Application of escape probability to line transfer in laser-produced plasmas

The application of an escape probability method to treat transfer of optically thick lines in laser-produced plasmas in plane-parallel geometry is described. The escape probability calculated is valid in a plasma with any locally linear velocity profile. It reduces to the standard result for a static plasma and to the result of Sobolev [Sov. Astron. Astrophys. J. 1, 678 (1957)] for large ion velocity gradients. As examples, escape probability is applied to model the effect of optically thick line trapping in both exploding foil and ablating massive slab targets. Compared to detailed numerical calculations for the line transfer in these targets, the escape probability gives quite accurate estimates. This can be understood in terms of the large Doppler shifts in the line profiles that are generated by the ion velocity in the plasmas.