Session 5E - ICF Plasma Physics.
ORAL session, Wednesday morning, November 13
Majestic Ballroom, Adam's Mark

Clean (debris-free) and efficient multi-kilovolt x-ray sources are needed for irradiating large military test objects and for use as backlighters in future Inertial Confinement Fusion experiments. Laser-plasma x-ray sources are particularly attractive for these uses since their spectrum can be controlled by proper choice of plasma material and laser intensity; and because many laser-plasma sources can be designed to produce little or no particulate debris. We report on an experiment in which we measured the production-efficiency, spectrum, and time history of 1-4 KeV x-rays from beryllium hohlraums which were filled with 1 and 2 atm of Xe gas and then irradiated by a 2-nsec pulse from the NOVA laser. It is predicted that 17be converted into > 4KeV x rays and 30history of >4KeV part of the spectrum is predicted to exhibit a dip in intensity whose depth and location vary with fill pressure and hohlraum size.. We also measured the debris produced by these sources. Work supported by the Defense Special Weapons Agency and the U.S. Department of Energy at LLNL under W-7405-ENG-48.

[5E.02] Density measurements of shock compressed liquid D_2 *

We will report on preliminary experiments to measure the shock density of liquid D_2 in the pressure range 1-3 Mbar. In the experiment a beam smoothed with a kinoform phase plate is used to drive a shock into a 100-200 um thick aluminum cell which contains liquid D_2. The trajectories of the aluminum/D_2 interface and shock front in D_2 were measured using side on radiography. This technique allows measurement of the density in two ways. First by measuring the absorption and using calculated opacities and second by measuring the compression ratio directly from the position of the shock front and aluminum/D_2 interface. We will present the results of these experiments which extend recent gas gun measurements which suggest a softening of the D_2 equation of state due to dissociation. *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.

[5E.03] A New Non-LTE Model based on Super Configurations

Non-LTE effects are vital for the simulation of radiation in hot plasmas involving even medium Z materials. However, the exceedingly large number of atomic energy levels forbids using a detailed collisional radiative model on-line in the hydrodynamic simulations. For this purpose, greatly simplified models are required. We implemented recently Busquet's model(M. Busquet, Phys. Fluids B, 5, 4191 (1993)) in NRL's RAD2D Hydro code in conservative form (M. Klapisch et al., Bull. Am. Phys. Soc., 40, 1806 (1995), and poster at this meeting.). This model is quick and the results make sense, but in the absence of precisely defined experiments, it is difficult to asses its accuracy. We present here a new collisional radiative model based on superconfigurations( A. Bar-Shalom, J. Oreg, J. F. Seely, U. Feldman, C. M. Brown, B. A. Hammel, R. W. Lee and C. A. Back, Phys. Rev. E, 52, 6686 (1995).), intended to be a benchmark for approximate models used in hydro-codes. It uses accurate rates from the HULLAC Code. Results for various elements will be presented and compared with RADIOM.

[5E.04] The hypercritical shock wave

When the shock velocity increases the shock temperature becomes high enough to induce a radiative precursor in the unperturbed material, the shock wave is no longer adiabatic and is said supercritical. Such a shock was evidenced in xenon gas by laser irradiating an Al foil in 1986. New experiments have been recently performed on the Octal- Heliotrope installation. A laser irradiated low density foam allowed us to get a 100 eV radiative precursor in the supercritical shock wave. At even higher velocities the shock temperature becomes high enough to make the shocked material transparent to its own radiation, as a consequence the shock wave tends to a near adiabatic regime. This regime is not permanent when the shock velocity remains constant. Furthermore, the radiative precursor propagation law is different from that of the supercritical case. For these reasons we call it the "hypercritical" shock wave. We present here the experimental results and show their good agreement with a simple analytical model and with 1D numerical calculations.

