P2 Asymmetry of Hard X-Ray Flux in Laser Heated High-Z Cavities and Its Effects on ICF Ignition Capsules

X-ray drive asymmetry is one of the main seeds of low-mode implosion asymmetry that blocks further improvement of the nuclear performance of “high- foot” experiments on the National Ignition Facility
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. More particularly, the P2 asymmetry of Au's M-band flux can also severely in fluence the implosion performance of ignition capsules
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. Due to the higher opacity of Uranium than Au at temperatures ~300eV, U hohlraums are being more widely used to achieve high temperature X-ray drives at NIF. Its M-band flux, however, is much harder and more asymmetric than that of Au hohlraums. Here we study the P2 asymmetry of U's M-band flux and its effects on ICF ignition targets, and explore the methods of smoothing Au and /or U M-band flux P2 asymmetries by mid- and/or high-Z dopants in ablator. Our results show (1) mid- or high-Z dopants absorb hard x-rays (M-band flux) and re-emit isotropically, which helps to smooth the asymmetric M -band flux arriving at the ablation front, therefore reducing the P2 asymmetries of the imploding shell and hot spot; (2) the smoothing effect of Ge-dopant is more remarkable than Si-dopant because of its higher opacity in M-band than the latter; and (3) placing the doped layer at a larger radius in ablator is more efficient. Applying this effect may not be a main measure to reduce the low-mode implosion asymmetry, but might be of significance in some critical situations such as Inertial Confinement Fusion (ICF) experiments very near the performance cliffs of asymmetric x-ray drives.