Session PO8: Compression and Burn II

Cryogenic Deuterium--Tritium Implosions on OMEGA

This talk summarizes the results on improving performance and the progress in theoretical understanding of cryogenic deuterium--tritium implosions on OMEGA. As shown recently,\footnote{I. V. Igumenshchev \textit{et al.,} Phys~. Plasmas \textbf{17}, 122708 (2010).} the cross-beam energy transfer (CBET) is one of the main factors limiting hydrodynamic efficiency of direct-drive implosions. To tune the CBET model used in hydro simulations, a series of warm plastic implosions was carried out on OMEGA. Based on the result of such a model, it was found that CBET contributes up to 15{\%} to the incident energy scattering and up to 20{\%} loss in hydro-efficiency in symmetric-drive implosions. Mitigation strategies for energy loss caused by CBET as well as recent improvements in target quality that resulted in significant improvement (by a factor of two) in target yields will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Cryogenic-DT-Implosion Performance with Improved Target-Surface Quality

High fuel areal densities (\textit{$\rho $R}) are routinely achieved in low-adiabat cryogenic deuterium--tritium (DT) implosions on the OMEGA laser.\footnote{ V. N. Goncharov\textit{ et al.}, Phys. Rev. Lett. \textbf{104}, 165001 (2010).} While these \textit{$\rho $R}'s agree with 1-D hydrocode predictions, the measured yields have been lower than predictions, even when cross-beam energy transfer is included. The yield deficit is believed to be caused by nonuniformities on the target surfaces and in the laser-drive symmetry.\footnote{S. X. Hu\textit{ et al.}, Phys. Plasmas \textbf{17}, 102706 (2010).} A significant source of outer-surface perturbations has been identified and partially mitigated-condensable gases that freeze out on the surface of the capsule. Mitigating this source led to a consistent 2$\times $ improvement in the measured primary yield. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Two-Dimensional Analysis of Crossed-Beam Energy Transfer (CBET) in Direct-Drive ICF Implosions

Crossed-beam energy transfer (CBET) causes pump and probe beams to exchange energy via stimulated Brillouin scattering.\footnote{W. L. Kruer, \textit{The Physics of Laser Plasma Interactions}, Frontiers in Physics (Westview Press, Boulder, CO, 2003), p. 45.} Experimental backscattered streaked spectra suggest that CBET can result in a significant energy loss. One-dimensional \textit{LILAC} simulations,\footnote{I. V. Igumenshchev\textit{ et al.}, Phys. Plasmas \textbf{17}, 122708 (2010).} modeling CBET, show that it occurs primarily in the central portion of the far-field spot, resulting in decreased absorption near the critical surface via energy transfer into the tails of outgoing crossing beams. CBET is incorporated into the 2-D hydrodynamics code \textit{DRACO}\footnote{P. B. Radha\textit{ et al.}, Phys. Plasmas \textbf{12}, 056307 (2005).} as an integral part of the full 3-D ray trace. CBET is treated self-consistently as a feedback on the hydrodynamic evolution. \textit{DRACO} simulation results employing CBET will be discussed and compared to experimental results. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Preheat Studies Using Low-Adiabat Plastic-Shell Implosions with Triple-Picket Pulses on OMEGA

The effect of laser--plasma interactions in the underdense coronal plasma on direct-drive target performance has been systematically studied on OMEGA. Room-temperature D$_{2}$-filled, 27-\textit{$\mu $}m-thick plastic shells were irradiated using triple-picket laser pulse shapes. The intensity on the main pulse is varied between 3.5 $\times $ 10$^{14}$ W/cm$^{2}$ and 1.1 $\times $ 10$^{15}$~W/cm$^{2}$, while picket energies are kept nominally the same to maintain similar shell adiabat in all designs. Time-resolved reflected light and its spectrum, neutron-rate histories, areal densities, ion temperatures, neutron yield, and time-resolved hard x-ray signals have been simultaneously measured on these implosions. At lower intensities below the two-plasmon-decay (TPD) threshold, only cross-beam transfer induced by laser--plasma interactions influences target performance, whereas both affect target performance at higher intensities. In particular, fast-electrons generated by TPD can potentially preheat the shell reducing compression at high intensities ($\sim $1 $\times $ 10$^{15}$ W/cm$^{2})$. This work was supported by the U.S. Department of Energy under Cooperative Agreement Nos. DE-FC02-04ER54789 and DE-FC52-08NA28302.   

