Session 6S - ICF Physics: Experiments.
POSTER session, Wednesday afternoon, November 13
Exhibit Hall - Concourse Level, Adam's Mark

Initial experiments to study spall from aluminium at high strain rates (107 s-1)using one arm of the HELEN laser (532nm, 200ps) to irradiate a compositeDelrin/Aluminium target at an irradiance of around 5 x 1013 W/cm2 failed to showspallation. Our 1-D NYM hydrocode simulations suggest tensions of 50kb at adepth of 505m from the aluminium surface. Spallation should therefore haveoccurred based purely on a peak tension argument, however, comparison with acumulative damage model such as that of Tuler-Butcher indicates that this is not thecase. A second experimental design using an all aluminium target gave higher peaktensions which persist for longer and the cumulative damage spall criterion isgreatly exceeded. The experimental radiograph shows a well defined spall of 605m thickness. We believe that this is the first occasion on which direct evidenceof spallation has been obtained from a dynamic radiograph in a laser driven experiment.

[6S.02] Characterization of preheat in laser-driven targets using interferometry*

We will discuss preliminary results of experiments using interferometry to measure the level of preheat-induced expansion present in laser-driven targets. A single beam of Nova at .53 \mum and 8 ns square is used to directly drive a strong (10-20 Mbar) shock through various target materials (aluminum,silicon). A Michelson interferometer is used to image the position of the rear surface of the target and the output is temporally resolved with a streak camera. Motion of the fringes prior to the arrival of the shock at the rear surface is indicative of the presence of preheating and the velocity of the surface can be used to quantify the temperature. \scriptsize *Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

[6S.03] Brominated Plastic Equation of State Measurements by Indirectly Driven Shocks

AWE is conducting experiments to obtain high pressure Hugoniot equation of state(EOS) data by the impedance match method. A hohlraum is used to drive shockpressures up to 10Mbar in the aluminium reference material which produces 4Mbar in plastic. EOS data is calculated from shock velocities which are obtainedby observing the visible light emitted on break-out from the surface of the targetusing optical streak cameras.We have performed measurements on brominated plastic samples from LLNL. Thismaterial is used extensively in, e.g., Rayleigh-Taylor instability growthexperiments and in the NIF capsule point design. The pressures reached in theHELEN experiments correspond to those generated by the foot of shaped NOVA orNIF pulses and thus our data can help to address uncertainties in modelling thetargets referred to above.

[6S.04] Two-parameter measurements of the principal Hugoniots of x-ray transmissive materials up to 40 Mbar*

We will present results of experimental measurements of principal hugoniots of x-ray transmissive materials shocked to pressures up to 40 Mbar. Using side-on x-radiography, two-section samples shocked by a Nova hohlraum x-ray drive are followed in time. The shock front is tracked by the difference in transmission between the compressed and uncompressed material. The interface between the two sections is also observed as the shock accelerates it. The two speeds inferred from these measurements define a point on the principal hugoniot. *Work performed under the auspices of the U. S. Department of Eenrgy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

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[6S.05] Magnetically Suspended Pellet (MSP) for Laser Fusion Scheme -Active Damper for MSP-

To reduce the vibration amplitude and the damping time constant, we introduce active damper. We apply optical forces based on the preceding analysis. The active damper is composed of position sensors, LDs, their drivers and focusing optics. The drivers vary optical forces according to the signal of the position sensors. In case of no active damper, the vibration amplitude and the decay time constant become larger, as pressure decreases. By using the active damper in high vacuum, the former is effectively reduced to less than few micrometers. The latter is reduced by two orders.

[6S.06] Measurements of Rear Surface Shock Breakouts from Flat Targets with Highly Uniform Illumination by the Nike Laser

The NRL Nike laser is a 56-beam, multiplexed KrF laser (\lambda = 248\,nm) designed for very uniform illumination of flat targets. The RMS spatial variation of a single 4-ns beam has been measured to be \sim1%, and the net smoothing is better when up to 44 beams at \sim2\,kJ and \sim10^14\,W/cm^2 are overlapped onto a target. Calibrated time-resolved measurements of the absolute intensity of the target rear surface visible emission allow brightness temperatures to be inferred. Streak camera recording of the rear surface light intensity with a time resolution of better than 15 ps gives the timing and uniformity of the shock breakout. A comparison of some of these measurements with hydrodynamic simulations will be made.

