Central structure of low-nBalmer lines in dense plasmas

Stark-broadened profiles of the Balmer lines H_α and H_β have been measured by means of a high-pressure electromagnetically driven shock tube, at electron densities Nₑ~10¹⁷ cm^-3 and temperatures κT~1.5 eV. The measured profiles of H_α, 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_β, 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_β 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_β and the He i 4471-{} line are consistent with these conclusions.