Characterization of collisionally pumped optical-field-ionization soft X-ray lasers

We give an overview of recent experimental 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 8×1017 Wcm-2 into a low-density gas cell containing Xe or Kr, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d95d(1 S 0)–4d95p(1 P 1) transition at 41.8 nm in Xe8+, and on the 3d94d(1 S 0)–3d94p(1 P 1) transition in Kr8+ at 32.8 nm. Under optimum pumping conditions the Xe8+ laser provides about (5±2)×109 photons per pulse whilst the Kr8+ laser delivers up to (2.5±1)×109 photons per shot. The repetition rate of these soft X-ray lasers is 10 Hz. The beam wavefront of the Xe8+ laser has been measured by a Shack–Hartmann soft X-ray wavefront sensor, and the pulse duration by a cross-correlation technique.