Investigation of particle diffusion and suprathermal electrons in a magnetized helium plasma column

Abstract

Studying radiative properties of magnetized helium plasma via high-resolution spectroscopy identified close correlations between the particle diffusion and suprathermal electrons for different modes of operation of the MISTRAL installation. The standard diagnostic emission lines in neutral helium (1s3d 3D-1s2p 3P, 1s3s 3S-1s2p 3P, 1s3d 1D-1s2p 1P, and 1s3s 1S-1s2p 1P) show anomalous ratios that are related to enhanced particle diffusion and suprathermal electron generation. The supplementary investigation of singlet/triplet Rydberg series (transitions 1snd 3D-1s2p 3P and 1s5p 1P-1s2s 1S) as well as ionic lines (HeII, transitions n = 3–4 at 469 nm and n = 4–6 at 656 nm) allowed quantitative characterization. Simulations carried out with the atomic physics code SOPHIA demonstrate that simultaneous implementation of diffusion processes and suprathermal electrons matches all experimental findings. Single consideration, however, of either diffusion or hot electrons is in contradiction to the proposed extended set of HeI and HeII emission lines. The high precision achieved with the LSJ-split level structure of SOPHIA coupled to Langmuir probe measurements allowed to conclude to a Bohm type diffusion in MISTRAL.