Measurement of the ion dynamic effect on theHαandDαlines at low electron densities

The line profiles of the Balmer α transitions of hydrogen and deuterium were measured simultaneously in a theta-discharge plasma. The characteristics of the light source were the following: peak electron density of the order of 7×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}3}$, electron temperatures between 15 and 20 eV, and neutral and ion temperatures between 2.5 and 3.5 eV. The filling gas was helium with a 1% admixture of equal parts of hydrogen and deuterium. The experimental profiles were compared with a theoretical spectrum that was calculated taking into account the Stark broadening, independent Gaussian widths for each line, a fine-structure correction, and the instrumental broadening. In the data reduction a correction of the experimental data was made to allow for a radial inhomogeneity of the plasma. The measurements confirm the existence of large deviations from calculated profiles that do not account for ion dynamical effects. It has also been found that a model to include ion dynamical effects as a Gaussian broadening partly corrects the discrepancies. Finally, about 10% agreement is obtained when Lorentzian profiles based on relaxation theory calculations are used for the Stark broadening. Similar agreement is obtained with numerically simulated profiles.