Search

Self-induced plasma gratings are now routinely used in inertial confinement fusion experiments at the National Ignition Facility (NIF) to achieve precise spatio-temporal control of the laser energy deposition directly inside the fusion targets. This novel capability is enabled by applying a few Å wavelength shifts between different groups of beams, in order to control the direction and level of light scattering off these plasma gratings. A new wavelength tuning capability was added on the NIF in January 2020 to extend the range of applications of plasma gratings; such applications include the control of several asymmetry modes during the implosion, and mitigation of backscatter from stimulated Raman or Brillouin scattering.

We report on the first multilocation electron temperature (T_{e}) and flow measurements in an ignition hohlraum at the National Ignition Facility using the novel technique of mid-Z spectroscopic tracer "dots." The measurements define a low resolution "map" of hohlraum plasma conditions and provide a basis for the first multilocation tests of particle and energy transport physics in a laser-driven x-ray cavity. The data set is consistent with classical heat flow near the capsule but reduced heat flow near the laser entrance hole. We evaluate the role of kinetic effects, self-generated magnetic fields, and instabilities in causing spatially dependent heat transport in the hohlraum.

Current inertial confinement fusion experiments on the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)] are attempting to demonstrate thermonuclear ignition using x-ray drive by imploding spherical targets containing hydrogen-isotope fuel in the form of a thin cryogenic layer surrounding a central volume of fuel vapor [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. The fuel is contained within a plastic ablator layer with small concentrations of one or more mid-Z elements, e.g., Ge or Cu. The capsule implodes, driven by intense x-ray emission from the inner surface of a hohlraum enclosure irradiated by the NIF laser, and fusion reactions occur in the central hot spot near the time of peak compression. Ignition will occur if the hot spot within the compressed fuel layer attains a high-enough areal density to retain enough of the reaction product energy to reach nuclear reaction temperatures within the inertial hydrodynamic disassembly time of the fuel mass ...

The National Ignition Campaign (NIC) on the National Ignition Facility plans to use an indirectly driven spherical implosion to assemble and ignite a mass of D-T fuel. The NIC is currently in the p...

We experimentally demonstrate that application of laser smoothing schemes including smoothing by spectral dispersion (SSD) and polarization smoothing (PS) increases the intensity range for efficient coupling of frequency doubled (527 nm) laser light to a long scalelength plasma with n c /n er = 0.14 and T e = 2 keV.

Experiments show that application of laser smoothing schemes including smoothing by spectral dispersion and polarization smoothing effectively suppresses stimulated Raman scattering from a 2ω (527 nm) laser beam in a low average-gain plasma with a steep density gradient. Full-wave simulations reproduce the observed trends in the data and show that the scattering reduction is an indirect result of suppressing active filamentation. © 2009 American Institute of Physics.

A demonstration of the ability to control the flow of laser energy in a dense plasma by tuning the colour of multiple laser beams injected into it could be useful in the development of laser-driven fusion.

Targets intended to produce ignition on the National Ignition Facility (NIF) are being simulated, and the simulations are used to set specifications for target fabrication. Recent design work has focused on incorporating the implications of NIF experiments that were done in fall 2009 and planning for the campaign in 2010. Near-term experiments will use Ge-doped CH, although Be and diamond are still under active consideration for 2011 and beyond. The emphasis in this paper will be on changes in the requirements over the last year, the characteristics of the 2010 CH-ablator design, and the designs for 2011 and beyond. Capsule defects of particular interest are surface perturbations on the CH ablator and composition variations in the Be shells. Complete tables of specifications are regularly updated for all of the targets. All the specifications are rolled together into an error budget indicating adequate margin for ignition with all of the designs.

Experiments demonstrate the ~77× amplification of 0.5 to 3.5-ps pulses of seed light by interaction with Langmuir waves in a low density (1.2 × 1019 cm−3) plasma produced by a 1-ns, 230-J, 1054-nm pump beam with 1.2 × 1014 W/cm2 intensity. The waves are strongly damped (kλD = 0.38, Te = 244 eV) and grow over a ~ 1 mm length, similar to what is experienced by scattered light when it interacts with crossing beams as it exits an ignition target. The amplification reduces when the seed intensity increases above ~1 × 1011 W/cm2, indicating that saturation of the plasma waves on the electron kinetic time scale (

Ignition Set to Go One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys ). Glenzer et al. (p. 1228 , published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231 , published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings.

Targets intended to produce ignition on the National Ignition Facility are being simulated, and the simulations are used to set specifications for target fabrication. Recent design work has focused...

