Sub-10-nm wavelength Ni-like-ion collisional x-ray lasers

We report on recent progress achieved in x-ray laser research at the University of Bern. Using the existing 10-TW Nd:glass CPA (chirped-pulse amplification) laser system in the grazing-incidence pumping (GRIP) scheme, saturated x-ray lasing is demonstrated on the 4d → 4p, J = 0-1 line of Ba at a wavelength of 9.2 nm, using a main pulse energy of 9 J. A small-signal gain coefficient of ~30 cm-1 and a gain-length product of ~16 at saturation have been measured. Crucial to these results was the introduction of a second, relatively intense (~10%) prepulse less than ~100 ps before the 1.5-ps duration main pulse, in addition to the 2.8% prepulse that irradiated the target 2.4 ns earlier. Travelling-wave excitation was used throughout. For handling convenience, compound targets (BaF2, LaF3) were used, either in the form of windows or coated onto glass slides.