On the stability of the zinc X-ray laser beam quality using a half cavity

At the Prague Asterix Laser System Center (PALS) the Asterix laser, which delivers up to 700 J in 0.5 ns, is used as a pump source for X-ray laser experiments and applications. The prepulse technique, which is known to improve the neon-like X-ray laser at the J = 0 - 1 transition dramatically, was applied. Since Zn slab targets were used, the output wavelength was 21.2 nm. A prepulse having up to 20 J precedes the main pulse by 10 ns. The main beam and the prepulse beam are focused by two different optical systems separately and their foci are superimposed at the target surface. By implementing a half-cavity for double-pass amplification using a Mo/Si multilayer mirror - which can be used for 100 shots - the X-ray laser output was more than 10 times stronger than at the single pass in a 3 cm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse was at least 150 ps longer than the round trip time (about 260 ps) in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing about 4 mJ of energy, which was proved to be sufficient for applications. In this contribution, such X-ray laser features divergence in two dimensions, beam quality (symmetry), and pointing angle, are investigated over 110 shots. To characterize the stability of the X-ray laser, the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots of a one-day-series, these values are given; a statistical analysis carrying out a chi-squared test characterizes the Zn X-ray laser as a robust tool suitable for future applications.