic modulator was adopted. High-speed images of the jet provide the basis for approximating the density profile by a Gaussian, which simplifies the reconstruction of the volumetric density of the plasma jet considerably. The problem posed by the practical difficulty of aligning the axis of the gun with the axis of the diagnostics system is alleviated in a limited way by the use of two chords for the laser interferometric system. In the future, multiple arrays of multiple chords are planned in order to fully address this misalignment problem.
Y et al. (2026) studied this question.