Multi-keV x-ray conversion efficiencies of laser-preexploded titanium foils

In the context of target design for multi-keV x-ray laser-produced experiments, the concept of exploding metallic thin foils by two laser pulses delayed in time has been tested at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al. , Phys. Plasma 3, 2108 (1996)]. The first laser pulse creates an underdense plasma ( n e ∕ n c ≈ 0.2 ) , and the second laser pulse heats the plasma plume which produces strong line emission from the titanium K shell ( He α at 4.7 keV and H α at 4.9 keV). Six OMEGA beams (500-ps duration) for the prepulse and nine beams (1-ns duration) for the heating pulse irradiate one side of the foil. Different experimental conditions have been investigated in order to optimize the conversion efficiency enhancement on titanium foils. The influences of the foil thicknesses (5 and 6 μm), the delays (3, 4, and 5 ns) between the laser pulses, and the laser intensities (1.3 and 2.2 × 10 15 W cm − 2 ) have been tested. The absolute output power was measured by a set of filtered x-ray diodes, giving conversion efficiencies (CEs) up to 3.6% in 2 π for energies above 4 keV with a preformed plasma, to be compared to the case without a prepulse where the CE is 1.5%. This double-pulse concept in this case shows an increase of CE by a factor of 2.4 for titanium thin foils. CE up to 4.9% has been reached with a laser intensity of 2.2 × 10 15 W cm − 2 .