Evidence of nonlinear processes from x-ray spectra of CO2 laser-irradiated targets

CH2, Al, and Pb targets have been irradiated at a maximum flux of 3×1013 W cm−2 with a focused CO2 laser in order to study the effects of nonlinear processes on x-ray emission. A multichannel x-ray spectrometer measured the x-ray spectrum from 1 to 50 keV. Above a critical flux of 6×1012 W cm−2, the results show a change in the slope of soft x-ray intensity versus laser flux, a change in the power-law dependence of hot temperature versus flux, an anisotropy of soft x-ray emission, and a strong production of energetic electrons. With increasing Z, there is no reduction of the effects of nonlinear phenomena, which appeared stronger with Al. The comparison with experiments performed at 1.06 μm suggests that the fraction of laser energy present in fast electrons scales as φλ2.