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 8X10 Wcm 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 4d5d(S0)--4d5p(P1) transition at 41.8 nm in Xe, and on the 3d4d(S0)--3d4p(P1) transition in Kr at 32.8 nm. Under optimum pumping conditions the Xe laser provides about (5c2)X10 photons per pulse whilst the Kr laser delivers up to (2.5c1)X10 photons per shot. The repetition rate of these soft X-ray lasers is 10 Hz. The beam wavefront of the Xe laser has been measured by a Shack--Hartmann soft X-ray wavefront sensor, and the pulse duration by a cross-correlation technique.