[6S.07] Shock speed and steadiness measurements in shock compressed liquid D_2*

We will present preliminary measurements of shock steadiness for laser-driven shock pressures in the range of 1-3 Mbar propagating through liquid D_2. A single beam of Nova, smoothed with a kinoform phase plate, is used to generate a shock first into a 100-250 \;\mum thick aluminum pusher and ultimately into liquid D_2. We measure the in-flight shock speed in the D_2 using a velocity interferometer arrangement which measures the Doppler shift of a probe beam reflected from the advancing shock front. We will compare these results to predictions of the shock speed and steadiness from hydrodynamic simulations. *Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

[6S.08] Temperature measurements of shock compressed liquid D_2*

We will present preliminary temperature measurements of shock-compressed liquid D_2 in the temperature range of 0.5-3 eV and the pressure range of 1-3 Mbar. A single beam of Nova, smoothed with a kinoform phase plate, is used to generate a shock first into a 100-250 \;\mum thick aluminum pusher and ultimately into liquid D_2. We determine the in-flight shocked D_2 color temperature and emissivity by measuring the spectral radiance at several discrete wavelengths and fitting the data to a gray body. We will compare these results to recent lower temperature and pressure gas gun measurements. *Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

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[6S.09] Reducing DT Surface Roughness for Cryogenic Ignition Targets

ICF experiments and modeling during the past few years have lead to a better understanding of the growth of instabilities during capsule implosion and well defined specifications for ignition targets with solid D-T fuel layers. Recent work has demonstrated that the natural D-T layering process can reduce DT surface non-uniformity's to the point where NIF baseline target designs will ignite in simulations. We report several methods for improving these DT surfaces to provide margin for uncertainties in modeling as well as surface statistics. These improvements involve providing additional bulk heating to the D-T solid by selective infrared absorption, subjecting the ice surface to a heat flux, and growing the solid fuel layer on very low density polymer foam. *Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

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[6S.10] Planar, Ablative Rayleigh-Taylor Instability Growth in Copper Foils*

We have completed a study of planar-geometry ablative Rayleigh-Taylor (R-T) instability growth in copper. The linear and non-linear phases of ablative R-T growth have been measured in bare, flat Cu foils (with various surface perturbations) as these foils are ablated off of a NOVA scale 1 laser hohlraum. We have measured transmission through the foil, transmission across the foil, and acceleration of the foil. Measurements have been corrected for detector image resolution. These measurements have been compared to planar 1-D and 2-D predictions of ablative R-T instability growth, as modeled by the RAGE code and the LASNEX code. RAGE is an Eulerian, AMR, 2-T, grey diffusion code; LASNEX is a Lagrangian, 3-T, multigroup diffusion code. Additionally, we have compared these codes' predictions of planar, ablative R-T instability growth for different input physics. *This work was performed under the auspices of the U.S. Department of Energy by the Los Alamos National Laboratory under contract No. W-7405-ENG-36.

[6S.11] Analysis of the Ablative Rayleigh-Taylor Instability in Cylindrical Convergent Geometry using Indirect Drive.

We are studying the ablative Rayleigh-Taylor instability in a convergent geometry by imploding cylinders using indirect drive in a hohlraum illuminated by eight beams of the Nova laser. This geometry provides two-dimensional implosions with convergent effects but excellent diagnostic views. We have made radiation temperature and drive measurements using a filtered x-ray diode array and passive shock breakout measurements of an aluminum wedge in the hohlraum with cylinder. This will be compared to 3-dimensional calculations of the expected radiation drive. Radiographs transverse to the implosion axis will be compared to simulations of the x-ray transmission from hydrodynamic calculations. Analysis is presented of the perturbation growth at the ablation surface seeded by 0.5 and 1.0 micron sinusoidal perturbations with mode numbers 10 and 14 on the outer surface of a brominated polystyrene layer that feed-in to a chlorinated polystyrene marker layer. Comparison to predictions of mode growth from hydrodynamic calculations will be presented. This work supported by DOE Contract W-7405-ENG-36.

