Measurements of the Stark Broadening of Two Neutral Helium Lines in a Plasma

A detailed study has been made of the broadening of the lines λ3889 {} (width only) and λ5016 {} (shift and width) of neutral helium in the plasma produced in an electromagnetically driven T tube operating in helium-hydrogen gas mixtures. The electron density in the plasma was found from the broadening of H_β, while the temperature was determined from the ratio of the intensity of H_β to that of the underlying continuum. Temperatures were in the range 20 000-30 000^∘{}K; electron densities varied from 10¹⁶ to 6×{}10¹⁷ cm^-3. The broadening of the line λ3889 {} was found to increase linearly with electron density; and it compared well with calculations based on the generalized impact theory, except for (constant) correction factors ranging from about 0.9 to 1.0 (depending on which of the various calculations is referred to). For the line λ5016 {}, neither of the previous calculations, nor a correction discussed here, predicts either the width or the shift variations satisfactorily; all the calculations give significantly wider lines above electron densities of 10¹⁷ cm^-3 than are found experimentally. Also, as the electron density increases, the calculated shifts all become increasingly larger than those found experimentally. However, at the low densities most deviations are small ({}20%).