Advances in collisionally pumped optical-field-ionization soft x-ray lasers

We give an overview of recent results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8 x 1017 Wcm-2 into a low-density gas cell containing xenon or krypton, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d95d(1S0) - 4d95p(1P1) transition at 41.8 nm in Pd-like Xe, and strong lasing at 32.8 nm on the 3d94d(1S0) - 3d94p(1P1) transition in Ni-like Kr has been demonstrated. Under optimum pumping conditions the Xe IX laser provides about 5 x 109 photons per pulse while the Kr IX laser delivers up to (2-3) x 109 photons per shot. The repetition rate of these X-ray lasers is 10 Hz. The experimental results are discussed and compared to computational simulations in which issues related to plasma formation, electron energy distribution, and atomic processes are addressed.