Absence of Dicke narrowing in x-ray laser plasmas

Abstract

Present x‐ray laser plasmas are in the intermediate coupling regime (0.1<Γ<4, Γ≡Z2e2/kTa, with a the ion sphere radius), where the binary collision based arguments of low density plasma kinetic theory can be misleading. The difficulty of modeling the spectral output from neonlike lasers at wavelengths near 200 A has led to the suggestion, based on a low density, electron screened, plasma model, that collisional (‘‘Dicke’’) narrowing may be important. Molecular dynamics results, which include ion collisions and screening effects, are presented here. They indicate that the free‐particle Doppler approximation is still quite accurate for λ∼200 A and Γ≲1. Collisional narrowing will become important at somewhat stronger couplings (Γ∼5) and at this wavelength the diffusion approximation is reasonable for Γ≳10. Collisional narrowing is found to be small even for the higher density (Γ≊4) nickel‐like lasers operating near 45 A.