Multimillijoule, highly coherent x-ray laser at 21 nm operating in deep saturation through double-pass amplification

The deeply saturated operation of a double-pass Ne-like zinc soft x-ray laser emitting at a wavelength of 21.2 mm is reported, along with results of a study and optimization of its beam parameters and coherence. The active medium is a 3-cm-long plasma column produced by a separately delivered prepulse and a main pulse with a duration of {~}450 ps, supplied by a 1.315-{μ}m laser. To generate plasma with reduced lateral density gradients, the {~}130-{μ}m-wide main pulse focus, producing a pump irradiance of ~2.8×10¹³Wcm^-2, is placed on top of a much broader prepulse focus. The x-ray laser emission emerges as a narrowly collimated beam possessing high spatial quality and coherence. The dependence of the output on the prepulse conditions, the pump power, and the setup of the half-cavity mirror are investigated and discussed. The study of the beam transverse coherence demonstrates its strong dependence on pump energy. The large gain-length product attained through half-cavity operation allows for an efficient extraction of the energy stored in the active medium. With a small-signal gain of 7(±{}0.5) cm^-1, the double-pass beam is deeply saturated and provides {~}4 mJ of energy in one pulse, corresponding to peak power in excess of 40 MW, and implies that this system is the most powerful x-ray laser yet demonstrated. The inherent engineering conception of the device is compatible with a repetition rate in a hertz domain.