[6S.12] Convergent Geometry Foam Buffered Direct Drive Experiments*

A serious concern for directly driven ICF implosions is the asymmetry imparted to the capsule by laser drive non-uniformities, particularly the ``early time imprint'' remaining despite the use of random phase plates and smoothing with spectral dispersion. The use of a foam buffer has been proposed as a means to reduce this imprint. Two types of convergent geometry tests of the technique to correct static nonuniformities have been studied to date; cylindrical implosions at the Trident and Vulcan lasers, and spherical implosions at the NOVA laser, all using 527 nm laser drive. Cylindrical implosions used end on x-ray backlighter imaging of inner surface disruption due an intentional hole in the drive footprint, using 50 mg/cc acyrlate foam with a thin Au preheat layer. Spherical implosions used 50 mg/cc polystyrene foam plus Au to study yield and imploded core symmetry of capsules with and without a foam buffer, in comparison to ``clean 1D'' calculations. For thick enough layers, all cases showed improvement. Details of the experiments and theoretical unpinnings will be shown. *Work performed under US DOE Contract No. W-7405-Eng-36.

[6S.13] Two Dimensional Studies of Foam-Buffered Laser Imprint Smoothing

Experiments on LANL's TRIDENT laser facility have shown that low density foam layers can significantly mitigate the perturbing effects of beam non-uniformities affecting the acceleration of thin shells. We have studied this smoothing phenomenology parametrically with 2D LASNEX. A typical ``buffered'' target consists of a 10 \mum CH foil, covered with 50 \mum of 50 mg/cc foam and a 250 Ålayer of gold. When exposed to a 1 ns flat topped green pulse at I = 10^14 W/cm^2 with 60% perturbations, bare CH shells are severly disrupted by 1 ns, while the buffered shells manifest minimal distortion at 3 ns. We have found calculationally that: Smoothing derives critically from the high thermal conductivity of the foam. An opaque outer cap for the foam is essential for suppression of early imprinting. Foam thickness should exceed the wavelength of the perturbing disturbance. I \ge 5 \times 10^13W/cm^2 is needed for assured stability exceeding 2 ns. Longer foams (\sim 200 \mum), and/or higher intensities are needed for stability for shorter wavelengths. The early-time solid structure of 10 mg/cc foams may be crucial to their success with 3 ømega light. We are assessing the overall need for a gold layer, which may cause decompressive preheat problems.

\parindent 0pt \vskip 3pt Work supported by the USDOE.

[6S.14] Laser Imprinting and Hydrodynamic Instability Experiments with the Nike KrF Laser \*

Krypton-fluoride lasers (KrF) are of interest to laser fusion because they have both the large bandwidth capability ( > 1 Thz) desired for rapid beam smoothing and the short laser wavelength needed for good laser-target coupling. Nike is a 56 beam KrF laser and target facility that was built to study the physics and means for controlling hydrodynamic instability in ablatively accelerated targets.\footnote S. P. Obenschain et al, Phys. Plasmas 3, 2098 (1996) Better than 1% effective illumination uniformity is achieved with Nike by overlapping many (up to 44) ISI-smoothed beams onto planar targets. In this paper we will discuss current and planned experiments to study laser imprinting and hydrodynamic instability using the Nike facility. \vspace0.1 in This work was supported by the U.S. Department of Energy.

[6S.15] Investigation of Target Acceleration by X-ray Sidelighting on the Nike Laser.

Thin planar targets are ablatively accelerated by the Nike laser to investigate the Rayleigh-Taylor instability growth. Nike is a 56-beam KrF laser designed to study the critical physics issues of the direct-drive Inertial Confinement Fusion scheme. Nike is routinely producing 2kJ of laser energy on target and 500-600J in back- and side-lighter beams in a 4 nsec long pulse. 12 beams are diverted for x-ray diagnostics. Currently 6 beams are used for backlighting the accelerated target for an x-ray pinhole framing camera. The remaining 6 beams are used to illuminate the planar target edge-on with hard x-rays, to study its ablative acceleration. Results of improved imaging, and analysis of mass density and ablative pressure evolution will be presented at the meeting.

