PLASMA DISPERSION IN ULTRASHORT-PULSE X-RAY LASERS

The effect of plasma dispersion in longitudinally pumped x-ray lasers prepared by optical-field-induced ionization is investigated. Dispersion is shown to bound the x-ray peak intensity along the laser and to stretch the pulse in time, thus coincidentally giving a saturated output energy fluence that increases linearly with laser length as in the nondispersive regime. High output efficiencies eta > 10(-4) in Li-like Ne at 98 angstrom are thus accessible in x-ray lasing plasmas of high density n(e) > 5 X 10(20) cm-3, low temperature T(e) < 40 eV, and large radius a > 20 mum, despite significant dispersion.