X-ray lasing - The experiments

The Novette laser is used to produce a population inversion in an exploding foil plasma, and substantial amplification of spontaneous emission at soft-X-ray wavelengths is demonstrated. Targets consist of thin selenium or yttrium foils deposited on one side of a Formvar substrate. The on-axis emission spectrum was measured by a microchannel-plate grazing-incidence spectrometer (MCPIGS) and a transmission-grating streak spectrometer (TGSS). A second MCPIGS, placed at an angle of 77 degrees to the long axis, was used to confirm the anisotropy of the amplification. The amplification of at least four 3p to 3s transitions was observed, the largest gain being detected for the J = 2 to J = 1 lines at wavelengths of 20.9 nm (59 eV) and 20.6 nm (60 eV) in selenium and at 15.5 nm (80 eV) in yttrium. The optimum amplification occurred at a target thickness of 75 nm and a pulse length of 450 ps or longer. By varying the target lengths, the exponential growth with increasing length that characterizes stimulated emission was observed. It is estimated that lasing at 8 nm wavelengths is attainable with the present collision excitation scheme.