Stark–Zeeman spectroscopy for spatially resolved characterization of plasma jet with beta exceeding unity

A high-resolution polarimetric spectroscopy diagnostic is developed to provide a quantitative characterization of the plasma density and magnetic field structures of magnetically driven plasma jets generated in Caltech magnetohydrodynamic jet experiments. The diagnostic distinguishes elliptically polarized emission components by the Zeeman effect and measures the Stark broadening of the Ar II 480.602 nm spectral line [3s23p4(3P)4s 4P5/2–3s23p4(3P)4p 4P5/2°] with sub-millimeter and sub-microsecond resolution, enabling reconstruction of radial profiles of plasma density and vector magnetic fields across the axially uniform jet column. A model incorporating spectroscopic effects, line integration, and a multi-parameter plasma description reconstructs the spectra collected by 128 channels of an optical fiber system. A Markov chain Monte Carlo method inferred model parameters of plasma density, poloidal magnetic flux, and poloidal electric current by minimizing the difference between synthetic and measured data. A comparison with a non-linear regression analysis without model constraints confirmed that the densities obtained directly from line-integrated spectra underestimate the actual local density. The splitting of wavelengths in a polarimetric system is in accordance with the Zeeman effect. The observations show that the classic pinch force is inadequate to balance the radial pressure gradient force. Instead, the pressure gradient force is balanced by a Bernoulli-like inward radial force associated with a stagnating inward radial E×B velocity.