Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session CI2: ICF Theory and Diagnostics |
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Chair: Patrick McKenty, University of Rochester Room: Adam's Mark Hotel Plaza Ballroom EF |
Monday, October 24, 2005 2:00PM - 2:30PM |
CI2.00001: New findings on the control of filamentation of intense laser beams propagating in underdense plasma Invited Speaker: In indirect drive ICF ignition designs, the laser energy is delivered into the hohlraum through the laser entrance holes (LEH), which are sized as small as practicable to minimize X-ray radiation losses. On the other hand, deleterious laser plasma processes, such as filamentation and stimulated back-scatter, typically increase with laser intensity. Ideally, therefore, the laser spot shape should be a close fit to the LEH, with uniform (envelope) intensity in the spot and minimal energy at larger radii spilling onto the LEH material. This keeps the laser intensity as low as possible consistent with the area of the LEH aperture and the power requirements of the design. This can be achieved (at least for apertures significantly larger than the laser's aberrated focal spot) by the use of custom-designed phase plates. However, outfitting the 192 beam NIF laser with multiple sets of phase plates optimized for a variety of different LEH aperture sizes is an expensive proposition. It is thus important to assess the impact on laser-plasma interaction processes of using phase plates with a smaller than optimum focal spot (or even no phase plates at all!) and then de-focussing the beam to expand it to fill the LEH and lower its intensity. We find significant effects from the lack of uniformity of the laser envelope out of the focal plane, from changes in the characteristic sizes of the laser speckle, and on the efficacy of additional polarization and/or SSD beam smoothing. We quantify these effects with analytic estimates and simulations using our laser plasma interaction code pF3D. [Preview Abstract] |
Monday, October 24, 2005 2:30PM - 3:00PM |
CI2.00002: Rayleigh--Taylor Growth Measurements of 3-D Modulations in Nonlinear Regime Invited Speaker: The nonlinear growth of 3-D broadband nonuniformities was measured near saturation levels\footnote{S. W. Haan, Phys. Rev. A, Gen. Phys. \textbf{39}, 5812 (1989).}$^{,}$\footnote{ V. A. Smalyuk\textit{ et al.}, Phys. Rev. Lett. \textbf{81}, 5342 (1998).} using x-ray radiography in planar foils accelerated by the OMEGA laser. An understanding of the nonlinear evolution of the Rayleigh--Taylor instability is essential in inertial confinement fusion and astrophysics. The initial target modulations were seeded by laser nonuniformities and subsequently amplified by the Rayleigh--Taylor instability. The nonlinear saturation velocities are measured in Fourier space and are found to be in excellent agreement with Haan predictions.\footnote{V. A. Smalyuk \textit{et al.}, ``Fourier-Space, Nonlinear Rayleigh--Taylor Growth Measurements of 3-D Laser-Imprinted Modulations in Planar Targets,'' submitted to Phys. Rev. Lett.} The measured growth of long-wavelength modes is in good agreement with models of enhanced, nonlinear, long-wavelength generation in ablatively-driven targets.$^{3}$ In a real space analysis, bubble merger is quantified by the evolution of distributions of the bubble size, amplitude, and velocity. A self-similar evolution of the distribution of bubble sizes is measured and the bubble-merging rate inferred. This talk will describe the nonlinear evolution of the 3-D modulations and compare them with theoretical models. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: J. A. Delettrez, D. D. Meyerhofer, S. P. Regan, and T. C. Sangster; O. Sadot (also Ben Gurion University); D. Shvarts, Negev, Israel. [Preview Abstract] |
Monday, October 24, 2005 3:00PM - 3:30PM |
CI2.00003: Late-time Radiography of Beryllium Ablators in Long-pulse Gas-filled Hohlraums Invited Speaker: We have obtained the first-ever late-time ($>$10 ns) radiographs of perturbation growth in a doped beryllium ablator, which is one candidate for the inertial-confinement-fusion capsule ablator at the National Ignition Facility (NIF). To do so, we have designed, deployed, and characterized a 6-ns radiation drive for an Omega hohlraum, filled with methane to inhibit the inward flow of high-Z wall material. The laser drive consists of two separate laser pulse shapes melded together into a uniquely shaped composite. Total input energy is $\sim $4.25 kJ. The radiation drive temperature, characterized by soft x-ray spectroscopy (Dante) and laser Doppler velocimetry, increases to 50 eV in 0.5 ns then gently ramps up to a peak value exceeding 140 eV at 5.6 ns. Side-on x-radiography of the Be sample ejected from the hohlraum provides an additional verification of the drive. Backscatter losses from laser-plasma instabilities into the lens from this surrogate NIF hohlraum are $<$5{\%}. We find that methane has the desired effect of holding the Au wall back for $>$10 ns in these experiments. Active and passive shock-break-out diagnostics show that 40-$\mu $m thick Be-Cu (0.9{\%} Cu by atom) samples are preheated even with this soft drive with $\sim $1{\%} M-band contribution. Be samples at the rear of the hohlraum have taken the form of both sputtered and powder-pressed planar disks, sinusoids with 100-$\mu $m period and 2.5-$\mu $m amplitude, and steps (30 and 60 $\mu $m thick). This longer-than-average Omega radiation drive approximates conditions expected for the NIF capsule's first shock, where the effects of ablator microstructure are expected to be most significant. [Preview Abstract] |
Monday, October 24, 2005 3:30PM - 4:00PM |
