Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session GI1: Direct Drive Inertial Confinement Fusion and Z Pinches |
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Chair: Craig Sangster, University of Rochester, Laboratory for Laser Energetics Room: Rosen Centre Hotel Junior Ballroom |
Tuesday, November 13, 2007 9:30AM - 10:00AM |
GI1.00001: Performance of Direct-Drive Cryogenic Targets on OMEGA Invited Speaker: Cryogenic D$_{2}$ and DT targets are routinely imploded on the OMEGA Laser System to study ignition-relevant, high-convergence-ratio, direct-drive physics. High-performance target designs depend crucially on the accuracy of the physics models used to simulate the implosions. To validate the predictive capabilities of the physics models, computer simulations have been benchmarked against a variety of precision measurements, including laser-energy absorption, x-ray emission spectra, neutron and charged-particle yields and spectra, core emission spectra, and time-resolved hard-x-ray spectra ($>$20 keV). The target designs are characterized by the shell adiabat \textit{$\alpha $} (ratio of the fuel pressure to the Fermi-degenerate pressure) and the peak drive intensity. Targets for the OMEGA experiments included both ``ice--CD'' designs, consisting of a 65- to 100-\textit{$\mu $}m ice layer with a 3- to 13-\textit{$\mu $}m CD overcoat, and 40- to 70-\textit{$\mu $}m wetted-foam designs with a 2- to 5-\textit{$\mu $}m CH overcoat. The cryogenic shells were driven using high-contrast-ratio (up to 100) pulse shapes, including picket pulses to shape the adiabat inside the shell to improve stability. This talk will review the results of OMEGA cryogenic implosions with shell adiabats in the range 1 $<$ \textit{$\alpha $} $<$ 10 and peak intensities varying from 2 $\times $ 10$^{14}$ to 1.5 $\times $ 10$^{15}$ W/cm$^{2}$. High-areal-density, cryogenic fuel assembly (\textit{$\rho $R} $>$ 0.2 g/cm$^{2}$,$^{ }$\textit{$\rho $}$_{D2} \quad \sim $ 100 g/cm$^{3}$, 500 $\times $ LD) is achieved when the excessive hot-electron and radiation preheat is mitigated. The experimental database of cryogenic targets imploded on the OMEGA laser will be used to design direct-drive-ignition targets for the National Ignition Facility. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460. \newline \newline Contributors: R.S. Craxton, J.A. Delettrez, D.H. Edgell, R. Epstein, V. Yu. Glebov, D.R. Harding, S.X. Hu, I.V. Igumenshchev, R. Janezic, R.L. Keck, J.P. Knauer, S.J. Loucks, L.D. Lund, J.R. Marciante, J.A. Marozas, F.J. Marshall, D.N. Maywar, R.L. McCrory, P.W. McKenty, D.D. Meyerhofer, P.B. Radha, S.P. Regan, R.G. Roides, T.C. Sangster, W. Seka, V.A. Smalyuk, J.M. Soures, C. Stoeckl, and S. Skupsky, \textit{UR/LLE}; J.A. Frenje, C.K. Li, R.D. Petrasso, \textit{MIT-PSFC}; D. Shvarts, \textit{NRCN (Israel)}. [Preview Abstract] |
Tuesday, November 13, 2007 10:00AM - 10:30AM |
GI1.00002: OMEGA Experiments on the Shock-Ignition ICF Concept Invited Speaker: Shock ignition\footnote{ R. Betti \textit{et al}., Phys. Rev. Lett. \textbf{98}, 155001 (2007).} is an ICF concept that assembles thermonuclear fuel to high areal densities and then ignites it by launching a strong shock wave into the compressed fuel. The low-adiabat fuel assembly implodes with a velocity that is less than that required for hot-spot ignition. An intensity spike at the end of the main drive pulse generates a strong shock that is timed to meet the return shock bouncing back from the capsule center in the hot spot. The resulting fuel assembly is non-isobaric and will ignite with less energy than a conventional isobaric implosion.$^{1}$ Experiments to study the shock-ignition concept were performed on the OMEGA Laser System using 40-\textit{$\mu $}m-thick, 0.9-mm-diam plastic shells filled with D$_{2}$ gas. The targets were driven by a relaxation adiabat-shaping laser pulse with a short picket pulse\footnote{ K. Anderson and R. Betti, Phys. Plasmas \textbf{11}, 5 (2004).