Exploding-Foil Technique for Achieving a Soft X-Ray Laser

We describe a design for producing a soft x-ray laser via $3p\ensuremath{-}3s$ transitions in Ne-like selenium (wavelength of about 200 \AA{}). A 0.53-\ensuremath{\mu} m laser, focused in a 1.2\ifmmode\times\else\texttimes\fi{}0.02-cm spot to \ensuremath{\sim} 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$, heats and burns through a thin foil of Se. Besides ionizing the Se to a Ne-like state, the laser explodes the foil, creating a region of uniform electron density. This allows propagation of the x rays down the 1-cm-long gain direction without debilitating refraction. Gains of 4 to 10 ${\mathrm{cm}}^{\ensuremath{-}1}$ are predicted for various transitions.