Multi-millijoule, deeply saturated x-ray laser at 21.2 nm for applications in plasma physics

We report on the development of a multi-millijoule, ultra high-brightness Ne-like soft x-ray laser (XRL) at the wavelength of 21.2 nm, recently undertaken at the PALS Centre. The device has been implemented as a routine radiation source delivering about 4 mJ of energy in pulses with a duration of ∼100 ps, in a narrowly collimated beam exhibiting high spatial quality and possessing high spatial coherence. The peak power of about 40 MW, generated by this device, is the largest value attained by an XRL to date. The same holds true for the peak spectral brightness, amounting to 10²⁷ photons s⁻¹ mm⁻² mrad⁻² 0.1%BW⁻¹. The active medium consists of a 3 cm long plasma column, sequentially generated from a slab target by an energy of ∼500 J delivered by the iodine 1.315 μm driving laser. The population inversion is produced by focusing tightly down to the line ∼500 J of energy into a much wider preplasma column, which makes it possible to create a lasing medium with a reduced lateral density gradient and thereby to minimize the lateral refraction of the x-ray beam. The multi-millijoule output is generated in double-pass amplification regime, achieved by a carefully designed half cavity using a flat multilayer Mo : Si mirror positioned a few millimetres near the plasma end. We describe the measured characteristics of the output beam generated by double-pass amplification, and illustrate its spatial beam coherence obtained using a Fresnel wavefront splitting interferometer. Given the available energy, peak power and brightness in a single pulse, the demonstrated XRL constitutes a tool for novel applications in both solid state and plasma physics. Examples of such applications are the generation of high-energy density, low-temperature plasmas relevant to laboratory astrophysics, x-ray interferometry of plasmas and solids with spatial resolution ranging from a micron to a few nanometres according to the specific experimental arrangement, or study of soft x-ray photoionization.