} and a high-intensity spike. The implosion was optimized by measuring the fuel assembly performance as a function of the timing of the picket pulse and the spike. Neutron-averaged areal densities of $\sim $200 mg/cm$^{2}$ were measured. The shock-generated implosion showed fusion product yields enhanced by a factor of $\sim $4 compared to an implosion without the spike. The measured neutron yield for a 25-atm fill, an adiabat of 1.6, and 17 kJ of laser energy was $\sim $10{\%} of the 1-D simulation prediction. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreements DE-FC52-92SF19460 and DE-FC02-04ER54789. Contributors: R. Betti,$^{\ast }$ C. Stoeckl, K.S. Anderson,$^{\ast }$ J.A. Delettrez, V.Yu. Glebov, F.J. Marshall, D.N. Maywar, R.L. McCrory, D.D. Meyerhofer, P.B. Radha, T.C. Sangster, V.A. Smalyuk, A.A. Solodov,$^{\ast }$ B. Yaakobi, and C.D. Zhou, \textit{UR/LLE}; J.A. Frenje, C.K. Li, R.D. Petrasso, and F.H. S\'{e}guin, \textit{MIT-PSFC}; L.J. Perkins, \textit{LLNL}; D. Shvarts, \textit{NRCN (Israel)}. $^{\ast }$Also at the Fusion Science Center for Extreme States of Matter and Fast Ignition. [Preview Abstract] |
Tuesday, November 13, 2007 10:30AM - 11:00AM |
GI1.00003: Time-Resolved Absorption in Cryogenic and Room-Temperature, Direct-Drive Imploding Targets Invited Speaker: Time-resolved absorption has been measured in direct-drive-implosion experiments for various targets and pulse shapes using OMEGA's UV Laser System. These experiments reveal a number of interaction processes beyond inverse bremsstrahlung absorption. During the first 100 to 200 ps, evidence of enhanced absorption points toward resonance absorption. Depending on target material and pulse shapes, the absorption at times $t \quad >$ 0.7~ns is reduced compared to predictions by hydrodynamic simulations with flux-limited electron heat transport. This discrepancy may be partly due to uncertainties in the heat transport model. Scattered light spectra further indicate that beam-to-beam energy transfer with gain provided by stimulated Brillouin scattering (SBS) may also contribute. Evidence for two-plasmon-decay (TPD) instability is seen in almost all direct-drive-implosion experiments as evidenced by hard-x-ray and 3\textit{$\omega $}/2 emission. The TPD instability is driven particularly hard when the laser burns through the CD shell during the laser pulse in a cryogenic target implosion with the concomitant possibility of fast-electron preheat. This wealth of interaction processes will be discussed along with implications for future larger-scale experiments. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460. Contributors: D.H. Edgell, V.N. Goncharov, I.V. Igumenshchev, J.A. Delettrez, J. Myatt, A.V. Maximov, R.W. Short, C. Stoeckl, and T.C. Sangster. [Preview Abstract] |
Tuesday, November 13, 2007 11:00AM - 11:30AM |
GI1.00004: Monoenergetic Proton Radiography of Electromagnetic Fields and Areal Density in Implosions and in Laser-Plasma-Interaction Experiments$^{\ast ,\dag}$ Invited Speaker: An isotropic monoenergetic proton backlighter source with matched detector has been utilized on the OMEGA laser system to accurately and sensitively study the following: First, MG fields generated by laser plasma interactions [1,2], both in the growth and decay phase, the latter associated with the development of instabilities that break 2-D symmetry; Second, the reconnection of MG fields in interacting, laser-generated magnetic bubbles [3]; Third, the fields and areal density evolution for cone-in-shell implosions[4]; and Fourth, the fields and areal density evolution of spherical implosions [5,6]. Complex field structures are observed during the implosions. Because of the precise energy of the 14.7 (3.0) MeV P backlighter, a result of the fusion reaction of D and $^{3}$He (and DD) in an exploding pusher, a quantitative relationship exists between particle energy loss and areal density, and between particle deflections and field strength. Results of these experiments, as well as those currently being planned, such as accurate stopping power measurements in warm dense matter, will be presented. \newline [1] C. K. Li \textit{et al.}, PRL 97 2006~; \newline [2] C. K. Li \textit{et al.}, PRL 99 2007; \newline [3] C. K. Li \textit{et al.} (to be published in PRL); \newline [4] J. R. Rygg \textit{et al.}, this conference; \newline [5] F. H. Seguin \textit{et al.}, this conference; \newline [6] C. K. Li \textit{et al.}, this conference. \newline \newline * In collaboration with scientists from MIT, LLNL, and LLE. \newline {\dag} Supported by NLUF Contract DE-FG52-2005NA26011; and the Univ. of Rochester/Fusion Science Center Contract 412761-G. [Preview Abstract] |
