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Plasma crowbars in cylindrical flux compression experiments

Description: We have done a series of one- and two-dimensional calculations of hard-core Z-pinch flux compression experiments in order to study the effect of a plasma on these systems. These calculations show that including a plasma can reduce the amount of flux lost during the compression. Flux losses to the outer wall of such experiments can be greatly reduced by a plasma conducting sheath which forms along the wall. This conducting sheath consists of a cold, dense high ..beta.., unmagnetized plasma which has enough pressure to balance a large field gradient. Flux which is lost into the center conductor is not effectively stopped by this plasma sheath until late in the implosion, at which time a layer similar to the one formed at the outer wall is created. Two-dimensionl simulations show that flux losses due to arching along the sliding contact of the experiment can be effectively stopped by the formation of a plasma conducting sheath.
Date: May 18, 1979
Creator: Suter, L.J.
Partner: UNT Libraries Government Documents Department

Prospects for fluorescence based imaging/visualization of hydrodynamic systems on the National Ignition Facility

Description: The next generation of large, high power lasers, such as the National Ignition Facility (NIF) [1] in the United States, Laser Mega Joule [2] in France or Helen Successor [3] in the United Kingdom offer the prospect of x-ray fluorescence based diagnosis of hydrodynamic experiments The x-ray fluorescence could be pumped by at least two techniques One technique is to use a sizable fraction of these facilities` high power to efficiently make multi-kilovolt x-rays which, in turn, causes dopants placed in experimental packages to fluoresce We call this ``externally pumped x-ray fluorescence`` The second technique is to use the sizable multi-kilovolt photon background that we expect to be present in many hohlraum based experiments, while the driving laser is on, to pump x-ray fluorescence The fluorescing medium could be a dopant in an experimental package or, possibly, a relatively thick slab of material in the hohlraum wall which could serve as a backlighter We call this ``hohlraum hot-corona pumped fluorescence``.
Date: June 4, 1998
Creator: Suter, L. J., LLNL
Partner: UNT Libraries Government Documents Department

Target designs for energetics experiments on the National Ignition Facility

Description: The goal of the first hohlraum energetics experiments on the National Ignition Facility (NIF) [G. H. Miller et al , Optical Eng. 43, 2841 (2004)] is to select the hohlraum design for the first ignition experiments. Sub-scale hohlraums heated by 96 of the 192 laser beams on the NIF are used to emulate the laser-plasma interaction behavior of ignition hohlraums. These ''plasma emulator'' targets are 70% scale versions of the 1.05 MJ, 300 eV ignition hohlraum and have the same energy-density as the full-scale ignition designs. Radiation-hydrodynamics simulations show that the sub-scale target is a good emulator of plasma conditions inside the ignition hohlraum, reproducing density ne within 10% and temperature Te within 15% along a laser beam path. Linear backscatter gain analysis shows the backscatter risk to be comparable to that of the ignition target. A successful energetics campaign will allow the National Ignition Campaign to focus its efforts on optimizing ignition hohlraums with efficient laser coupling.
Date: August 31, 2007
Creator: Meezan, N B; Glenzer, S H & Suter, L J
Partner: UNT Libraries Government Documents Department

Laser Plasma instability reduction by coherence disruption

Description: The saturation levels of stimulated scattering of intense laser light in plasmas and techniques to reduce these levels are of great interest. A simple model is used to highlight the dependence of the reflectivity on the coherence length for the density fluctuations producing the scattering. Sometimes the coherence lengths can be determined nonlinearly. For NIF hohlraum plasmas, a reduction in the coherence lengths might be engineered in several ways. Finally, electron trapping in ion sound waves is briefly examined as a potentially important effect for the saturation of stimulated Brillouin scattering.
Date: April 19, 2006
Creator: Kruer, W l; Amendt, P A; Meezan, N & Suter, L J
Partner: UNT Libraries Government Documents Department

