Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session NI2: ICF Implosions |
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Chair: Mordecai Rosen, Lawrence Livermore National Laboratory Room: Ballroom DE |
Wednesday, October 31, 2012 9:30AM - 10:00AM |
NI2.00001: Inertial confinement fusion implosions at 500 TW laser drive on NIF Invited Speaker: John Kline The scaling of nuclear performance with increasing laser power and hohlraum drive has been measured in high-velocity implosions of inertial confinement fusion capsules filled with equimolar deuterium-tritium fuel. These experiments use the highest laser powers employed to date to demonstrate signatures of alpha heating and significant 14.1 MeV fusion neutron yield. Besides laser power, the experiments further scale the capsule ablator thickness and hohlraum wall material. Specifically, the first depleted uranium hohlraums for layered implosion experiments have shown an unambiguous improvement in capsule drive, which is equivalent to 25 TW over gold hohlraums at laser peak powers of 320 TW. To assess proximity to the ignition regime, we analyze measurements of the primary DT yield and the ratio of down scattered neutrons that are observed at energies of 10-12 MeV and which are a signature of the areal density of the implosion. We report on DT areal densities of 1.2 g cm$^{-2}$ exceeding 90{\%} of the required value for fully tuned ignition implosions. This achievement is the result of recent hohlraum and capsule tuning experiments by fielding long-drive laser pulses that avoid coasting of the implosion. The data indicate pressures of more than 100 Gbar. Comparisons with radiation-hydrodynamic simulations indicate that this value is currently within a factor of three required for reaching the ignition regime and that further improvements in implosion performance can be achieved at higher power drive. [Preview Abstract] |
Wednesday, October 31, 2012 10:00AM - 10:30AM |
NI2.00002: Improving Cryogenic-DT Implosion Performance on OMEGA Invited Speaker: T.C. Sangster Although cryogenic-DT implosion performance has improved both in absolute terms and relative to hydro simulations, a number of long-standing discrepancies remain unresolved. Absolute yield performance increased with higher-quality capsule and ice surfaces, routine delivery of low-adiabat ($\alpha$ $\sim $ 2) laser pulses at specification, and more-accurate target alignment with respect to the beam pointing (typically less than 10-$\mu$m rms for all 60 beams). Higher implosion velocities using thinner ice and constant mass ablators have resulted in additional increases in measured yields and ion temperatures. However, ion temperatures remain systematically below the hydro predictions suggesting higher-than-predicted imprint levels (note that $T_{i} \sim T_{e}$ for all implosions except for cases where fuel motion artificially enhances $T_{i})$. Imprint reduction is being addressed using dopants (small at.{\%} of silicon) in the outer part of the ablator. To preserve the ablator mass, doped shells are necessarily thinner than undoped shells and recent compression results show a clear inverse relation between the inferred areal density and the measured yields. This suggests more radiative preheat with the thinner ablators (the areal densities are about 70{\%} of predictions---below what is expected based on burn truncation). While improved nonlocal thermal transport and cross-beam energy transfer models resolved a persistent discrepancy between predicted and measured bang times, the measured burn width is longer than predicted. Furthermore, core x-ray emission below 2.5 keV is consistently higher than predictions. These discrepancies, combined with improved modeling, implicate shell stability and suggest that thicker ablators and thinner ice (to preserve the overall payload mass) may lead to improved ignition hydro equivalency. This talk will show the latest experimental results using thicker ablators and ablators doped with silicon, and compare these results with the latest hydro simulations.\\[4pt] 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 V. N. Goncharov, R. Betti, T. R. Boehly, R. Epstein, C. Forrest, V. Yu. Glebov, S. X. Hu, I. V. Igumenshchev, D. H. Froula, R. L. McCrory, D. D. Meyerhofer, P. B. Radha, W. Seka, W. T. Shmayda, S. Skupsky, C. Stoeckl (Laboratory for Laser Energetics, U. of Rochester), J. A. Frenje, D. T. Casey, and M. Gatu-Johnson (Plasma Science and Fusion Center, MIT). [Preview Abstract] |
Wednesday, October 31, 2012 10:30AM - 11:00AM |
NI2.00003: Mix mitigation experiments on cryogenic DT layered implosions on the NIF Invited Speaker: Tilo Doeppner Inertial confinement fusion implosion experiments compress a deuterium-tritium (DT) ice fuel layer inside a low-Z ablator capsule shell, forming a central hot spot inside a high areal density DT layer. One possible obstacle to achieving ignition is mixing of ablator material into the hot spot, leading to radiative and conductive losses, cooling the hot spot, and extinguishing the incipient burn. Mix results from hydrodynamic instabilities seeded by perturbations at the outer ablator surface, the ablator-ice interface, and the inner fuel surface. We will report on a series of cryogenic DT layered implosion experiments that studied the effects of the capsule ablator thickness and silicon doping levels on the appearance of ablator mix into the hot spot, and its impact on implosion yield, temperature, and fuel areal density, as a function of the applied drive history. We use Ge dopant K-shell spectroscopy and Ross-pair imaging of the self emission to quantify the mix mass in the hot spot. We observe significant mixing when the remaining ablator mass drops below $\sim$ 0.25 mg for ablators with large preheat shielding, which is evidence that the observed mix is mainly due to instabilities seeded by ablation front surface imperfections, consistent with simulations. In the presence of significant mix we observe a distinct drop of hot spot ion temperature ($<$ 2 keV), a 2-4 fold neutron yield decrease, and increased hot spot x-ray yield. In order to understand and minimize mixing, we are varying the capsule ablator thickness and DT ice layer thickness, varying the ablator dopant concentration and distribution, and varying the laser pulse shape and power. An extensive series of DT layered implosion experiments will be presented, and the effect of ablator mix into the hot spot described. \\[4pt] This work was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, October 31, 2012 11:00AM - 11:30AM |
