Conversion efficiency of soft X-ray from 0.35 μm pulse laser-irradiated aluminium planar target at laser intensities 1013-1015W/cm2 on the Xingguang-Ⅱ facility (laser energy 5- 90 J, focal spot～φ 200μm full width of half maximun (FWHM) 400 -800 ps) was measured.A simple model was given to explain soft X-ray conversion efficiency.In this model, because of the heat conduction from the laser-heated spot, the conversion was very small at lower irradiance limit, while at higher limit it was bounded by the energy lost in blow off plasma.Consequently, at the laser intensity around 2 × 1014 W/cm2 , the X-ray conversion efficiency reaches a maximum.
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