Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session BI3: The National Ignition Campaign |
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Chair: Karl Krushelnick, University of Michigan Room: Ballroom AC |
Monday, November 14, 2011 9:30AM - 10:00AM |
BI3.00001: Cryogenic thermonuclear fuel implosions on the National Ignition Facility Invited Speaker: The first inertial confinement fusion implosion experiments with cryogenic fuel layers have been fielded in preparation for ignition experiments on the National Ignition Facility. These experiments use mega joule laser energies that compress fusion capsules in indirect dive hohlraums to test initial hot spot formation and thermonuclear fuel assembly. Hydrogen-rich fuel (THD) provides a relatively low yield and diagnostics rich environment that allows us to measure the implosion core, neutron yield, temperatures and fuel areal density from a suite of x-ray and neutron diagnostics. These experiments have successfully demonstrated the control of the implosion shape using ignition grade cryogenic fuel layers, laser pulse shaping, and nonlinear plasma optics. The implosions show scaling of the DT fusion yield with ion temperature over more than one order of magnitude to a yield in excess of 10$^{14}$ neutrons. Recent implosion performance improvements due to shock timing tuning have led to high Lawson confinement parameters. Additional tuning experiments are being performed with the goal to increase hot spot temperatures and to observe alpha particle heating with pure DT fuel. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, November 14, 2011 10:00AM - 10:30AM |
BI3.00002: The Velocity Campaign for Ignition on NIF Invited Speaker: Achieving ignition requires a high velocity implosion since the energy required for ignition scales like 1/v$^8$. Beyond ignition, a higher velocity produces more robust performance, which will be useful for applications of ignition. In the velocity campaign, we will explore three methods for increasing implosion velocity: increased laser power and energy, optimized hohlraum and capsule materials, and optimized capsule thickness. The main issue with increasing the laser power and energy is the way in which LPI (laser plasma interactions) and hot electron preheat will change as we increase the laser power. Based on scalings from previous data and theory, we expect to couple 80-85\% of 1.5 MJ at 475-500 TW. We can also increase the velocity by optimizing the hohlraum and capsule materials. In this campaign, we will explore depleted uranium hohlraums to reduce wall loss and optimize the capsule dopant by replacing the germanium dopant with silicon. Those two changes are expected to increase velocity by 6-7\%. Finally, we will optimize the capsule thickness. The optimal capsule thickness is a trade-off between velocity and mix. A thinner capsule has higher velocity, but is more susceptible to mix of the ablator material into the hotspot due to hydrodynamic instabilities seeded by ablation surface imperfections. Once we have achieved adequate capsule areal density, we will optimize the velocity/mix trade off by varying the capsule thickness. We will also make direct measure of Rayleigh-Taylor instability growth by backlighting the growth of engineered features on the surface of the capsule. This will allow us to benchmark our models of mix. In this paper, we will describe the designs and experimental results of the velocity campaign. [Preview Abstract] |
Monday, November 14, 2011 10:30AM - 11:00AM |
BI3.00003: NIF Convergent Ablator Performance Measurements Invited Speaker: Measuring ablator performance around the time of peak implosion velocity is critical to assessing the progress towards ignition in inertial confinement fusion capsules. A back-lit radiography technique has been implemented on the National Ignition Facility to determine a suite of time-resolved ablator parameters from a series of gated x-ray images. Measurements on implosions driven by laser energies up to 1.4 MJ indicate that the velocity of the Ge-doped CH ablator is lower and the remaining mass is comparable to or higher than that given by integrated 2D simulations. The lower velocities and delayed radius versus time ablator trajectories are consistent with the later-than-expected x-ray peak emission times seen on these and other shots. Distinctly improved performance is observed when Ge was replaced with Si as the CH dopant. This work was performed in collaboration with the National Ignition Campaign Team. The work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, November 14, 2011 11:00AM - 11:30AM |