With the next generation of high-power laser facilities for inertial fusion coming online1,2, ensuring laser beam propagation through centimetre-scale plasmas is a key physics issue for reaching ignition. Existing experimental results3,4,5 including the most recent one6 are limited to small laser spots, low-interaction laser beam energies and small plasma volumes of 1–2 mm. Here, we demonstrate the propagation of an intense, high-energy, ignition-size laser beam through fusion-size plasmas on the National Ignition Facility (NIF) and find the experimental measurements to agree with full-scale modelling. Previous attempts to apply computer modelling as a predictive capability have been limited by the inherently multiscale description of the full laser–plasma interaction processes7,8,9,10,11. The findings of this study validate supercomputer modelling as an essential tool for the design of future ignition experiments.

Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication and other program elements. Recent design work has focused on designs that assume only 1.0 MJ of laser energy instead of the previous 1.6 MJ. To perform with less laser energy, the hohlraum has been redesigned to be more efficient than previously, and the capsules are slightly smaller. Three hohlraum designs are being examined: gas fill, SiO2 foam fill, and SiO2 lined. All have a cocktail wall, and shields mounted between the capsule and the laser entrance holes. Two capsule designs are being considered. One has a graded doped Be(Cu) ablator, and the other graded doped CH(Ge). Both can perform acceptably with recently demonstrated ice layer quality, and with recently demonstrated outer surface roughness. Complete tables of specifications are being prepared for both targets, to be completed this fiscal year. All the specifications are being rolled together into an error budget indicating adequate margin for ignition with the new designs. The dominant source of error is hohlraum asymmetry at intermediate modes 4–8, indicating the importance of experimental techniques to measure and control this asymmetry.

Targets intended to produce ignition on NIF are being simulated and the simulations used to set specifications for target fabrication. Recent design work has focused on refining the designs that use 1.0 MJ of laser energy, with ablators of Be(Cu), CH(Ge), and diamond-like C. The main-line hohlraum design now has a He gas fill, a wall of U-Au layers, and no shields as were formerly used between the capsule and the laser entrance holes. The emphasis in this presentation will be on changes in the requirements over the last year, and on the characteristics of the diamond-ablator design. Complete tables of specifications have been prepared for all of the targets. All the specifications are rolled together into an error budget indicating adequate margin for ignition with all of the designs.

Deposition of maximum laser energy into a small, high-Z enclosure in a short laser pulse creates a hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technology 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, the Raman backscatter spectrum contains features consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Physical Review Letters 89, 015003 (2002)]. Also, NIF Early Light diagnostics indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light.

Several experiments using either CO 2 or propane gas filled halfraums [i.e. hohlraums with a single laser entry hole (LEH)] have been shot at the National Ignition Facility (NIF) in a joint LosAlamos/Livermore collaboration. The experiments have been modeled by the Lasnex code. The possibility of beam spray due to filamentation of the incident laser beam is assessed through simulations which parametrically decrease the f-number of the beam at times of high intensity. The uncertainty in heat transport is evaluated through parametric variations in the electron thermal flux limit (fe). Each calculation in the resulting two parameter set is post-processed to simulate outputs which can be compared with Dante detector results for the soft X-ray flux through the LEH, and gated, framed images of hard X-rays (FXI) through the hohlraum side walls. Simulations which well match the data for both gases indicate that the laser energy is penetrating the gas filled hohlraum even towards the end of the pulse. This suggests that the gas fill is useful in keeping the hohlraum open to laser energy throughout the pulse.

Gas filled halfraum experiments were conducted at the National Ignition Facility which provided an excellent test of the tools needed to understand halfraum energetics in an ignition relevant regime. The experiments used a highly shaped laser pulse and measured large levels of backscattered laser energy. These two components challenge the ability of radiation hydrodynamic simulations to model the experiments. The results show good agreement between experimental measurements and simulations.

An experimental campaign to maximize radiation drive in small-scale hohlraums has been carried out at the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA, USA) and at the OMEGA laser at the Laboratory for Laser Energetics (Rochester, NY, USA). The small-scale hohlraums, laser energy, laser pulse, and diagnostics were similar at both facilities but the geometries were very different. The NIF experiments used on-axis laser beams whereas the OMEGA experiments used 19 beams in three beam cones. In the cases when the lasers coupled well and produced similar radiation drive, images of x-ray bumthrough and laser deposition indicate the pattern of plasma filling is very different.

Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication. Recent design work has focused on designs that assume only...

