Collisional-radiative model for neutral helium in plasma revisited

Abstract

The collisional-radiative (CR) model code for neutral helium (T. Fujimoto, JQSRT 21, 1979) has been improved. The spin–orbit interaction gives rise to mixing of the wavefunctions of the singlet and triplet states. The degree of the mixing depends on the magnetic field strength, and at the field strength of the level-anticrossings complete mixing, or complete breakdown of the L– S coupling scheme, occurs. This effect is taken into account. The spontaneous transition probabilities and the excitation and ionization cross sections by electron impacts have been updated. Measurements of the electron temperature T e and density n e which make use of intensity ratios among three emission lines, λ 728.1 nm (2 1P – 3 1S) , λ 667.8 nm (2 1P – 3 1D) and λ 706.5 nm (2 3P – 3 3S) , have been attempted for the plasma boundary region in the Large Helical Device. These parameters are obtained in a wide range of the averaged electron density n ̄ e measured with the interferometer and it is found that T e and n e have a negative- and positive-dependences on n ̄ e , respectively. We have also measured the intensity of other emission lines, λ 501.6 nm ( 2 1S – 3 1P ), λ 388.9 nm ( 2 3S – 3 3P ) and λ 587.6 nm ( 2 3P – 3 3D ), and have compared them with the results of our code. The agreement is satisfactory, confirming the reliability of our present model code.