Measurements of DD Neutron Yield and Ion Temperature in DT Implosions on OMEGA

Measurements of the DD neutron yield and ion temperature in inertial confinement fusion experiments with DT-filled targets will provide additional information on the state of the compressed fuel to further constrain numerical hydrocode simulation models. Requirements and designs of neutron detectors capable of measuring the DD yield and ion temperature in DT implosions and their performance on the OMEGA Laser System will be discussed. Comparisons of experimental data with numerical simulations for cryogenic-DT implosions, and room temperature CH or glass targets filled with DT gas will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Numerical Evaluation of Sub-tangential Focusing in OMEGA Target Implosions

Crossed beam energy transfer (CBET) in direct-drive implosions\footnote{ D. H. Edgell\textit{ et al.}, Bull. Am. Phys. Soc. \textbf{54}, 145 (2009).}$^{,}$\footnote{ I. V. Igumenshchev \textit{et al}., Phys. Plasmas \textbf{17}, 122708 (2010).} removes energy from incoming laser light, lowering the laser energy reaching the target, and reducing overall target performance. One mitigation strategy is the use of sub-tangential beam focusing to reduce the energy flowing around the target. This reduction decreases the seed energy for CBET. This focusing can lead to higher levels of long-wavelength illumination nonuniformity and can reduce the effective beam overlap that leads to decreased smoothing of single-beam, short-wavelength nonuniformities The effect of sub-tangential focusing is investigated for several OMEGA target designs, including cryogenic and room-temperature capsules. The results of 2-D \textit{DRACO} simulations evaluating the effects of various levels of sub-tangential focusing on target performance, including laser absorption, neutron yield, fuel areal density, and core asymmetry, will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

High-Convergence-Ratio Polar-Drive Experiments on OMEGA

Polar-drive experiments are being performed on OMEGA in preparation for future ignition attempts using the same method at the National Ignition Facility (NIF). This work presents results from multiple-picket laser pulses from 40 OMEGA beams, driving gas-filled shells approximately reproducing the illumination conditions on the NIF. The beams of OMEGA are re-aimed to compensate for the asymmetric beam distribution and in some cases the beam energy is adjusted. A set of such symmetry tests has been performed and diagnosed with x-ray backlighting, fusion yield, and reaction particle spectra from which the implosion symmetry, areal density, and core conditions are inferred. The ability to accurately simulate these experiments, as is done with the two-dimensional hydrodynamics code \textit{DRACO}, gives confidence to the ability to predict conditions in polar-drive experiments on the NIF. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

OMEGA Polar-Drive Target Designs

Polar-drive (PD) ignition designs for the National Ignition Facility (NIF) rely on obliquely repointing beams to the equator, different pulse shapes for different rings of the NIF configuration, specialized phase plates for the equatorial beams, and optionally, target shimming. PD OMEGA implosions and cone-in-shell geometry are used to validate models of laser deposition, heat conduction, and nonuniformity growth. Forty of the 60 OMEGA beams emulate the NIF configuration. Simulated shock timing is in good agreement with observations. Initial experiments on high-convergence triple-picket warm plastic (CH) shells indicate that good control over the $\lambda $ = 2 mode is achieved with existing OMEGA phase plates by only repointing beams. Target designs that additionally incorporate several of the NIF relevant input variables are presented. The goal is to improve OMEGA PD target performance and validate models of laser-energy deposition and heat conduction. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Proton Radiography of Polar-Drive Implosions on OMEGA

Low-adiabat, polar-drive-implosion experiments performed on OMEGA are diagnosed for the first time with high-energy protons generated by the OMEGA EP laser. Warm D$_{2}$-gas--filled CH capsules are imploded with shaped laser pulses, keeping the main fuel layer on a low adiabat. The high-energy protons are used to radiograph the targets, with the goal of providing time-resolved two-dimensional images of the converging shell. The inferred shell shape and symmetry will be compared to those determined by x-ray radiography and radiation hydrodynamics modeling. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Polar-Direct-Drive Defect Implosions at OMEGA inPreparation for Experiments at NIF

The Defect-Implosion (DIME) campaign involves compressing perturbed spherical capsules with polar direct drive (PDD). For direct-drive implosions at NIF, PDD will be used. We have done simulations and experiments at OMEGA to test our modeling capability for equatorial-plane defects in fusion capsules and for PDD at NIF. Since PDD is anisotropic, we show the results of 0$^{th}$ hydrodynamics of implosions and perturbation-driven features near stagnation. Later presentations discuss defect-induced mix and neutronics, and laser pointing for NIF experiments. Prototype OMEGA shots used 865-$\mu $m diameter CH shells filled with 5 atm of D$_{2}$. Machined channels 30-$\mu $m wide and up to 9-$\mu $m deep formed the defects.   

