Session QI3: NIF Indirect Drive

The Hohlraum Drive Campaign on the National Ignition Facility

The Hohlraum drive effort on the National Ignition Facility (NIF) laser has three primary goals: 1) improve hohlraum performance by improving laser beam propagation, reducing backscatter from laser plasma interactions (LPI), controlling x-ray and electron preheat, and modifying the x-ray drive spectrum; 2) improve understanding of crossbeam energy transfer physics to better evaluate this as a symmetry tuning method; and 3) improve modeling in order to find optimum designs. Our experimental strategy for improving performance explores the impact of significant changes to the hohlraum shape, wall material, gasfill composition, and gasfill density on integrated implosion experiments. We are investigating the performance of a rugby-shaped design that has a significantly larger diameter (7 mm) at the waist than our standard 5.75 mm diameter cylindrical-shaped hohlraum but maintains approximately the same wall area. We are also exploring changes to the gasfill composition in cylindrical hohlraums by using neopentane at room temperature to compare with our standard helium gasfill. In addition, we are also investigating higher He gasfill density (1.6 mg/cc vs nominal 0.96 mg/cc) and increased x-ray drive very early in the pulse. Besides these integrated experiments, our strategy includes experiments testing separate aspects of the hohlraum physics. These include time-resolved and time-integrated measurements of cross-beam transfer rates and laser-beam spatial power distribution at early and late times using modified targets. Non-local thermal equilibrium modeling and heat transport relevant to ignition experiments are being studied using sphere targets on the Omega laser system. These simpler targets provide benchmarks for improving our modeling tools. This talk will summarize the results of the Hohlraum Drive campaign and discuss future directions.   

Dynamic Symmetry of Indirectly Driven ICF Capsules on NIF

In order to achieve ignition it is important to control the growth of low-mode asymmetries as the capsule is compressed. Understanding the time-dependent evolution of the shape of the imploding capsule, hot spot and surrounding fuel layer is crucial to optimizing implosion performance. A design and experimental campaign to examine the sources of asymmetry and to measure the symmetry throughout the implosion has been developed and executed on the NIF. For the first time on NIF, two-dimensional radiographs of the capsule during its implosion phase have been measured to infer the symmetry of the radiation drive [1]. Time dependent equatorial symmetry has been measured of gas-filled capsules and capsules with cryogenic DT layers. These measurements have been used to modify the hohlraum geometry and the wavelength tuning to improve the inflight implosion symmetry. The technique is being extended to study azimuthal symmetry by imaging along the hohlraum axis. We have also expanded our shock timing measurements [2] by the addition of extra mirrors inside the re-entrant cone to allow the simultaneous measurement of shock symmetry in three locations on a single shot, providing a measurement of asymmetries up to mode 4 in both the equatorial and azimuthal planes. The shape of the hot spot during final stagnation is measured using time-resolved imaging of the self-emission, and information on the shape of the fuel at stagnation can be obtained from Compton radiography [3] using a wire-backlighter. In addition to x-ray diagnostics, a series of neutron and proton measurements of the low-mode areal density of the fuel at peak compression and at shock-flash time have been made. This talk will discuss the new imaging techniques, the results, and the analysis of the experiments done to date and their implication for ignition on NIF. The sensitivity of the in-flight and final implosion symmetry to imposed changes will be presented and compared to model predictions. \\[4pt] [1] R. Rygg, et al, to be submitted to \textit{Phys. Rev. Lett.}\\[0pt] [2] H. F. Robey et al,~\textit{Phys. Rev. Lett.}~\textbf{108}, 215004 (2012)\\[0pt] [3] R. Tommasini, et al, \textit{Phys. Plasmas}, \textbf{18}, 056309 (2011)   

Hydrodynamic instability and mix experiments at National Ignition Facility

Hydrodynamic growth and its effects on implosion performance and mix are being studied in hohlraum-driven implosions using gas-filled plastic shells at the National Ignition Facility (NIF). These experiments are motivated by observed elevated amounts of plastic mixed into the hot spot, degrading the performance of high-compression cryogenic DT layered implosions on NIF. Spherical shells with pre-imposed 2D modulations are being developed to measure Rayleigh-Taylor (RT) instability growth in the acceleration phase of implosions using in-flight x-ray radiography. Ablation-front RT growth measurements will be carried out for mode numbers ranging from 30 to 80 at drive conditions relevant to high-compression cryogenic implosions. In addition, implosion performance and mix are being studied at peak compression using plastic ``Symcap'' shells filled with tritium gas and imbedding localized CD diagnostic layer in various locations in the ablator. Neutron yield and ion temperature of the DT fusion reactions are used as a measure of shell-gas mix, while neutron yield of the TT fusion reaction is used as a measure of implosion performance. Experimental results and comparisons with 1D and 2D simulations, including mix models, will be presented.\\[4pt] This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

The High-Foot Implosion Campaign on the National Ignition Facility

The 'High-Foot' platform manipulates the laser pulse-shape coming from the National Ignition Facility (NIF) laser to create an indirect drive 3-shock implosion that is significantly more robust against instability growth involving the ablator and also modestly reduces implosion convergence ratio. This tactic gives up on theoretical high-gain in an inertial confinement fusion implosion in order to obtain better control of the implosion and bring experimental performance in-line with calculated performance, yet keeps the absolute capsule performance relatively high. This approach is generally consistent with the philosophy laid out in a recent international workshop on the topic of ignition science on NIF [``Workshop on the Science of Fusion Ignition on NIF,'' \textit{Lawrence Livermore National Laboratory} Report, LLNL-TR-570412 (2012). \textit{Op cit.} V. Gocharov and O.A. Hurricane, ``Panel 3 Report: Implosion Hydrodynamics,'' LLNL-TR-562104 (2012)]. Side benefits our the High-Foot pulse-shape modification appear to be improvements in hohlraum behavior---less wall motion achieved through higher pressure He gas fill and improved inner cone laser beam propagation. Another consequence of the `High-Foot' is a higher fuel adiabat, so there is some relation to direct-drive experiments performed at the Laboratory for Laser Energetics (LLE) [V. Goncharov, et al. APS-DPP (2012)]. In this talk, we will cover the various experimental and theoretical motivations for the High-Foot drive as well as cover the experimental results that have come out of the High-Foot experimental campaign. Most notably, at the time of this writing record DT layer implosion performance with record low levels of inferred mix and excellent agreement with one-dimensional implosion models without the aid of mix models.