Characterization of a 10Hz double-pulse non-normal incidence pumped transient collisional Ni-like molybdenum soft x-ray laser for applications

Stable and reliable operation demonstrated and studied with a 10 Hz Ti:sapphire laser system proves the suitability of the double-pulse non-normal incidence pumping geometry for table-top high repetition soft x-ray lasers (SXRL) on the way to applications. In this experiment a nickel-like molybdenum 4d1S0 − 4p1P1 transient collisional soft X-ray laser at 18.9 nm is investigated which is pumped in a double-pulse non-normal incidence geometry [1] at a 10Hz laser system facility [2]. The two pumping pulses are generated in the front-end of the Ti:sapphire laser system in a Mach-Zehnder type setup. The stretched pulse is distributed into the two arms with an adjustable ratio via the combination of a wave plate and a polarizing beam splitter. One of the arms incorporates a delay line adjustable between 0.8 and 2.6 nanoseconds. The two pulses are then amplified through the chain of the Ti:sapphire CPA system which includes a regenerative amplifier, a 4-pass pre-amplifier and two 4-pass cryogenic power amplifiers. After the compression adjustment two pulses of equal duration between 2 and 16 picoseconds are formed and then sent to the experiment chamber. The focussing system consisting of a flat 45 degree mirror and a spherical mirror with a focal length of 500 mm off the normal incidence produces a line focus of 3.8 mm length and 50μm width with an intrinsic traveling wave speed of 1.2 c. Both pulses hit the target at the same grazing incidence angle Φ of 19 degrees. For the pump laser wavelength of 805 nm the electron density at which the energy is absorbed amounts to ne,abs ≈ 2.3× 10 cm, following ne = nc · sin 2 Φ with the critical density nc. The line foci optimization is done in the infra-red with a high resolution imaging device (RILF). The applied diagnostic consists of a monochromatic near-field imaging system that shows the soft-x-ray laser source size and position relative to the target surface and a far-field imaging system to provide information about the divergence and the coherence of the soft x-ray laser beam. The near-field is imaged by a spherical multi-layer mirror and a flat multi-layer mirror onto on a 16-bit back-thinned CCD camera behind a 2μm Al filter. A typical near-field image of the source, with its limitation in resolution of the 13μm camera pixel size, is shown in the upper part of Fig. 1. The measured source size is 4μm × 12μm. The far-field beam profile is recorded by another 16-bit back-thinned CCD camera after a deflection by a flat multi-