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

Abstract

A detailed study has been made of the broadening of the lines $\ensuremath{\lambda}3889$ \AA{} (width only) and $\ensuremath{\lambda}5016$ \AA{} (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 ${\mathrm{H}}_{\ensuremath{\beta}}$, while the temperature was determined from the ratio of the intensity of ${\mathrm{H}}_{\ensuremath{\beta}}$ to that of the underlying continuum. Temperatures were in the range 20 000-30 000\ifmmode^\circ\else\textdegree\fi{}K; electron densities varied from ${10}^{16}$ to 6\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The broadening of the line $\ensuremath{\lambda}3889$ \AA{} 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 $\ensuremath{\lambda}5016$ \AA{}, 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 ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ 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 (\ensuremath{\lesssim}20%).