Enhancement of He(I) and He(II) radiation from a helium plasma in a magnetic field in non-local thermodynamic equilibrium

Abstract

Emission intensity enhancements of He I and He II lines have been measured from a helium plasma column in a longitudinal magnetic field (0-700 Oe) at a neutral particle density ≈10¹⁴ cm^-3. The lines involve He I transitions n³D → 2³P (n = 3,4,5), n³S → 2³P (n = 3,4,5), n³P → 23S (n = 3,4), n¹D → 2¹P (n = 3,4,5,6), n¹P → 2¹S (n = 3,4), n¹S → 2¹P (n = 4,5) and He II transition i = 4 → I = 3. As the magnetic field is varied in the 0-700 Oe range, the axial electron density increases from ≈10¹⁰ cm^-3 to ≈10¹² cm^-3 and the electron temperature falls from ≈11 eV to ≈6 eV. An improved collisional-radiative model is developed which considers completely separated singlet and triplet sublevels of He I up to n = 5, combined He I levels for n = 6-12, and He II levels i = 1-13. The model provides a semi-quantitative interpretation of the magnetic field enhancement of helium line radiation. It also explains results of some other helium plasma experiments satisfactorily.