Simultaneous near-field and far-field imaging of the 11.9-nm Ni-like Sn soft-X-ray laser

We report on two-dimensional near-field imaging experiments of the 11.9-nm Sn x-ray laser that were performed with a set of Mo/Y multilayer mirrors having reflectivities of up to {approx}45% at normal and at 45{sup o} incidence. Second-moment analysis of the x-ray laser emission was used to determine values of the x-ray beam propagation factor M{sup 2} for a range of irradiation parameters. The results reveal a reduction of M{sup 2} with increasing prepulse amplitude. The spatial size of the output is a factor of {approx}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.