Development of Methods for Determining Near-Surface Plasma Parameters During Tests of Fusion Reactor First-Wall Prototypes Using the PLM Device

Plasma-surface interaction and high heat flux on plasma-facing materials in magnetic fusion devices cause surface ablation and degradation, while the influx of eroded materials into plasma can have a shielding effect. The reduction of the power load due to the plasma detachment effect over tungsten fuzz is an important phenomenon to be investigated for the ITER divertor problem. Measuring near-wall plasma parameters is a challenging task, requiring the development of improved and advanced techniques, including high-resolution spectroscopic methods. In this paper, we present study results of steady-state plasma over tungsten fuzz formed in plasma linear multicusp (PLM). The PLM device is a linear plasma trap composed of an eight-pole multicusp magnetic field with steady-state plasma discharge with parameters similar to the scrape-off layer and divertor plasma in a tokamak.

We used spectroscopic measurements to estimate spatial distributions of plasma radiation in the vicinity of the sample surface exposed to the plasma column. Thus, we obtained information on the temperature and composition of the boundary layer plasma and the temperature of the sample surface. Helium plasma exhibits ionization-type nonequilibrium even at atmospheric pressure, necessitating the use of specific methods to estimate its electron temperature T_e. When the helium ion spectral line He II 468.5 nm is present in the spectra, its intensity ratio to one of the atomic lines He I can be described by using coronal approximation. Spectrum analysis has shown that emitting helium ions are highly sensitive indicators of average electron energy  = 3kT_e/2. Therefore, utilizing intensity ratios of the strongest emitting lines in the ultraviolet-visible near-infrared range, He II 468.6 nm and several He I lines with well-known electron excitation functions were found to be a reliable T_e measurement method in the case of magnetized low-pressure helium plasma. We also propose a method for determining the concentrations of the metallic admixture in the plasma on the data on relative intensities of its spectral lines.