Measurement of the ion-mass effect on the central structure of H_β in a plasma produced by a combustion-driven shock tube

The central structure of the Stark-broadened Balmer line H_beta has been analysed in the plasma produced behind the reflected shock in a combustion-driven shock tube of internal diameter 10 cm. The incident shock is propagated at a Mach number typically of M=8 in a mixture of 0.5% H₂(D₂), 80% Ne, 20% He (Ar) at an initial pressure of 10 Torr creating in the reflected-shock region temperatures close to 10⁴K and electron number densities 10¹⁶<N_e<10¹⁷. Measuring times are 150 mu s. The variation of centre dip with reduced mass which has now been measured in two widely differing plasma sources strongly suggests that the discrepancy between theory and experiment for the central structure of H_beta is due to ion dynamical effects.