Laser-Produced plasmas at subcritical densities have proven to be efficient sources for X-ray production. In this context, we obtain experimental results from Kr and Xe gas-filled targets that were irradiated by the OMEGA (Laboratory for Laser Energetics, University of Rochester) laser. Nearly 40% of the laser energy was converted into X-rays in the L-shell-photon-energy range (≥ 1.6 keV) by a Kr-filled target. The conversion efficiency measurements were correlated with time-resolved plasma-temperature measurements done by means of a Thomson-scattering diagnostic. The measured range of temperatures, between 2-3.5 keV, is in good agreement with LASNEX radiation-hydrodynamics simulations. X-ray-cooling rates and charge-state distributions were computed using detailed atomic data from the HULLAC suite of codes. X-ray yields predicted by the cooling-rate calculations are compared to measured spectra, and good agreement is found for predictions made with highly-detailed atomic models. We find that X-ray conversion efficiency in Kr-filled targets is a strong function of temperature, and has an optimum density near 15% of the laser's critical density. © 2005 Elsevier Ltd. All rights reserved.

Recently the first laser–plasma interaction and hohlraum experiments have been performed at the National Ignition Facility (NIF) in support of indirect drive inertial confinement fusion designs. The effects of laser beam smoothing by spectral dispersion and polarization smoothing on the intense (2 × 1015 W cm−2) beam propagation in gas-filled tubes has been studied at up to 7 mm plasma scales as found in indirect drive gas filled ignition hohlraum designs. These experiments have shown the expected full propagation without filamentation and beam break up when using full laser smoothing. In addition, vacuum hohlraums have been irradiated with laser powers up to 6 TW, 1–9 ns pulse lengths and energies up to 17 kJ to activate several diagnostics, to study the hohlraum radiation temperature scaling with the laser power and hohlraum size, and to make contact with hohlraum experiments performed at the Nova and Omega laser facilities. Subsequently, novel long laser pulse hohlraum experiments have tested models of hohlraum plasma filling and long pulse hohlraum radiation production. The validity of the plasma filling assessment using in analytical models and radiation hydrodynamics calculations with the code LASNEX has been proven in these studies. The comparison of these results with modelling will be discussed.

Recent 2ω laser propagation and stimulated Raman backscatter (SRS) experiments performed on the Helen laser have been analyzed using the radiation-hydrodynamics code HYDRA [M. M. Marinak, G. D. Kerbel, N. A. Gentile, O. Jones, D. Munro, S. Pollaine, T. R. Dittrich, and S. W. Haan, Phys. Plasmas 8, 2275 (2001)]. These experiments utilized two diagnostics sensitive to the hydrodynamics of gasbag targets: a fast x-ray framing camera (FXI) and a SRS streak spectrometer. With a newly implemented nonlocal thermal transport model, HYDRA is able to reproduce many features seen in the FXI images and the SRS streak spectra. Experimental and simulated side-on FXI images suggest that propagation can be explained by classical laser absorption and the resulting hydrodynamics. Synthetic SRS spectra generated from the HYDRA results reproduce the details of the experimental SRS streak spectra. Most features in the synthetic spectra can be explained solely by axial density and temperature gradients. The total SRS backscatt...

The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2 × 1015 W cm−2. The targets were filled with 1 atm of CO2 producing up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 1020 cm−3 and temperatures of Te = 2 keV. The high energy in an NIF quad of beams of 16 kJ, illuminating the target from one direction, creates unique conditions for the study of laser–plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keV x-rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ~1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (~2 mm), increasing to 10–12% for ~7 mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modelling of the laser–plasma interactions at ignition-size scale lengths.

Studies on simulation and design of ignition targets for the National Ignition Facility (NIF) are described. Recent effort has emphasized the systematic exploration of the parameter space of possible ignition targets, providing comparisons as specific as possible between the various targets. This study aims at providing guidance for target fabrication R&D, and for other elements of the ignition program. Targets are being considered that span 250–350 eV drive temperatures, capsule energies from 150 to 600 kJ, cocktail and gold hohlraum spectra, and three ablator materials (Be[Cu], CH[Ge] and polyimide). Capsules with graded doped beryllium ablators are found to be very stable with respect to short-wavelength Rayleigh–Taylor growth. Sensitivity to ablator roughness, ice roughness and asymmetry is being explored, as it depends on ablator material, drive temperature and absorbed energy. Three-dimensional simulations are being used to ensure adequate radiation symmetry in three dimensions (3D), and to ensure that coupling of 3D asymmetry and 3D Rayleigh–Taylor does not adversely affect planned performance. Integrated 3D hohlraum simulations indicate that 3D features in the laser illumination pattern affect the hohlraums' performance, and the hohlraum has been redesigned to accommodate these effects.

