Efficient soft X-ray lasing at 6 to 8 nm with nickel-like lanthanide ions.

We report the demonstration of efficient soft x-ray lasing in nickel-like lanthanide elements (Nd, Sm, Gd, Tb, and Dy) covering the spectral range between 6 and 8 nm. A curved slab target was pumped by 1.053- $\ensuremath{\mu}$m-wavelength multiple laser pulses: two or three 100-ps-duration pulses separated by 400 ps. A gain coefficient of 3.1 ${\mathrm{cm}}^{\ensuremath{-}1}$ and a gain-length product of 7.8 have been achieved at 7.97 nm in the Nd ions with 250 J pumping energy on a 2.5-cm-length target. The result represents close to an order of magnitude reduction in the energy required to pump these soft x-ray lasers.