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 \ensuremath{\eta}g${10}^{\mathrm{\ensuremath{-}}4}$ in Li-like Ne at 98 \AA{} are thus accessible in x-ray lasing plasmas of high density ${\mathit{n}}_{\mathit{e}}$g5\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, low temperature ${\mathit{T}}_{\mathit{e}}$40 eV, and large radius ag20 \ensuremath{\mu}m, despite significant dispersion.