Central structure of low-nBalmer lines in dense plasmas

Abstract

Stark-broadened profiles of the Balmer lines $H_{\alpha }$ and $H_{\beta }$ have been measured by means of a high-pressure electromagnetically driven shock tube, at electron densities $N_e\sim {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and temperatures $\kappa T\sim 1.5$ eV. The measured profiles of $H_{\alpha }$, down to ∼5% of peak intensity, are in much better agreement with the theoretical profiles of Kepple and Griem than with the results of more recent computations by Vidal, Cooper, and Smith. This suggests that for hydrogen lines with significant upper- and lower-state broadening, only elastic scattering contributions to the upper-lower state interference term should be included in the line-broadening operator. For $H_{\beta }$, agreement is obtained with both theories except within the central dip, the discrepancies between measured and predicted modulations being somewhat larger than those obtained in recent arc experiments. Our results for $H_{\beta }$ can be interpreted as indicating the presence of a transition layer of much lower electron density near the walls rather than the necessity for including the effects of ion dynamics in the calculations. The first-order dynamical correction to the Holtsmark profile for a single Stark component is considered in analogy with stellar dynamics, and found to be of negligible importance under our conditions. Additional measurements of the central structure of $D_{\beta }$ and the He i 4471-Å line are consistent with these conclusions.