Session GO6: Compression and Burn II

Thermonuclear Ignition and the Onset of Propagating Burn in Inertial Fusion

Defining ignition in inertial confinement fusion (ICF) is an unresolved problem. In ICF, a distinction must be made between the ignition of the hot spot and the propagation of the burn wave in the surrounding dense fuel since most of the energy gain comes from burning the dense shell. In this work, we used a large 1-D simulation ensemble to show that it is possible to identify the onset of ignition through a unique value of the yield amplification defined as the ratio of the fusion yield including alpha-particle deposition to the fusion yield without alphas. Since the yield amplification is a function of the fractional alpha energy (total alpha energy/hot-spot energy, a measurable quantity), it appears possible not only to define ignition but also to measure the onset of ignition by the experimental inference of the fractional alpha energy and yield amplification. We also investigate the impacts of low- and mid-mode asymmetries on this ignition curve and how the definition of ignition is modified by these perturbations.    

Novel double-spike pulse shape for OMEGA cryogenic implosions

Models obtained through statistical analysis have demonstrated good predictive capability for the fusion yields of OMEGA cryogenic implosions. These models produced accurate correlations between the experimental fusion yields and several output variables from the 1-D radiation–hydrodynamic code LILAC. These statistical relations were used to design new implosion experiments, leading to tripling of the fusion yield on OMEGA. Here we describe how these statistical methods can be applied to predict other experimental observables including the areal density and hot-spot radius. These predictive correlations are then used to design new pulse shapes to increase the convergence ratio and the areal density for a new implosion campaign. Once an accurate predictive capability is established for both fusion yields and areal densities, it will be possible to design and field the optimum implosion compatible with the capabilities of the OMEGA laser.    

Three-dimensional modeling and hydrodynamic scaling of high density carbon ablator implosions on the National Ignition Facility

Recent implosion experiments using high density carbon ablators on the National Ignition Facility have achieved neutron yields exceeding 10¹⁶.  Three-dimensional, high-resolution modeling of these cryogenic implosions shows encouraging agreement with experiment in many respects.  This includes x-ray and neutron imaging data from the implosions, flange nuclear activation detector (fNAD) maps, and directional neutron spectral measurements, as well as scalar observables such as total neutron yield. This talk reviews the status of these high-resolution simulations, their comparison to experiment, and where discrepancies still remain.  Preliminary results using these simulations to assess hydrodynamic scaling to higher energy will also be discussed.   

Beryllium-ablator DT implosion performance at high case-to-capsule ratio on the National Ignition Facility

Using beryllium as an ablator material has several potential advantages for inertial fusion because of its low opacity and thus higher ablation rate. This could enable novel designs taking advantage of the reduced ablation-front growth rate or operating at lower radiation temperature. We report on a series of DT layered experiments using subscale beryllium implosions at high case-to-capsule ratio to isolate the implosion physics from hohlraum uncertainties. We find that the DT performance degrades substantially relative to 1-D clean predictions as the velocity increases by increased mass ablation. The performance trends are investigated with mix models, especially between the fuel and ablator.   

Capsule symmetry and hydrodynamic stability of high density carbon and plastic ablator implosion designs on the National Ignition Facility

Achieving areal density and temperature conditions necessary for hot spot ignition at the National Ignition Facility (NIF) requires careful control of capsule performance (implosion velocity, fuel adiabat) and hot spot shape. Capsules using high density carbon (HDC) and plastic ablators driven by adiabat-shaped X-ray profiles are designed to reach the alpha heating regime. A comparison of the relative stability of these designs was made using TROLL radiation  hydrodynamics simulations of single-mode perturbation linear growth at  the ablation front and fuel-ablator interface. Performing designs have to associate both hohlraum performance and capsule stability. Two-dimensional integrated simulations have been performed to evaluate how changes to the hohlraum (size, gas fill density, shape, beam balance) and to the laser pulse (picket, foot drive, pulse length) impact the HDC and plastic capsule performance and implosion symmetry.  The combination of all these effects is used to help guide future designs for NIF and the Laser Mega Joule facility.