Thomson scattering from inertial confinement fusion plasmas

Description: Thomson scattering has been developed at the Nova laser facility as a direct and accurate diagnostic to characterize inertial confinement fusion plasmas. Flat disks coated with thin multilayers of gold and beryllium were with one laser beam to produce a two ion species plasma with a controlled amount of both species. Thomson scattering spectra from these plasmas showed two ion acoustic waves belonging to gold and beryllium. The phase velocities of the ion acoustic waves are shown to be a sensitive function of the relative concentrations of the two ion species and are in good agreement with theoretical calculations. These open geometry experiments further show that an accurate measurement of the ion temperature can be derived from the relative damping of the two ion acoustic waves. Subsequent Thomson scattering measurements from methane-filled, ignition-relevant hohlraums apply the theory for two ion species plasmas to obtain the electron and ion temperatures with high accuracy. The experimental data provide a benchmark for two-dimensional hydrodynamic simulations using LASNEX, which is presently in use to predict the performance of future megajoule laser driven hohlraums of the National Ignition Facility (NIF). The data are consistent with modeling using significantly inhibited heat transport at the peak of the drive. Applied to NIF targets, this flux limitation has little effect on x- ray production. The spatial distribution of x-rays is slightly modified but optimal symmetry can be re-established by small changes in power balance or pointing. Furthermore, we find that stagnating plasma regions on the hohlraum axis are well described by the calculations. This result implies that stagnation in gas-filled hohlraums occurs too late to directly affect the capsule implosion in ignition experiments.
Date: July 8, 1997
Creator: Glenzer, S.H.; Back, C.A. & Suter, L.J.
Partner: UNT Libraries Government Documents Department

Efficient production of 2--10 keV x-rays by laser heated ``underdense radiators``

Description: The next generation of high power lasers offers the prospect of creating multi-kilovolt x-rays with {gt}10% efficiency. Such efficiencies are achieved with ``underdense radiators``, a non- traditional source of laser generated x-rays. Applications of these sources with the proposed National Ignition Facility (NIF) include volume preheating of experiments; bright, multi-keV backlighting; pumps for fluorescent imaging of capsule dopants and Doppler velocimetry; uniform irradiation of large test objects. This paper presents two-dimensional numerical simulations for these high power lasers with unprecedented efficiency.
Date: May 22, 1996
Creator: Suter, L.J.; Kauffman, R.L.; Maxon, M.S. & Davis, J.F.
Partner: UNT Libraries Government Documents Department

Radiation drive in laser heated hohlraums

Description: Nearly 10 years of Nova experiments and analysis have lead to a relatively detailed quantitative and qualitative understanding of radiation drive in laser heated hohlraums. Our most successful quantitative modelling tool is 2D Lasnex numerical simulations. Analysis of the simulations provides us with insight into the details of the hohlraum drive. In particular we find hohlraum radiation conversion efficiency becomes quite high with longer pulses as the accumulated, high Z blow-off plasma begins to radiate. Extensive Nova experiments corroborate our quantitative and qualitative understanding.
Date: November 3, 1995
Creator: Suter, L.J.; Kauffman, R.L. & Darrow, C.B.
Partner: UNT Libraries Government Documents Department

Hohlraum drive and implosion experiments on Nova. Revision 1

Description: Experiments on Nova have demonstrated hohlraum radiation temperatures up to 300 eV and in lower temperature experiments reproducible time integrated symmetry to 1--2%. Detailed 2-D LASNEX simulations satisfactorily reproduce Nova`s drive and symmetry scaling data bases. Hohlraums has been used for implosion experiments achieving convergence ratios (initial capsule radius/final fuel radius) up to 24 with high density glass surrounding a hot gas fill.
Date: September 8, 1994
Creator: Kilkenny, J.D.; Suter, L.J. & Cable, M.D.
Partner: UNT Libraries Government Documents Department

Predictive three dimensional modeling of Stimulated Brillouin Scattering in ignition-scale experiments

Description: The first three-dimensional (3D) simulations of a high power 0.351 {micro}m laser beam propagating through a high temperature hohlraum plasma are reported. We show that 3D linear kinetic modeling of Stimulated Brillouin scattering reproduces quantitatively the experimental measurements, provided it is coupled to detailed hydrodynamics simulation and a realistic description of the laser beam from its millimeter-size envelop down to the micron scale speckles. These simulations accurately predict the strong reduction of SBS measured when polarization smoothing is used.
Date: November 7, 2007
Creator: Divol, L; Berger, R; Meezan, N; Froula, D H; Dixit, S; Suter, L et al.
Partner: UNT Libraries Government Documents Department

