Intense water-window soft x-ray emission by spectral control using dual laser pulses.

We demonstrate intense emission in the water-window soft x-ray spectral region by controlling the spectral behavior through changing the balance between emissivity and self-absorption in an expanding plasma. The number of photons obtained from a dual laser irradiated target with a 150-ps pre-pulse was maximized at 3.8 × 1014 photons/sr in λ = 2.34 - 4.38 nm at a pulse separation time of 7 - 10 ns. Enhancement of the number of photons is attributed to efficient coupling with the main laser pulse while maintaining a tiny source size.

Selection of target elements for laser-produced plasma soft x-ray sources.

We demonstrated the upper limitation to the number of shots, i.e., target lifetime, together with the number of photons emitted in the water-window soft x-ray spectral region from a number of targets used as sources in this spectral region, for multi-shot irradiation at the same position on the target surface. The spectra involved result from unresolved transition arrays originating from n=3-n=4 transitions in medium-Z element plasmas and from n=4-n=4 transitions originating in high-Z plasmas. The output flux was maintained for the highest number of shots in the case of the high melting point element molybdenum, and the total output in the water window was 7.7×1013 photons/sr at a laser power density of 1.2×1014 W/cm2.