Saturation intensity for ultrashort-pulse X-ray laser schemes

Abstract

Analytic expressions are obtained for the saturation intensity in X-ray laser schemes based on short-pulse high-intensity drivers. For field-ionized plasma schemes, the specific mean saturation intensityJ _sat is time independent and depends only on atomic transition probabilities, level degeneracies, and transition energies. The analytic expression is found to be in good agreement with a detailed numerical calculation. Integrating over space and frequency gives a saturation intensityI _sat of order 2 × 10¹¹ W/cm² for lasing in Li-like Ne at 98 Å. The low input energy requirements for this scheme (< 1 J), associated with using a confocal geometry, give energy efficiencies of order 10⁻⁶ and greater. For inner-shell photo-ionization schemes, an accurate expression for a time-dependent saturation intensity is obtained. This scheme is calculated to have high saturation intensities,I _sat ≈ 10¹³ W/cm², at short wavelengths (5–15 Å). The requirement of a line focus geometry leads to higher input energies (≥5 J) and the short duration of lasing (≤50 fs) results in lower energy efficiencies (≈ 10⁻⁷). Repetition rates are important in determining appropriate applications for both schemes.