Dependence of soft x-ray conversion on atomic composition in laser-produced plasma of copper-gold mix-Z targets

In the indirect scheme of ICF, a high-Z ablator is used as the inside wall material of the hohlraum cavity. This choice is made because a high-Z plasma is an efficient x-ray convertor, and it offers a high opacity to radiation conduction into the cavity wall. Recently it had been shown that a mixture of two or more high-Z elements can provide a higher opacity than that of any single materials used, and optimum composition was predicted from computer simulations. It is expected that such a wall will provide maximum conversion of absorbed laser energy into soft x-ray radiation. In this paper, we present a comparative experimental study of soft x-ray emission from copper-gold mix-Z planar targets of different atomic compositions irradiated at a laser intensity of ~1013 W/cm2 (λ = 0.532μm). Spectral features of the soft x-ray radiation were recorded using a transmission grating spectrograph. Maximum enhancement in radiation intensity in the spectral region ~ 40 - 120 Å was observed for a mix-Z target of atomic composition of Cu 0.57 - Au 0.43. This is in broad agreement with theoretical results of opacity calculations performed for different atomic compositions using a screened hydrogenic average atom model.