Ionization‐induced refraction in recombination x‐ray lasers

Ionization‐induced refraction is examined in regimes relevant to recent and planned recombination x‐ray lasing experiments [D. C. Eder et al., Phys. Plasmas 1, 1744 (1994)]. A model that self‐consistently evolves the laser radiation as it ionizes a neutral gas is presented. The model allows for multiple ionization stages and for spatial variations in the neutral gas density appropriate for studying gas jets and laser evaporated gases. The length of highly stripped plasma is calculated in H‐like Li and Li‐like N systems, where the conditions are appropriate for recombination. It is found that for a given pump power there is always an optimal spot size and focal position with respect to the gas jet, which minimizes refraction and maximizes the length for which gain is possible.