Comparison of high-density carbon implosions in unlined uranium versus gold hohlraums

In Inertial Confinement Fusion (ICF) implosions, laser energy is converted to x-ray radiation in hohlraums with High-Z walls. At radiation temperatures near 300 eV relevant for ICF experiments, the radiative losses in heating the wall are lower for~U than for Au hohlraums [1]. Furthermore, the intensity of the ``M-band'' x-rays with photon energies h$\nu $ \textgreater 1.8 keV is lower for uranium, allowing for reduced capsule dopant concentrations employed to minimize inner ablator preheat and hence keep favorable fuel/ablator interface Atwood numbers. This in turn improves the ablator rocket efficiency and reduces the risk of polluting the hot-spot with emissive dopant material. The first uranium vacuum hohlraum experiments on the National Ignition Facility (NIF) with undoped high-density carbon (HDC, or diamond) capsules have demonstrated 30 {\%} lower ``M-band'' intensity relative to Au, resulting in lower inflight ablator thickness due to reduced preheat. In addition, fusion neutron yields are 2x higher in U than in Au hohlraums for D$_{\mathrm{2}}$-gas filled capsule implosions at ICF relevant velocities of 380 $+$/-~20 km/s. These results have led the NIF ICF implosions to routinely employ U hohlraums. [1] J. Schein, O.S. Jones, M. Rosen, E. Dewald, S. Glenzer, et al, \textit{Phys. Rev. Lett. }\textbf{98}, 175003 (2007). Prepared by LLNL under Contract DE-AC52-07NA27344.