NI2.00004: Inflight Properties of NIF Ignition Capsules Inferred from Convergent Ablator Experiments Invited Speaker: Nathan Meezan Convergent ablator (ConA) experiments on the National Ignition Facility (NIF) are indirect drive implosions that study the inflight dynamics of an imploding capsule. Side-on, back-lit radiography provides data used by the National Ignition Campaign to infer time-dependent properties of the capsule ablator, including its center of mass radius, velocity, unablated mass, shell thickness, and peak density. Previously, Callahan\footnote{D. A. Callahan \textit{et al.}, Phys. Plasmas \textbf{19}, 056305 (2012)} and Hicks reported ConA experiments demonstrating velocities approaching those required for ignition. Here, we present the findings from a full year of NIF ConA experiments where we have shot more than 20 targets at energies greater than 1 MJ to study the inflight dynamics of ignition-like implosions. These include: \begin{itemize} \item Studies of ablator center of mass motion vs. time, suggesting that the drive history differs substantially from that predicted by standard modeling \item Pulse shape scalings studying the dynamics of a ``fifth shock'' that can significantly increase the entropy of the DT fuel in an ignition implosion. \item Performance of different ablators, including CH ablators with graded Si doping, CH ablators with uniform Si doping, and other ablators. \item ConA experiments using capsules with cryogenic ice layers, demonstrating that gas-filled capsules are adequate surrogates for DT layered implosions. \item Studies of thicker capsules shot at powers and energies surpassing 500 TW and 1.8 MJ as we work to meet the ignition implosion velocity requirement in the presence of hydrodynamic instabilities. \end{itemize} Finally, we describe insights into hydrodynamic instabilities that we have gained through this large database, from variations in capsule performance (neutron yield and T$_{\textrm{ion}}$) as well as from the impact of mix on observed late-time ablator properties. [Preview Abstract] |
Wednesday, October 31, 2012 11:30AM - 12:00PM |
NI2.00005: X-ray Thomson scattering measurements of temperature and density from multi-shocked CH capsules Invited Speaker: Luke Fletcher To achieve the high level of compression, at low entropy, needed for inertial confinement fusion currently requires the use of multiple and precisely timed shock waves [1]. While x-ray Thomson scattering has previously been applied to isochorically heated matter in many planar shocked systems [2], we have performed proof-of-principle measurements of the electron densities, temperatures, and ionization states of spherically compressed multi-shocked CH capsules through the use of spectrally resolved x-ray Thomson scattering. A total of 13.5 kJ incident on a CH shell (45 beams at the Omega laser system), are used to compress a 70 micron thick CH shell above solid-mass density using three coalescing shocks. Separately, a laser-produced Zinc He alpha x-ray source at 9 keV delayed 200 ps - 800 ps in time after maximum compression is used to probe the plasma under a non-collective scattering geometry. The data show high compression of less than 8 g/cc consistent with radiation-hydrodynamic simulations that use adequate coalescence of the three shocks. These results are compared with independent experiments in CH that use counter-propagating shocks or highly compressed implosions. We show that x-ray Thomson scattering allows probing extreme states of Warm Dense Matter and enables a complete description of the time-dependent hydrodynamic evolution of shock-compressed CH.\\[4pt] [1] S. W. Haan et al., Nucl. Fusion 44, S171 (2004).\\[0pt] [2] S. H. Glenzer et al., Rev. Mod. Phys. 81, 1625 (2009). [Preview Abstract] |
Wednesday, October 31, 2012 12:00PM - 12:30PM |
NI2.00006: Polar-Drive--Implosion Physics on OMEGA and the NIF Invited Speaker: P.B. Radha Polar drive (PD) permits the execution of direct-drive--ignition experiments on facilities that are configured for x-ray drive such as the National Ignition Facility (NIF) and Laser M\'{e}gajoule. Experiments on the OMEGA laser are used to develop and validate models of PD implosions. Results from OMEGA PD shock-timing and warm implosions are presented. Experiments are simulated with the 2-D hydrodynamic code \textit{DRACO} including full 3-D ray trace to model oblique beams. Excellent agreement is obtained in shock velocity and catch-up in PD geometry in warm, plastic shells. Predicted areal densities are measured in PD implosion experiments. Good agreement between simulation and experiments is obtained in the overall shape of the compressing shell when observed through x-ray backlighting. Simulated images of the hot core, including the effect of magnetic fields, are compared with experiments. Comparisons of simulated and observed scattered light and bang time in PD geometry are presented. Several techniques to increase implosion velocity are presented including beam profile variations and different ablator materials. Results from shimmed-target PD experiments will also be presented. Designs for future PD OMEGA experiments at ignition-relevant intensities will be presented. The implication of these results for NIF-scale plasmas is discussed. Experiments for the NIF in its current configuration, with indirect-drive phase plates, are proposed to study implosion energetics and shell asymmetries. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
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