Hard X-ray Imaging for Measuring Laser Absorption Spatial Profiles on the National Ignition Facility

Description: Hard x-ray (''Thin wall'') imaging will be employed on the National Ignition Facility (NIF) to spatially locate laser beam energy deposition regions on the hohlraum walls in indirect drive Inertial Confinement Fusion (ICF) experiments, relevant for ICF symmetry tuning. Based on time resolved imaging of the hard x-ray emission of the laser spots, this method will be used to infer hohlraum wall motion due to x-ray and laser ablation and any beam refraction caused by plasma density gradients. In optimizing this measurement, issues that have to be addressed are hard x-ray visibility during the entire ignition laser pulse with intensities ranging from 10{sup 13} to 10{sup 15} W/cm{sup 2}, as well as simultaneous visibility of the inner and the outer laser drive cones. In this work we will compare the hard x-ray emission calculated by LASNEX and analytical modeling with thin wall imaging data recorded previously on Omega and during the first hohlraum experiments on NIF. Based on these calculations and comparisons the thin wall imaging will be optimized for ICF/NIF experiments.
Date: April 25, 2006
Creator: Dewald, E L; Jones, O S; Landen, O L; Suter, L; Amendt, P; Turner, R E et al.
Partner: UNT Libraries Government Documents Department

Three dimensional modeling of Laser-Plasma interaction: benchmarking our predictive modeling tools vs. experiments

Description: We have developed a new target platform to study Laser Plasma Interaction in ignition-relevant condition at the Omega laser facility (LLE/Rochester)[1]. By shooting an interaction beam along the axis of a gas-filled hohlraum heated by up to 17 kJ of heater beam energy, we were able to create a millimeter-scale underdense uniform plasma at electron temperatures above 3 keV. Extensive Thomson scattering measurements allowed us to benchmark our hydrodynamic simulations performed with HYDRA[2]. As a result of this effort, we can use with much confidence these simulations as input parameters for our LPI simulation code pF3d[3]. In this paper, we show that by using accurate hydrodynamic profiles and full three-dimensional simulations including a realistic modeling of the laser intensity pattern generated by various smoothing options, whole beam three-dimensional linear kinetic modeling of stimulated Brillouin scattering reproduces quantitatively the experimental measurements(SBS thresholds, reflectivity values and the absence of measurable SRS). This good agreement was made possible by the recent increase in computing power routinely available for such simulations. These simulations accurately predicted the strong reduction of SBS measured when polarization smoothing is used.
Date: November 8, 2007
Creator: Divol, L; Berger, R; Meezan, N; Froula, D H; Dixit, S; Suter, L et al.
Partner: UNT Libraries Government Documents Department

Debris and Shrapnel Mitigation Procedure for NIF Experiments

Description: All experiments at the National Ignition Facility (NIF) will produce debris and shrapnel from vaporized, melted, or fragmented target/diagnostics components. For some experiments mitigation is needed to reduce the impact of debris and shrapnel on optics and diagnostics. The final optics, e.g., wedge focus lens, are protected by two layers of debris shields. There are 192 relatively thin (1-3 mm) disposable debris shields (DDS's) located in front of an equal number of thicker (10 mm) main debris shields (MDS's). The rate of deposition of debris on DDS's affects their replacement rate and hence has an impact on operations. Shrapnel (molten and solid) can have an impact on both types of debris shields. There is a benefit to better understanding these impacts and appropriate mitigation. Our experiments on the Omega laser showed that shrapnel from Ta pinhole foils could be redirected by tilting the foils. Other mitigation steps include changing location or material of the component identified as the shrapnel source. Decisions on the best method to reduce the impact of debris and shrapnel are based on results from a number of advanced simulation codes. These codes are validated by a series of dedicated experiments. One of the 3D codes, NIF's ALE-AMR, is being developed with the primary focus being a predictive capability for debris/shrapnel generation. Target experiments are planned next year on NIF using 96 beams. Evaluations of debris and shrapnel for hohlraum and capsule campaigns are presented.
Date: September 4, 2007
Creator: Eder, D; Koniges, A; Landen, O; Masters, N; Fisher, A; Jones, O et al.
Partner: UNT Libraries Government Documents Department

