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We report two-dimensional near-field imaging experiments of the 11.9-nm Sn X-ray laser that were performed with a set of novel Mo/Y multilayer mirrors having reflectivities of up to ∼ 40% at normal and at 45 • incidence. Second-moment analysis of the X-ray laser emission was used to determine values of the X-ray beam propagation factor M 2 for a range of irradiation parameters. The results reveal a reduction of M 2 with increasing prepulse intensity. The spatial size of the output is a factor of ∼ 2 smaller than previously measured for the 14.7-nm Pd X-ray laser, while the distance of the X-ray emission with respect to the target surface remains roughly the same.PACS 42.55.Vc IntroductionSince the first demonstration of substantial X-ray laser gain in neon-like selenium [1], the electron-collisional excitation scheme has proven to be the most successful method on the route towards high output power and saturated gain. Saturated operation of soft-X-ray lasers is important because it assures the maximum stimulated-emission power extraction from a given volume of excited plasma. Clear evidence of saturation has been reported in the last few years by a number of authors for the 4 d → 4p, J = 0-1 transition in nickel-like elements at wavelengths between 14.7 and 7.3 nm [2][3][4]. The efficiency with which saturation is achieved in the Ni-like lasers has recently been improved by exploiting the prepulse and multiple-pulse techniques [5][6][7] and, on the other hand, by reducing the pump pulse duration to the picosecond range, combined with traveling-wave irradiation [8]. Reports on the properties of these lasers have included measurements of the output energy, pulse duration, near-and far-field intensity distributions, and divergence [9]. The recent development of novel Mo/Y multilayer X-ray mirrors [10] has opened up a new wavelength range from 7 to 12 nm for imaging experiments of soft X-rays not accessible so far. In previous experiments, we have demonstrated saturated gain on the 4 d → 4p, J = 0-1 transition in Ni-like Pd, Ag, and Sn at 14.7, 13.9, and 11.9 nm, respectively, with a pump irradiation scheme utilizing a 0.5% prepulse that irradiated flat-slab targets 5-6 ns before the main pulse. Both the prepulse and the main pulse had a duration of 100 ps. In this work we report a series of simultaneous near-and far-field imaging experiments for the Sn 11.9-nm X-ray laser using four different prepulse schemes: a 0.5% prepulse at −6 ns, a 2.8% prepulse at −2 ns, a 8% prepulse at −1 ns, and a 16% prepulse at −0.8 ns. Experimental setupThe experiments were performed using the 1054-nm Nd : glass laser system at the Institute for Applied Physics (IAP) of the University of Berne. The laser, having a final amplifier of 90-mm diameter, is capable of delivering up to 40 J at a pulse duration of 100 ps (FWHM). The output beam is focused to give a line focus of 2.5-cm length and approximately 100-µm width using a combination of a 500-mm focal length aplanatic doublet and a −1700-mm focal length cylindrical lens. Targ...

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