Highly Directive 18.9 nm Nickel-like Molybdenum X-Ray Laser Operating at 150 mJ Pump Energy

We experimentally demonstrate that by longitudinally pumping 2 mm long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-x-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires a pump laser energy of 150 mJ.

Figure 1

Diagram of the experimental setup. The symbols shown in the figure correspond to spherical lens (SL), cylindrical lens (CL), molybdenum target (T), position that is spatially imaged onto the spectrometer focal plane (I), cylindrical gold-coated mirror (M), Hitachi flat-field grating (G), 0.65 μm thick Al foil (F), and photocathode camera (PC).

Figure 2

Intensity trace of the on-axis soft-x-ray spectrum between 15 and 25 nm. The background was subtracted from the trace to obtain the actual intensity profile.

Figure 3

Spectrum image of the on-axis emission for plasma lengths of (a) 2.0 mm and (b) 0.5 mm. The sensitivity of the photocathode camera was increased by about 8 times for (b).

Figure 4

Calculated spatial distribution of the 18.9 nm gain coefficient for main pump pulse with $1\times {10}^{-5}$ contrast ratio. The ordinate is the direction normal to the target surface, and the abscissa is the direction parallel to the x-ray laser axis. This figure shows the gain experienced by an 18.9 nm x-ray pulse that begins propagation at $z=0\mu \mathrm{m}$ at a time 1.0 ps after the main pulse.