SATURATION INTENSITY FOR ULTRASHORT-PULSE X-RAY LASER SCHEMES

Analytic expressions are obtained for the saturation intensity in X-ra y laser schemes based on short-pulse high-intensity drivers. For field -ionized plasma schemes, the specific mean saturation intensity J(sat) is time independent and depends only on atomic transition probabiliti es, level degeneracies, and transition energies. The analytic expressi on is found to be in good agreement with a detailed numerical calculat ion. Integrating over space and frequency gives a saturation intensity I(sat) of order 2 x 10(11) W/cm2 for lasing in Li-like Ne at 98 angst rom. The low input energy requirements for this scheme (< 1 J), associ ated with using a confocal geometry, give energy efficiencies of order 10(-6) and greater. For inner-shell photo-ionization schemes, an accu rate expression for a time-dependent saturation intensity is obtained. This scheme is calculated to have high saturation intensities, I(sat) almost-equal-to 10(13) W/CM2 , at short wavelengths (5-15 angstrom). The requirement of a line focus geometry leads to higher input energie s (greater-than-or-equal-to 5 J) and the short duration of lasing (les s-than-or-equal-to 50 fs) results in lower energy efficiencies (almost -equal-to 10(-7)). Repetition rates are important in determining appro priate applications for both schemes.