Thomson scattering from Inertial Confinement Fusion targets

Description: We have applied ultraviolet Thomson scattering to accurately measure the electron and ion temperature in high-density gas-filled hohlraums at the Nova laser facility. The implementation of a short-wavelength probe laser that operates at 263 nm (4{omega}) has allowed us for the first time to investigate scalings to high gas fill densities and to characterize the hohlraum conditions of the low-Z gas plasma. as well as of the high-Z wall plasma. These measurements have provided us with a unique data set that we use to make critical comparisons with radiation-hydrodynamic modeling using the code LASNEX. This code is presently being applied to design fusion targets for the National Ignition Facility. The Thomson scattering experiments show the existence of electron temperature gradients in the gas plasma that are well modeled when including a self-consistent calculation of magnetic fields. The fields are of relatively small strength not affecting the Thomson scattering spectra directly but limiting the electron thermal transport in the gas resulting into temperature gradients consistent with the experimental observations. In addition, the ion temperature data show that the stagnation time of the gas plasma on the hohlraum axis, which is driven by the radial inward flowing plasma, is sensitive to the gas fill density and is well described by the calculations.
Date: July 22, 1999
Creator: Baldis, H A; Estabrook, K G; Glenzer, S H & Suter, L J
Partner: UNT Libraries Government Documents Department

Exploring the Limits to NIF Capsule Coupling

Description: Our original ignition ''point designs'' (circa 1992) for the National Ignition Facility (NIF) were made energetically conservative to provide margin for uncertainties in laser absorption, x-ray conversion efficiency and hohlraum-capsule coupling. Since that time, extensive experiments on Nova and Omega and their related analysis indicate that NIF coupling efficiency may be almost ''as good as we could hope for''. Given close agreement between experiment and theory/modeling, we can credibly explore target enhancements which couple more of NIF's energy to an ignition capsule. We find that 3-4X increases in absorbed capsule energy appear possible, providing a potentially more robust target and {approx}10X increase in capsule yield.
Date: November 3, 1999
Creator: Suter, L.; Rothenberg, J.; Munro, D.; Van Wonterghem, B. & Haan, S.
Partner: UNT Libraries Government Documents Department

Green (2(omega)) Laser Beam propagation in high-temperature Hohlraum Plasmas

Description: We demonstrate propagation and small backscatter losses of a frequency-doubled (2{omega}) laser beam interacting with inertial confinement fusion hohlraum plasmas. The electron temperature of 3.3 keV, approximately a factor of two higher than achieved in previous experiments with open geometry targets, approaches plasma conditions of high-fusion yield hohlraums. In this new temperature regime, we measure 2{omega} laser beam transmission approaching 80% with simultaneous backscattering losses of less than 10%. These findings suggests that good laser coupling into fusion hohlraums using 2{omega} light is possible.
Date: October 26, 2007
Creator: Niemann, C; Berger, R; Divol, L; Froula, D H; Jones, O S; Kirkwood, R K et al.
Partner: UNT Libraries Government Documents Department

Direct Measurements of an increased threshold for stimulated Brillouin scattering with polarization smoothing in ignition hohlraum plasmas

Description: We demonstrate a significant reduction of stimulated Brillouin scattering by polarization smoothing. The intensity threshold is measured to increase by a factor of 1.7 {+-} 0.2 when polarization smoothing is applied. The results were obtained in a high-temperature (T{sub 3} {approx_equal} 3 keV) hohlraum plasma where filamentation is negligible in determining the backscatter threshold. These results are explained by an analytical model relevant to ICF plasma conditions that modifies the linear gain exponent to account for polarization smoothing.
Date: November 8, 2007
Creator: Froula, D; Divol, L; Berger, R L; London, R; Meezan, N; Neumayer, P et al.
Partner: UNT Libraries Government Documents Department

