Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session XI2: High Energy Density Physics and High Performance Capsule Implosions |
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Chair: Gail Glendinning, Lawrence Livermore National Laboratory Room: Centennial I |
Friday, November 6, 2009 9:30AM - 10:00AM |
XI2.00001: Compressing Magnetic Fields with High-Energy Lasers Invited Speaker: Laser-driven, magnetic-field compression producing a magnetic field of many tens of megagauss (MG) is reported for the first time. A shock wave formed during the implosion of a cylindrical target traps an initial magnetic field that is amplified via conservation of magnetic flux. Such large fields are expected to magnetize the electrons in the hot central plasma leading to a cyclotron frequency exceeding the collision frequency. The Omega Laser Facility was used to implode cylindrical CH targets filled with deuterium gas and seeded with an external field ($>$50 kG) from a magnetic pulse generator. This seed field is trapped and rapidly compressed by the imploding shell, minimizing the effect of resistive flux diffusion. The compressed field was predicted to achieve values of several tens of MG and was probed via proton deflectrometry using the 14.7-MeV protons from the D + $^{3}$He fusion reaction emitted by an imploding glass microballoon. Protons crossing the hot spot of the imploded cylindrical target undergo the largest deflection since the compressed field peaks within the hot spot. Line-averaged magnetic fields of the imploded core were measured to between 30 and 40 MG. Experimental data were analyzed with both an MHD version of the 1-D hydrocode \textit{LILAC} and the particle propagation code Geant4. This work supported by the U.S. DOE Office of FES under Grants DEFG02-04ER54768 and DE-FC02-ER54789 and by the Office of ICF under Cooperative Agreement No. DE-FC52-08NA28302. In collaboration with O.V. Gotchev, P.Y. Chang, R. Betti, D.D. Meyerhofer, O. Polomarov (LLE), F.H. S\'{e}guin, J.A. Frenje, C.K. Li, M.J.-E. Manuel, R.D. Petrasso (MIT), and J.R. Rygg (LLNL). [Preview Abstract] |
Friday, November 6, 2009 10:00AM - 10:30AM |
XI2.00002: Observations of subsonic and supersonic shear flows in laser driven high-energy-density plasmas Invited Speaker: Shear layers containing strong velocity gradients appear in many high-energy-density (HED) systems and play important roles in mixing and the transition to turbulence. Yet few laboratory experiments have been carried out to study their detailed evolution in this extreme environment where plasmas are compressible, actively ionizing, often involve strong shock waves and have complex material properties. Many shear flows produce the Kelvin-Helmholtz (KH) instability, which initiates the mixing at a fluid interface. We present results from two dedicated shear flow experiments that produced overall subsonic and supersonic flows using novel target designs. In the subsonic case, the Omega laser was used to drive a blast wave along a rippled interface between plastic and foam, shocking both the materials to produce two fluids separated by a sharp shear layer. The interface subsequently rolled-upped into large KH vortices that were accompanied by bubble-like structures of unknown origin. This was the first time the evolution of a well-resolved KH instability was observed in a HED plasma in the laboratory. We have analyzed the properties and dynamics of the plasma based on the data and fundamental models, without resorting to simulated values. In the second, supersonic experiment the Nike laser was used to drive a supersonic flow of Al plasma along a rippled, low-density foam surface. Here again the flowing plasma drove a shock into the second material, so that two fluids were separated by a shear layer. In contrast to the subsonic case, the flow developed shocks around the ripples in response to the supersonic flow of Al. Collaborators: R.P. Drake, O.A. Hurricane, J.F. Hansen, Y. Aglitskiy, T. Plewa, B.A. Remington, H.F. Robey, J.L. Weaver, A.L. Velikovich, R.S. Gillespie, M.J. Bono, M.J. Grosskopf, C.C. Kuranz, A. Visco. [Preview Abstract] |
Friday, November 6, 2009 10:30AM - 11:00AM |
XI2.00003: Measurement of fast electrons inside the dielectric material via observation of its Cherenkov radiation Invited Speaker: Direct measurement of extremely high energy density electrons created in ultra-intense laser plasma interactions is crucial issue for Fast Ignition. Cherenkov radiation is studied to obtain the energy distribution of electrons because the emission angle depends on the electron energy. However in the previous studies [F. Brandl et al. Europhys. Lett. 61, 632 (2003), M. Manclossi et al. Phys. Rev. Lett. 96, 125002 (2006)], the experimental configurations using a planer target raised issues of spatially overlapping among the light from the different energy electrons as well as the other emissions such as transition radiation. We developed a novel prism shaped target in which Cherenkov light emitted from different energy electrons are spatially separated, realizing an absolute measurement of the energy spectrum by counting the light intensities in each observed position. In the experiment we observed a clear expected horseshoe pattern indicating a portion of Cherenkov ring image. In addition, we found that many parts of electrons are considered to propagate along laser axis from the blur of the outer edge of the pattern. The calibrated energy spectrum agrees extremely well with a PIC calculation qualitatively. A large discrepancy between the magnetic electron spectrometer and the Cherenkov results infers the strength of the electrostatic sheath potential at the rear boundary which also well agrees with the calculation. [Preview Abstract] |
Friday, November 6, 2009 11:00AM - 11:30AM |
XI2.00004: High Performance Capsule Implosions on the Omega Laser Facility with Rugby Hohlraums Invited Speaker: Rugby-shaped hohlraums have been proposed as a method for x-ray drive enhancement for indirectly-driven capsule implosions [1]. This concept has recently been tested in a series of shots on the OMEGA laser facility at the Laboratory for Laser Energetics at the University of Rochester. In this talk, experimental results are presented comparing the performance of D$_{2}$-filled capsules between standard cylindrical Au hohlraums and rugby-shaped hohlraums. Not only did the rugby hohlraums demonstrate 18{\%} more x-ray drive energy as compared with the cylinders, but the high-performance design of these implosions (both cylinder and rugby) also provided $\approx $ 20X more DD neutrons than any previous indirectly-driven campaign on Omega (and $\approx $ 3X more than ever achieved on Nova implosions driven with nearly twice the laser energy). This increase in performance enables, for the first time, a measurement of the neutron burn history of an indirectly-driven implosion. Previous DD neutron yields had been too low to register this key measurement of capsule performance and the effects of dynamic mix. A wealth of additional data on the fuel areal density from the suite of charged particle diagnostics was obtained on a subset of the shots that used D$^{3}$He rather than D$_{2}$ fuel. Comparisons of the experimental results with numerical simulations are shown to be in excellent agreement. The design techniques employed in this campaign, e.g., smaller NIF-like laser entrance holes and hohlraum case-to-capsule ratios, provide added confidence in the pursuit of ignition on the National Ignition Facility. \\[4pt] [1] P. Amendt, C. Cerjan, D. E. Hinkel, J. L. Milovich, H.-S. Park, and H. F. Robey, ``Rugby-like hohlraum experimental designs for demonstrating x-ray drive enhancement'', \textit{Phys. Plasmas} \textbf{15}, 012702 (2008). [Preview Abstract] |
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