Conversion of laser light into x rays in thin foil targets

Planar gold layers of thickness 50–200 nm, both free-standing and supported on substrates of 1-μm polypropylene, were irradiated with 3-ns laser pulses of 0.53 μm wavelength at ${10}^{12}$ and ${10}^{13}$ W/${\mathrm{cm}}^2$. The x-ray emission from the rear and front of the target was measured with symmetrically placed x-ray diodes and x-ray transmission grating film spectrometers. The transmitted laser light was also monitored. Under optimized conditions it is possible to obtain up to 30% conversion of the laser energy in the form of soft x rays from the rear surface of the target with less than 2% laser light transmission. The measured overall conversion efficiency is approximately in agreement with hydrodynamic simulations. The spectral and temporal features of the emission are discussed with respect to the opacity of the hot dense target material.