Tuesday, November 13, 2007 11:30AM - 12:00PM |
GI1.00005: Radiation energetics of inertial confinement fusion relevant wire-array z pinches Invited Speaker: The scaling of the radiation power and energy of z-pinch sources as a function of current and implosion time is of interest for z-pinch-driven, high-yield inertial confinement fusion applications [R.A. Vesey et al., Phys. Plasmas 14, 056302 (2007)]. Short implosion-time 20-mm diameter, 300-wire tungsten arrays maintain high peak x-ray powers on the 20 MA, 100-ns Z pulsed-power facility despite a reduction in peak current from 19 to 13 MA. The implosion kinetic energy is estimated using multiple diagnostics, including the first measurement of the imploding mass density profile of a wire- array z-pinch. The main radiation pulse (i.e., not including the late-time radiation) on tests with a 1-mm on-axis rod to limit the convergence may be explained solely by the kinetic energy flux. However, bare-axis tests require sub-mm convergence of the magnetic field and/or enhanced resistive heating. Sub-mm convergence is never seen in these arrays in the $\sim$450 eV x-ray emission characteristic of the peak of the blackbody emission. Sub-mm widths are seen only in high- energy $>1$ keV emission diagnostics. The latter images are characteristic of the high-energy tail in the emission spectrum that accounts for a substantial fraction of the total radiated energy and appears to be associated with small-area, high- temperature sources. The radiography and imaging data discussed here are presently being used to provide strong constraints for simulations beyond just the radiated power and energy. In collaboration with: M.E. Cuneo, S.V. Lebedev (Imperial College), R.W. Lemke, E.M. Waisman, W.A. Stygar, B. Jones, M.C. Jones, J.L. Porter, and D.F. Wenger. [Preview Abstract] |
Tuesday, November 13, 2007 12:00PM - 12:30PM |
GI1.00006: Investigation of trailing mass in Z-pinch implosions and comparison to experiment Invited Speaker: Wire-array Z pinches represent efficient, high-power x-ray sources with application to inertial confinement fusion, high energy density plasmas, and laboratory astrophysics. The first stage of a wire-array Z pinch is described by a mass ablation phase, during which stationary wires cook off material, which is then accelerated radially inwards by the JxB force. The mass injection rate varies axially and azimuthally, so that once the ablation phase concludes, the subsequent implosion is highly 3D in nature. In particular, a network of trailing mass and current is left behind the imploding plasma sheath, which can significantly affect pinch performance. In this work we focus on the implosion phase, electing to model the mass ablation via a mass injection scheme. Such a scheme has a number of injection parameters, but this freedom also allows us to gain understanding into the nature of the trailing mass network. For instance, a new result illustrates the role of azimuthal correlation. For an implosion which is 100{\%} azimuthally correlated (corresponding to an azimuthally symmetric 2D r-z problem), current is forced to flow on the imploding plasma sheath, resulting in strong Rayleigh-Taylor (RT) growth. If, however, the implosion is not azimuthally symmetric, the additional azimuthal degree of freedom opens up new conducting paths of lower magnetic energy through the trailing mass network, effectively reducing RT growth. Consequently the 3D implosion experiences lower RT growth than the 2D r-z equivalent, and actually results in a more shell-like implosion. A second major goal of this work is to constrain the injection parameters by comparison to a well-diagnosed experimental data set, in which array mass was varied. In collaboration with R. Lemke, M. Desjarlais, M. Cuneo, C. Jennings, D. Sinars, E. Waisman [Preview Abstract] |
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