Investigations of collisionally pumped optical field ionization soft-x-ray lasers

We report recent investigations of optical field ionization soft-x-ray lasers. We generated the amplifying medium by focusing a circularly polarized 760-mJ, 30-fs 10-Hz Ti:sapphire laser system into a gas cell a few millimeters long filled with xenon or krypton gas. A gain of 67 cm-1 on the 4d95p–4d95d transition at 41.8 nm in Pd-like xenon and a gain–length product of 15 have been inferred at saturation. This source delivers ∼5×109 photons per pulse. More recently we demonstrated lasing at 32.8 nm in Ni-like krypton. The influence of the pumping energy and the laser polarization on the lasing output as well as on the far-field pattern of the x-ray laser beam are reported and discussed.