Highly-Resolved Simulations of Omega Double-Shell Targets

Summary form only given. Double-shell (DS) targets are being actively pursued as a complementary approach to the cryogenic baseline design. This renewed interest is motivated by the simplicity of room temperature preparation, the potential for lower laser backscatter, and the reduced need for careful shock timing. In spite of these benefits, precise assembly of the hemispherical outer shells is required to meet the strict design criteria necessary for avoiding asymmetric implosions and the accompanying reduction in thermonuclear yield. Furthermore, smooth surface roughness is required to avoid large seeding of unwanted instabilities which are difficult to control in the absence of various stabilization. An experimental campaign at the Omega laser facility is underway to address the potential effect of these issues on the National Ignition Facility DS ignition design. Simultaneously, a computational approach with the objective of understanding the performance of the Omega DS and refining the design criteria is also being pursued. To date, this approach has identified and corrected a potentially damaging instability on the outer surface of the inner shell. This methodology is also being applied to studying imperfections that arise during the challenging fabrication and assembly process. This approach has allowed us to shed some light on the existing DS database as well as to predict the performance of future DS experiments. In this paper we will present results of simulations of two previous experimental campaigns to identify possible sources of neutron yield degradation. The first campaign used all-plastic DS targets which produced unprecedented compressional neutron yields up to 35% of the clean two-dimensional simulated yields, leaving an unexplained 65%. The other DS campaign used all-glass inner shells which gave consistently poor performance. Our simulations suggest that the major mechanism for yield reduction is imprinting of the outer-shell hemispherical join- onto the inner shell by the traversing shocks. Specifically, marginally prepared joints from misaligned or poorly bonded hemispherical outer shells are the major cause of asymmetric implosions and severely degraded neutron production. Finally, we present an improved glass inner-shell design for an upcoming Omega experimental campaign