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 × 10 ¹⁷ 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 4d 5d(¹So) - 4d ⁹5p(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 × 10⁹ photons per pulse while the Kr IX laser delivers up to (2-3) x 10 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.