For several years we have been exploring the possibility of using green (2ω) light for indirect drive ignition on National Ignition Facility (NIF). This paper is a comprehensive review of our progress in this investigation and was the subject of a Teller lecture when one of the authors (LJS) was honoured with the Edward Teller Medal at the IFSA03 conference on September 12th, 2003, at Monterey, CA. While much of the work presented here has been previously published (Suter L.J. et al 2004 Phys. Plasmas 11 2738) and is included for completeness of the review, this paper also includes new research examining the possibility of higher temperature (300 eV) ignition hohlraums driven by green light (section 7).

A copper doped beryllium ablator capsule design is geometrically scaled from 190 kJ to 600 kJ absorbed energy for use as an ignition capsule driven at 250 eV on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. The capsule design was previously optimized for 190 kJ fixed capsule absorbed energy. The optimization is confirmed at 377 kJ. Two-dimensional simulations are reported that determine surface roughness requirements and tolerance to radiative drive asymmetry over this absorbed energy range.

The National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], operating at green (2ω) light, has the potential to drive ignition targets with significantly more energy than the 1.8 MJ it will produce with its baseline, blue (3ω) operations. This results in a greatly increased “target design space,” providing a number of exciting opportunities for fusion research. These include the prospect of ignition experiments with capsules absorbing energies in the vicinity of 1 MJ. This significant increase in capsule absorbed energy over the original designs at ∼150 kJ could allow high-gain, high yield experiments on NIF. This paper reports the progress made exploring 2ω for NIF ignition, including potential 2ω laser performance, 2ω ignition target designs, and 2ω laser plasma interaction studies.

Indirect drive ignition target simulations are described as they are used to determine target fabrication specifications. Simulations are being used to explore options for making the targets more robust, and to develop more detailed understanding of the performance of a few point designs. The current array of targets is described. A new target is described with radially dependent Cu dopant in Be. This target has significantly looser specifications for high-mode perturbations than previous targets. Current estimates of size limitations for fill tubes, holes, and isolated defect are discussed. Recent 3D simulations are described.

The 1990 National Academy of Science final report of its review of the Inertial Confinement Fusion Program recommended completion of a series of target physics objectives on the 10-beam Nova laser at the Lawrence Livermore National Laboratory as the highest-priority prerequisite for proceeding with construction of an ignition-scale laser facility, now called the National Ignition Facility (NIF). These objectives were chosen to demonstrate that there was sufficient understanding of the physics of ignition targets that the laser requirements for laboratory ignition could be accurately specified. This research on Nova, as well as additional research on the Omega laser at the University of Rochester, is the subject of this review. The objectives of the U.S. indirect-drive target physics program have been to experimentally demonstrate and predictively model hohlraum characteristics, as well as capsule performance in targets that have been scaled in key physics variables from NIF targets. To address the hohlrau...

The role of a high-Z radiation cavity or hohlraum in inertial confinement fusion is to convert laser energy into soft x-ray energy, in a highly spatially symmetric manner, so that a centrally located capsule containing deuterium and tritium can be uniformly imploded. In practice, however, the asymmetry introduced by the small number of high intensity laser beams can introduce significant perturbations in the drive uniformity. Experiments performed on Nova (10 beams) [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] and Omega (using 40 beams) [J. M. Sources, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] demonstrate a significant improvement in symmetry and target performance from a fourfold increase in the number of laser beams.

X-ray sources are created at the Nova and Omega laser by irradiating a confined volume of Ar and Xe gas. The gas is heated by 20–35 kJ of 0.35 μm laser light and becomes a highly ionized mm-sized x-ray source which emits K-shell or L-shell x rays. The radiator is “underdense,” meaning that the initial electron density is lower than the critical density of the laser, nc∼1022 cm−3. It is heated primarily by inverse bremsstrahlung, which produces a supersonic ionization wave. In this paper, x-ray conversion efficiency and imaging from time-resolved and time-integrated diagnostics are compared over a range of experimental parameters. This work represents an important, new method for development of efficient, large-area, tailored multi-keV x-ray sources.

In experiments at the Omega laser facility, x-ray framing and streak cameras were used to explore a technique for simultaneously measuring the relative x-ray burnthrough and re-emission properties of pure Au and high-Z mixture “cocktail” foils exposed to a Hohlraum radiation field. For the Au:Dy:Nd cocktail used in these preliminary experiments, the burnthrough measurements indicated a cocktail opacity ∼1.5 times that of pure Au. The x-ray re-emission fluxes from the cocktail and Au appeared to be equivalent. In the future, we propose to use this experimental arrangement to compare the relative x-ray burnthrough and re-emission properties of other potential wall materials proposed for use in National Ignition Facility Hohlraums.

