Relative intensity of helium spectral lines as a function of electron temperature and density

The variation with electron temperature (2-11 ev) and density (10 10 -10 19 cm -3 ) of the intensity ratios of spectral lines from helium in subsequent ionization stages is calculated for a steady helium plasma which is optically thin towards these lines. Results are given for the ratio of the intensity of the triplet line of He I at 5876 Å to the intensity of the He II line at 4686 Å. The plasma is assumed to be uniform and fully contained. Known data are used from existing calculations of the ionization degree in hydrogen and of the excited state densities of helium atoms and hydrogenic ions. The effect on the density ratio He III/He II of self-absorption of He II Lyman a radiation is investigated using a simplified model for the processes He II ⇌ He III. The calculations may not be valid for particle densities less than about 10 15 d -1 cm -3 , where d is the size of the plasma in centimetres, or when the plasma is inhomogeneous or transient.