BI3.00004: Precision Shock Timing Measurements to set the Fuel Adiabat in Ignition Implosions Invited Speaker: An experimental campaign to tune the initial shock compression sequence of capsule implosions on the National Ignition Facility (NIF) was initiated in late 2010. The experiments use a NIF ignition-scale hohlraum and capsule that employs a re-entrant cone to provide optical access to the shocks as they propagate in the liquid deuterium-filled capsule interior. The strength and timing of the shock sequence is diagnosed with velocity interferometry that provides target performance data used to set the pulse shape for ignition capsule implosions that follow. From the start, these measurements yielded significant new information on target performance, leading to improvements both in the target design and in the physics packages in the radiation-hydrodynamic codes used to design and model these targets. We can set an accurately tuned pulse shape within a series of approximately 5 shots. The results and interpretation of these tuning experiments will be described. In collaboration with: T.R. Boehly, H.F. Robey, J.L. Kline, D.R. Farley, S. Le Pape, J.D. Moody, R.E. Olson, D.H. Munro, J.L. Milovich, P.A. Sterne, O.S. Jones, D.A. Callahan, A. Nikroo, J.J. Kroll, J.B. Horner, A.V. Hamza, S.D. Bhandarkar, J.H. Eggert, R.F. Smith, D.G. Hicks, H.-S Park, B.K. Young, W.W. Hsing, G.W. Collins, O.L. Landen and the NIC team. [Preview Abstract] |
Monday, November 14, 2011 11:30AM - 12:00PM |
BI3.00005: Hot-Spot Mix in Ignition-Scale Implosions at the National Ignition Facility Invited Speaker: Ignition of an inertial confinement fusion target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume.\footnote{ S. W. Haan\textit{ et al.}, Phys. Plasmas \textbf{18}, 051001 (2011).} The ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive.\footnote{ D. S. Clark\textit{ et al.}, Phys. Plasmas \textbf{17}, 052703 (2010).} Richtmyer--Meshkov and Rayleigh--Taylor hydrodynamic instabilities seeded by high-mode (50 $< \quad \lambda \quad <$ 200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase.\footnote{ B. A. Hammel\textit{ et al.}, Phys. Plasmas \textbf{18}, 056310 (2011).} As the shell decelerates, it compresses the fuel vapor, forming a hot spot. K-shell line emission from the ionized Ge that has penetrated into the hot spot provides an experimental signature of hot-spot mix. The Ge emission from tritium--hydrogen--deuterium (THD) and DT cryogenic targets and gas-filled plastic-shell capsules, which replace the THD layer with a mass-equivalent CH layer, was examined. The amount of hot-spot mix mass, estimated from the Ge K-shell line brightness using a detailed atomic physics code,\footnote{J. J. MacFarlane\textit{ et al.}, High Energy Density Phys. \textbf{3}, 181 (2007).} is typically below the 100-ng allowance for hot-spot mix.$^{1}$ Predictions of a simple mix model, based on linear growth of the measured surface-mass modulations, are consistent with the experimental results. The measured dependence of hot-spot mix on the implosion velocity and on the high-mode ablator-surface perturbations 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. In collaboration with the National Ignition Campaign Mix Working Group. [Preview Abstract] |
Monday, November 14, 2011 12:00PM - 12:30PM |
BI3.00006: The simulation basis for cryogenic layered implosion experiments on the National Ignition Facility Invited Speaker: The National Ignition Campaign (NIC) uses non-igniting \textquotedblleft{}THD\textquotedblright{} capsules to study and optimize the hydrodynamic assembly of the fuel without burn. These capsules are designed to simultaneously reduce DT neutron yield and to maintain hydrodynamic similarity with the DT ignition capsule. We will discuss nominal THD performance and the associated experimental observables. We will show the results of large ensembles of numerical simulations of THD and DT implosions and their simulated diagnostic outputs. These simulations cover a broad range of both nominal and off-nominal implosions. We will focus on the development of an experimental implosion performance metric called the experimental ignition threshold factor (ITFX). We will discuss the relationship between ITFX and other integrated performance metrics, including the ignition threshold factor (ITF), the generalized Lawson criterion (GLC; see Zhou and Betti, Physics of Plasmas, 15, 10, 2008), and the hot spot pressure (HSP). We will then consider the experimental results of the recent NIC THD campaign. We will show that we can observe the key quantities for producing a measured ITFX and for inferring the other performance metrics. We will discuss briefly the upcoming tuning campaign aimed at taking the next steps in performance improvement on the path to ignition on NIF. [Preview Abstract] |
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