[6S.16] Mix, Temperature, And Density Capsule Conditions in a 1-Ns Square Pulse Convergence Study

A set of indirect drive capsule implosion experiments have been performed using a 1-ns square pulse. A variety of capsules were fielded, all with basically the same fabrication, but with different gas fills, ranging from 2 atmospheres to 50 atmospheres (DD). Neutron diagnostics included yield, ion-temperature, and fuel Rho-R. X-Ray diagnostics included K-shell streaked spectroscopy of Ar and Cl dopants, gated x-ray imaging, and absolutely calibrated time-integrated pinhole camera images. We compare these experimental data against various 1 and 2D calculations with and without mix.

This work is supported by the U.S. Department of Energy.

[6S.17] Measurements of Rayleigh-Taylor growth in ablatively driven converging hemispherical targets at Nova

We have started experiments using the Nova laser to investigate the effects of

convergence on single-mode perturbation growth due to the Rayleigh-Taylor (RT)

instability on targets ablatively accelerated with an x-ray drive. In back-to-back experiments, planar CH(Br) foils and hemispherical CH(Ge) capsules are mounted on the wall of a standard Nova hohlraum, so that they see identical x-ray drive histories. Each target had identical initial perturbations, either a 2D \lambda=70 \mum sinusoidal ripple, or a 3D k_x=k_y(l_x=l_y=100 \mum) pattern. The effects of convergence in the hemispherically imploding target should cause the density at the ablation front to remain steep and the effective wavelength of the perturbation to decrease, each of which will affect the overall RT evolution. Initial results of the experiments and modeling will be presented.

[6S.18] X-ray backlit imaging of indirect drive implosions to measure in-flight capsule aspect ratio and convergent hydrodynamics

Both the efficiency of an implosion and the growth rate of hydrodynamic instabilities increase with the aspect ratio of an implosion. Doped ablators are used to study the physics of implosions with high Rayleigh-Taylor growth factors, by minimizing x-ray preheat and shell decompression, and hence increasing the in-flight aspect ratio. We present x-ray backlit images of indirectly-driven capsules, and measurements of the in-flight aspect ratios for doped and undoped casules. The 4.7 keV images of the full capsule are recorded throughout the implosion phase with 50 ps and 15\;\mu/m resolution. We inferred the radial density profile as a function of time by Abel inverting the x-ray transmission profiles. We also extended this technique to image the spherically convergent Rayleigh-Taylor growth of preimposed modulations on the surface of a capsule. Comparison will be made with simulation.

[6S.19] Turbulent Richtmyer-Meshkov Experiments,*

The Richtmyer-Meshkov instability is investigated at high compression using the Nova laser to produce Mach > 10 shocks. The target has two components at solid density with Atwood number \sim -0.88. The growth of imposed 3D random interfacial perturbations is measured radiographically using x-ray opaque diagnostic tracers in two configurations. The turbulent mixing width h is found to increase with the displacement of the interface \deltaZ as h \sim \deltaZ^0.5 in accordance with the large structure model of Shvarts and Alon. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

[6S.20] High Resolution Monochromatic X-Ray Imaging of Targets Irradiated by the Nike KrF Laser

Planar CH foils were accelerated by the main Nike laser driving beams and were backlit by Si plasmas. A spherically-bent quartz crystal (R=25cm, 2d=6.68703 Åimaged the radiation of He-like Si resonance line that was transmitted through the target foil. The intensity of the backlighted image (6 beams, 250 J) was 160 times greater than the self emission from a driven CH foil. The magnified (X9.6) images on DEF film had the resolution about 5 \mu m (3 \mu m with R=10cm). The resulting 10 \mu m spatial resolution in the gated images was determined by 100 \mu m resolution of the framing camera. Images of CH targets were recorded 2 nsec after the peak of the laser pulse and revealed the growth of Rayleigh-Taylor instabilities that were seeded by patterns with amplitude as small as 0.25 \mu m. The image of driven smooth CH target was quite smooth compared to those of the patterned CH foil. A future imaging instrument will have multiple backlighter plasmas and better quality bent crystals that can be bent into a toroidal shape. Up to four images with higher magnification and spatial resolution of 5 \mu m may be recorded on the framing camera. This work supported by the US Department of Energy

Part 6 of program listing