Analysis of 2011 Defect Imaging Capsule Implosions

Los Alamos is engaged in a project to design high neutron fluence feature-driven mix experiments for the National Ignition Facility in 2012. These results will be relevant for determining how much imperfection capsules can have in inertial fusion energy. To prepare for NIF, we fielded shots on the Omega laser in January and July 2011 with a 40 beam polar direct drive configuration similar to what we will employ on NIF. The capsules were 15 to 17 micron CH plastic shells about 880 microns in diameter filled with 5 atm of D$_{2}$ gas. We fielded capsules with different dopant layers and different depth equatorial grooves that were about 30 microns wide. We obtained radius versus time plots, radiographs, neutron yields, ion temperatures, burn widths, streak spectra, among other data. Preliminary calculations show that we match the radius versus time plots within about the data error and we have reasonable matches to the other data. We will will present these results and additional detailed comparisons of calculations to data. Work performed by Los Alamos National Laboratory under contract DE-AC52-06NA25396 for the National Nuclear Security Administration of the U.S. Department of Energy.   

Progress Toward Modeling Spectroscopic Signatures of Mix on Omega and NIF

Defect-induced mix processes may degrade the performance of ICF and ICF-like targets at Omega and NIF. An improved understanding of the relevant physics requires an experimental program built on a foundation of radiation-hydrodynamic simulations plus reliable synthetic diagnostic outputs. To that end, the Applications of Ignition (AoI) and Defect Implosion Experiment (DIME) efforts at LANL have focused on directly driven plastic capsules containing high-Z dopants and manufactured with an equatorial ``trench'' defect. One of the key diagnostic techniques for detecting and diagnosing the migration of dopant material into the hot core is Multi-Monochromatic X-ray Imaging (MMI). This talk will focus on recent efforts to model spectroscopic signatures of mix processes in AoI/DIME capsules via simulated MMI-type diagnostic instruments. It will also include data from recent Omega shots and calculations in support of Tier 1 experiments at NIF in FY2012.   

Streaked X Ray Spectra from Polar Direct Drive Capsules with an Equatorial Defect

In the Defect Implosion Experiment (DIME) on Omega, capsules with an equatorial ``trench'' defect have been imploded to study defect-induced mix processes. The capsules contain layers doped with titanium and/or vanadium, with doped layers in contact with the deuterium fill gas on some targets, and separated from the gas by a layer of undoped plastic in others. Streaked x-ray spectra from the capsule implosions provide information on conditions in the mix layer. Polar direct drive was utilized in preparation for experiments planned for the National Ignition Facility in 2012.   

Defect Induced Mix Experiments (DIME) for NIF

Los Alamos National Laboratory will be performing FY12 NIF experiments using polar direct drive to measure the effects of high mode number defects on ICF implosion hydrodynamics and yield. The effect of equatorial groove features will be assessed using both x-ray backlighting and spectrally resolved imaging of higher-Z dopant layers in 2.2 mm diameter (30 microns thick) CH capsules using a multiple monochromatic imager (MMI). By placing thin, 2 micron thick, layers containing $\sim $1.5{\%} of either Ge or Se at different depths in the capsule, we will be able to characterize the mixing and heating of these layers in both perturbed and unperturbed regions of the capsule. Precursor experiments have been performed on Omega to validate these measurement methods using Ti and V layers. An overview of our current results from Omega and design efforts for NIF will be presented. Work performed by Los Alamos National Laboratory under contract DE-AC52-06NA25396 for the National Nuclear Security Administration of the U.S. Department of Energy.   

Initial results from the Omega Asymmetric Burn Experiment -- ABEX

A new experiment has been designed to explore fusion burn degradation mechanisms in asymmetric laser-driven implosions. This presentation will present experimental design considerations, goals for the first series of tests, and a summary of the results from that series conducted at the Omega laser facility on April 5, 2011. The manufacturing technique will be summarized and issues to be addressed with several control capsules will be presented. Example data from time-integrated and time-resolved x-ray self-emission imaging along with time-resolved fusion burn rates and total neutron yield as a function of asymmetry will be presented. The scaling of measured yield degradation with calculated enstrophy derived from hydro-code simulations will be examined.