Experimental evaluation of the x-ray preheat of fast-ignition cones

In planned fast-ignition implosions, a gold cone will be inserted into the capsule to allow a short-pulse laser to directly irradiate the compressed fuel without having to propagate through the ablated shell plasma [M. Tabak et al., Fus. Science and Technol., 49, 254 (2006)]. For the case of indirect-drive implosions, L-band line emission from the gold hohlraum wall (8$\sim $13 keV) can penetrate though the shell and heat the outer 2-6 $\mu $m of the cone, causing the gold to expand and mix with the fuel. Since mixing of high-$Z$ material with the fuel reduces the margin for achieving ignition, it is important to quantify the L-shell emission from Au hohlraums and to evaluate the effects on the Au cone. We measured the absolute x-ray flux from thin-wall (5 $\mu $m thick) gold hohlraums at the OMEGA laser facility, and we have observed the expansion of a surrogate gold surface with time-gated radiography. Results from these experiments will be discussed.