Enchanced hohlraum radiation drive through reduction of wall losses with high-Z mixture "cocktail" wall materials

Description: We present results from experiments, numerical simulations and analytic modeling, demonstrating enhanced hohlraum performance. Care in the fabrication and handling of hohlraums with walls consisting of high-Z mixtures (cocktails) has led to our demonstration, for the first time, of a significant increase in radiation temperature (up to +7eV at 300 eV) compared to a pure Au hohlraum, in agreement with predictions and ascribable to reduced wall losses. The data extrapolated to full NIF suggest we can expect an 18% reduction in wall loss for the current ignition design by switching to cocktail hohlraums, consistent with requirements for ignition with 1MJ laser energy.
Date: May 17, 2006
Creator: Schein, J; Jones, O; Rosen, M; Dewald, E; Glenzer, S; Gunther, J et al.
Partner: UNT Libraries Government Documents Department

Experimental basis for laser-plasma interactions in ignition hohlraums at the National Ignition Facility

Description: A series of laser plasma interaction experiments at OMEGA (LLE, Rochester) using gas-filled hohlraums shed light on the behavior of stimulated Raman scattering and stimulated Brillouin scattering at various plasma conditions encountered in indirect drive ignition designs. We present detailed experimental results that quantify the density, temperature, and intensity thresholds for both of these instabilities. In addition to controlling plasma parameters, the National Ignition Campaign relies on optical beam smoothing techniques to mitigate backscatter. We show that polarization smoothing is effective at controlling backscatter. These results provide an experimental basis for forthcoming experiments on National Ignition Facility.
Date: November 12, 2009
Creator: Froula, D H; Divol, L; London, R A; Berger, R L; Doeppner, T; Meezan, N B et al.
Partner: UNT Libraries Government Documents Department

Demonstration of Enhanced Radiation Drive in Hohlraums Made with High-Z Mixture "Cocktail" Wall Material

Description: We present results from experiments, numerical simulations and analytic modeling, demonstrating enhanced hohlraum performance. Care in the fabrication and handling of hohlraums with walls consisting of high-Z mixtures (cocktails) has led to our demonstration, for the first time, of a significant increase in radiation temperature compared to a pure Au hohlraum that is in agreement with predictions and is ascribable to reduced wall losses. This data suggests that a NIF ignition hohlraum made of a U:Au:Dy cocktail should have {approx}17% reduction in wall losses compared to a similar gold hohlraum.
Date: February 6, 2007
Creator: Schein, J; Jones, O; Rosen, M; Dewald, E; Glenzer, S; Gunther, J et al.
Partner: UNT Libraries Government Documents Department

X-ray Conversion Efficiency of high-Z hohlraum wall materials for indirect drive ignition

Description: We measure the conversion efficiency of 351 nm laser light to soft x-rays (0.1-5 keV) for Au, U and high Z mixtures 'cocktails' used for hohlraum wall materials in indirect drive ICF. We use spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates to achieve constant and uniform laser intensities of 10{sup 14} and 10{sup 15} W/cm{sup 2} over the target surface that are relevant for the future ignition experiments on NIF. The absolute time and spectrally-resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses is subtracted. After {approx}0.5 ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 10{sup 14} W/cm{sup 2} laser intensity and of 80% at 10{sup 15} W/cm{sup 2}. The M-band flux (2-5 keV) is negligible at 10{sup 14} W/cm{sup 2} reaching {approx}1% of the total x-ray flux for all target materials. In contrast, the M-band flux is significant and depends on the target material at 10{sup 15} W/cm{sup 2} laser intensity, reaching values between 10% of the total flux for U and 27% for Au. Our LASNEX simulations show good agreement in conversion efficiency and radiated spectra with data when using XSN atomic physics model and a flux limiter of 0.15, but they underestimate the generated M-band flux.
Date: February 22, 2008
Creator: Dewald, E; Rosen, M; Glenzer, S H; Suter, L J; Girard, F; Jadaud, J P et al.
Partner: UNT Libraries Government Documents Department