[5E.05] Density Profiling of High-Z, X-ray Ablated Targets

An accurate determination of the density profiles of high-Z targets irradiated by a thermal x-ray source is important in a variety of topical problems. For example, it is important to be able to calculate low density plasma profiles in order to assess the degree of filling of a hohlraum, of constricting apertures, or of diagnostic holes. In addition, density scalelengths close to the ablation surface must be well determined if an informed assessment is to be made of the degree of ablative stabilisation of the RT instability. Radiography data will be presented for Au disc targets irradiated by the x-ray flux from a well-characterised HELEN hohlraum. Backlighter energies of 3.2, 4.7 keV were used to probe different regions of the blowoff plasma. Hydrocode simulations indicated that target densities of up to 0.2g/cc were detectable - i.e. within one scalelength of the ablation surface. The flux distribution on the foil was calculated using a time dependent, 3D viewfactor code to be sufficiently planar to allow quasi-1D expansion. Detailed comparison of the data to 1D and 2D radiation-hydrocode simulations will be presented, addressing the consequences of using various levels of calculational sophistication. The extension of these techniques to more advanced targets will be discussed.it intact.

[5E.06] Surrogate Cryogenic Target Implosion Experiments Performed with the OMEGA Laser System

The primary goal of the OMEGA laser target physics program is the validation of the direct-drive approach to ICF using cryogenic-fuel capsules that are hydrodynamically equivalent to high-performance capsules. Preliminary to these experiments scheduled to start in late 1999, experiments will be performed on surrogate cryogenic targets, initially avoiding the complex and difficult target preparation of cryogenic targets. We will report on the first such experiments performed on the 30-kJ, 60-beam OMEGA laser system. Targets consist of deuterated polystyrene (CD) shells, \sim900 \mum diam, with wall thicknesses of \sim10\ to 15 \mum, overcoated with \sim5\ to 20 \mum of parylene acting as an ablator. Diagnosis of target compression will be principally by two methods: space-resolved continuum absorption x-ray spectroscopy and secondary neutron spectroscopy. Both techniques measure the shell areal density, while the first technique also measures the core electron temperature (T_e), and the second technique measures the core ion temperature (T_i). This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460. *Also Lawrence Livermore National Laboratory.

[5E.07] Double-Shell Ignition Targets

Most targets proposed for ignition and gain with the National Ignition Facility use a single shell containing a cryogenic layer of deuterium-tritium ice with a vapor-pressure gaseous D-T central core. We have designed a double-shell target as an alternate concept for NIF ignition. The target uses a Cu-doped Be outer shell and a Cu-hardened Au inner shell, with 50 mg/cc CH foam in between. The main advantages of the double-shell target concept are both that cryogenic fuel layers and accurate, high-dynamic-range pulse shaping are not needed. To date we have performed two types of calculations: finely zoned capsule studies with realistic multi-mode surface perturbations to examine implosion stability; and integrated calculations of the laser, hohlraum, and capsule to examine symmetry and overall target performance. Both types of calculations have predicted ignition.

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

[5E.08] ``Shinethrough'' Experiments Using 50-ps Laser Pulses

``Shinethrough"(D. K. Bradley et al., in Laser Interaction and Related Plasma Phenomena, ed. by H. Hora and G. H. Miley (Plenum Press, NY, 1991), Vol. 9, p. 323.) is the penetration of the laser light into shell material before the creation of a critical surface in the plasma. This effect can be mitigated through the use of a thin, UV-opaque, barrier layer on the surface of the target. This, however, potentially limits the use of optical diagnostics to characterize the fuel. ``Shinethrough" will be studied using 1- and 40-ps laser pulses at 351 nm with intensities up to \sim10^14 W/cm^2 focused through a distributed phase plate. The transmitted energy fraction and spatial distribution for a variety of barrier materials will be studied as a function of pulse rise time and peak intensity. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[5E.09] Performance of Planar Foam-Buffered Targets on the OMEGA Laser System

The instrumentation that measured the growth of perturbations on the Rayleigh-Taylor unstable ablation front has been used to characterize the effect of an initial foam layer on planar foils. These experiments were performed with UV (351-nm) irradiation from the OMEGA laser system with a peak intensity of 2 \times 10^14 W/cm^2. The laser pulse shapes were both Gaussian and non-Gaussian. Foams with initial densities of 10 mg/cc and 50 mg/cc were used as the illumination surface for the laser. The effect of an initial metal layer was also studied. Experiments were conducted with single-mode irradiation nonuniformity to quantify the effect of the foam layer. This work was supported by the U. S. Department of Energy Office of Inertial Confinement Fusion under cooperative Agreement No. DE-FC03-92SF19460.

