Spectral linewidth of a Ne-like Ar capillary discharge soft-x-ray laser and its dependence on amplification beyond gain saturation

We report the measurement of the linewidth and temporal coherence of a λ=46.9 nm neon-like argon capillary discharge soft-x-ray laser and its variation with plasma column length. A wave-front-division interferometer was used to resolve the 3p1S ₀-3s1P ₁ laser line, resulting in measured relative linewidths of Δλ/λ=(3-4)×10 ^-5. The measurements do not observe saturation rebroadening when this clearly dominantly Doppler-broadened inhomogeneous line is amplified beyond the intensity corresponding to gain saturation. Model simulations indicate this is the result of a comparatively small collisional broadening that sufficiently homogenizes the line profile to practically eliminate inhomogeneous saturation rebroadening. Collisional redistribution is computed to play only a very minor role in homogenizing the line profile.