CI2.00004: Inertial confinement fusion neutron images Invited Speaker: Neutron images of DT-filled capsules are now routinely acquired with a 20-micron spatial resolution for direct drive implosions performed at the Laboratory for Laser Energetics. Images are recorded using a novel detector based on an array of 85-micron -diam capillary tubes filled with a liquid scintillator. Detector resolution of 650 micron is limited by track length of the elastically-scattered recoil protons. Replacing the hydrogen in the scintillator with deuterium improves detector spatial resolution to 325 micron, and makes a 6-7 micron source resolution achievable at the NIF and LMJ facilities. Detector sensitivity allows individual neutron events to be recorded. Coded ring apertures were recently implemented at LLE, and appear to be the most promising technique to achieve high signal-to-noise ratio. The technique reliability is established by comparing the experimental images acquired with penumbral and ring apertures. A signal-to-noise ratio near 30 at a yield of 3 x 10$^{13}$ 14-MeV neutrons confirms annular imaging capabilities with a 10 micron resolution filter. Images of DD filled capsules yielding 3 x 10$^{11}$ neutrons (2.45 MeV) have also been recorded. The consistency of the hot fuel burning area and the capsule shell revealed by the x-rays images is also discussed. [Preview Abstract] |
Monday, October 24, 2005 4:00PM - 4:30PM |
CI2.00005: Direct-Drive Shock-Wave-Timing Experiments in Planar Targets Invited Speaker: Inertial confinement fusion target designs use multiple shock waves to condition the target material for optimal performance. These designs require that the shock waves be accurately timed to coalesce at a particular point in the target. It is essential that the propagation and dynamics of these multiple shocks be understood and correctly modeled. The OMEGA Laser Facility at the University of Rochester is used to perform direct-drive experiments that measure the propagation and coalescence of two laser-driven shock waves propagating in planar targets made of CH or cryogenic D$_{2}$. Laser pulses with various temporal shapes generate two primary shocks that propagate in these transparent targets. The velocity and self-emission profiles of these shocks are temporally resolved and clearly show the first shock wave propagating through the material and then being overtaken by the second shock wave. The coalescence of these shocks forms a single, stronger shock that eventually arrives at the rear surface of the target. The measured velocity and emission profiles exhibit distinct features that include the decay rate of unsupported shocks, coalescence and shock breakout times, and curvature of the shock fronts. These results are presented for a number of drive conditions and compared to 1-D and 2-D hydrodynamic codes. The simulated velocity profiles and coalescence times are in good agreement with experimental observations, as are many of the two-dimensional effects. These experiments have also validated the dependence of laser--target coupling on the angle of incidence. The effect of preheat by x rays on these experiments is also discussed. Results of preliminary indirect-drive experiments are also shown. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: E. Vianello, J. E. Miller, R. S. Craxton, V. N. Goncharov, I. V. Igumenshchev, T. J. B. Collins \textit{LLE}; D. G. Hicks, P. M. Celliers, G.W. Collins\textit{, LLNL}; R. E. Olson, \textit{Sandia National Lab} [Preview Abstract] |
Monday, October 24, 2005 4:30PM - 5:00PM |
CI2.00006: High energy K-$\alpha $ radiography using high-energy, high-intensity, short-pulse lasers Invited Speaker: Many laser facilities utilizes x-ray backlighters to radiographically diagnose various stages of hydrodynamics experiments. These backlighters have x-ray energies $<\sim $9 keV generated by thermal plasmas from long-pulse lasers. However, many future high energy density experiments on new facilities such as Omega-EP and NIF will perform experiments that will require backlighters in the 15-100 keV range and better than $\sim $10 $\mu $m spatial resolution. High-energy K-$\alpha $ x-ray sources can be created through the interaction of high-energy electrons, generated by high-intensity short-pulse lasers, with target material. Not only K-$\alpha $ sources are more efficient way to generate high-energy photons but also advantageous in creating quasi-monochromatic backlighters by K-edge filters to provide required contrast. We have performed several experiments using the JanUSP, Vulcan 100 TW and Petawatt lasers to understand the K-$\alpha $ source characteristics and to implement a workable radiography. Our measurements show that the K-$\alpha $ size from a simple foil target is larger than 60 $\mu $m thus less than optimally efficient as a point source for high-resolution imaging. We also measured the total K-$\alpha $ yield is invariant to the target thickness verifying that refluxing plays a major role in photon yields and that smaller radiating volumes emit brighter K-$\alpha $ radiation [1]. Thus, 1-D radiography with the required brightness and spatial resolution can be obtained using small-edge-on foils. Subsequently, we employed this technique to perform an EOS experiment in a 400 $\mu $m thick Al sample. We tested many small volume sources in different geometrical shapes such as cone/wire and embedded wires to understand photon yields from these 2-D point sources. We will compare our measurements with hybrid-PIC LSP simulations. In addition, we are developing imaging detectors and diagnostics tools that are workable in the 15-100keV range. This paper will present the results. \newline \newline [1] H. S. Park et al., Rev. Sci. Instrum. 75, 4048 (2004) [Preview Abstract] |
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