[5E.10] Convergent Geometry Foam Buffered Direct Drive Experiments

Preliminary implosion tests of foam buffering at 527 nm on NOVA compared the yield and imploded core symmetry of capsules with and without foam to 1D Lasnex calculations. Glass shells 1300\mum diameter, approximately 4\mum thick were filled with 20 atm DT. Two covered with 200 \mum of 0.045 g/cc polystyrene foam and 25nm Au, gave 2.3% and 4.6% of clean calculated yield. To reach a similar convergence ratio and implosion time the laser energy was reduced on comparison targets. One GMB driven with 9kJ gave 1.1%. Its minimum diameter was the same as the foam (246\mum) and less than calculated (360\mum). Testing a similar mass shell, two microballoons covered with 10 \mum of full density CH and irradiated with 21kJ gave 0.1% and 0.3% Calculations for the foam and comparison capsules show two peaks in the neutron yield. The first is caused by heating when the first shock reaches capsule center; the second, by DT burn at the time of peak shell compression. Becaus! e ! the temperature of the burning DT is higher during the first shock peak than second, more degradation of the second peak causes the observed burn temperature to be higher than calculated for the full yield.

[5E.11] Fokker-Planck Simulations of Foam-Buffered Targets

The Rayleigh-Taylor (RT) instability has a potentially deleterious effect on the final implosion conditions of a direct-drive inertial confinement fusion target. The RT instability can be seeded by nonuniformities in laser illumination. Techniques, such as SSD and ISI, have improved the time-integrated laser illumination; however, preliminary calculations indicate that these techniques may be limited in effectiveness during the initial ``startup'' phase. During this phase the nonuniform laser irradiation is imprinted on the target surface. Recent experiments(M. Dunne et al., Phys. Rev. Lett. 75, 3858 (1995).) have been performed using foam-buffered targets that show a substantial reduction in the growth of the RT instability. The presence of the foam layer increases the standoff distance between the critical and ablation surfaces at early times, thus increasing the amount of smoothing of the instantaneous nonuniformities compared to conventional ``bare" targets. This paper will examine the role of nonlocal heat transport on foam-buffered targets. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[5E.12] Diffractive Calculation of the Intensity Distribution in a Direct-Drive Laser-Fusion Target Corona

Scalar diffraction theory is used to calculate the three-dimensional intensity distribution (``speckle pattern'') produced in a spherically symmetric target corona by a random-phase-plate laser beam. The incident electromagnetic field amplitude is resolved into spherical harmonics, and the spherical wave equation is solved for each component. Absorption is included by using a complex dielectric function. Intensity and absorption patterns will be shown near caustics and at high densities, where calculations based on ray-tracing or the paraxial approximation become invalid. The phase shifts of different spherical harmonic orders change differently as the corona evolves in time; the consequences of this for temporal irradiation smoothing will also be addressed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[5E.13] Correlation between High-Z-Low-Z Nuclear Reactions Yield and Hard = X-Rays Emission in Focused Discharges

Nuclear reactions with a high energy threshold (Ei>2 MeV) are detected in W=3D7 kJ plasma focus discharges with plasma composed of D_2 (90%) and O_2 (10%) at 5 Torr of total filling pressure in the chamber. The ^16O(d,n)^17F reaction occuring in plasma and ^65Cu(d,p)^66Cu reaction occuring in the external (side-on) target were measured by activation methods. The time resolved neutron pulse and hard X-ray pulse (Ex>50 keV) are detected by two shielded plastic scintillators (FWHM 8 ns) at different distances (0.12 amp; 6 m). Neutron amd X-ray signals were unfolded to the real time resolved signals. Neutron energy spectra were defined in the selected neutron emission intervals. Reaction yields, time emission structure and neutron energy spectra are discussed in terms of the Plasma-Domain-of-Enhanced-Nuclear-Reactivity model (J. S. Brzosko and V. Nardi, Phys.Lett. A155(1991)162 and A192(1994)250)

Part 5 of program listing