This paper is a general update on the target fab specs for ignition targets for the National Ignition Facility. A general overview of the status of all the requirements is presented, with a histori...

Analysis and design of indirect-drive National Ignition Facility double-shell targets with hohlraum temperatures of 200 eV and 250 eV are presented. The analysis of these targets includes the assessment of two-dimensional radiation asymmetry and nonlinear mix. Two-dimensional integrated hohlraum simulations indicate that the x-ray illumination can be adjusted to provide adequate symmetry control in hohlraums specially designed to have high laser-coupling efficiency [Suter et al., Phys. Plasmas 7, 2092 (2000)]. These simulations also reveal the need to diagnose and control localized 10–15 keV x-ray emission from the high-Z hohlraum wall because of strong absorption by the high-Z inner shell. Preliminary estimates of the degree of laser backscatter from an assortment of laser–plasma interactions suggest comparatively benign hohlraum conditions. The application of a variety of nonlinear mix models and phenomenological tools, including buoyancy-drag models, multimode simulations and fall-line optimization, in...

Several techniques for inferring the degree of flux symmetry in indirectly driven cylindrical hohlraums have been developed over the past several years for eventual application to the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. These methods use various ignition capsule surrogates, including non-cryogenic imploded capsules [Hauer et al., Phys. Plasmas 2, 2488 (1995)], backlit aerogel foamballs [Amendt et al., Rev. Sci. Instrum. 66, 785 (1995)], reemission balls [Delamater, Magelssen, and Hauer, Phys. Rev. E 53, 5240 (1996)], and backlit thinshells [Pollaine et al., Phys. Plasmas 8, 2357 (2001)]. Recent attention has focussed on the backlit thinshells as a promising means for detecting higher-order Legendre flux asymmetries, e.g., P6 and P8, which are predicted to be important sources of target performance degradation on the NIF for levels greater than 1% [Haan et al., Phys. Plasmas 2, 2490 (1995)]. A key property of backlit thinshells is the strong amplification of modal flux asymmetry imprinting with shell convergence. A simple single-parameter analytic description based on a rocket model is presented which explores the degree of linearity of the shell response to an imposed flux asymmetry. Convergence and mass ablation effects introduce a modest level of nonlinearity in the shell response. The effect of target fabrication irregularities on shell distortion is assessed with the rocket model and particular sensitivity to shell thickness variations is shown. The model can be used to relate an observed or simulated backlit implosion trajectory to an ablation pressure asymmetry history. Ascertaining this history is an important element for readily establishing the degree of surrogacy of a symmetry target for a NIF ignition capsule.

A necessary condition for igniting indirectly driven inertial confinement fusion (ICF) capsules on the National Ignition Facility (NIF) is controlling drive asymmetry to the 1% level [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. Even flux-asymmetry modes (e.g., Legendre modes P2, P4, P6, and P8) must be reduced by hohlraum design and laser beam pointing. Odd flux-asymmetry modes (e.g., Legendre modes P1, P3, P5, etc.) are theoretically removed by reflection symmetry across the hohlraum midplane [S. M. Pollaine and D. Eimerl, Nucl. Fusion 38, 1523 (1998)], but will be produced by power imbalance, laser beam pointing errors, and target fabrication errors. An experimental campaign is now being conducted on the University of Rochester’s Omega laser to measure higher order (P4 and higher) flux asymmetry modes inside hohlraums that approximate the conditions of a NIF hohlraum during the 90 eV early drive phase [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 24...

Recent changes in the manner of performing hohlraum drive experiments have significantly advanced the ability to diagnose, understand and control the x-radiation flux (or drive) inside a laser heated hohlraum. Comparison of modeling and data from a very broad range of hohlraum experiments indicates that radiation hydrodynamics simulation codes reproduce measurements of time dependent x-radiation flux to about ±10%. This, in turn, indicates that x-ray production and capsule coupling in ignition hohlraums will be very close to expectations. This article discusses the changes to experimental procedures and the broad variety of measurements and tests leading to these findings.

Measurements of radiation temperatures from empty and gas-filled hohlraums heated at the Nova Laser Facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] show efficient coupling of the laser power to the target when applying laser beam smoothing techniques. Scattering losses are reduced to the 3% level while the radiation temperatures increased by ∼15 eV for smoothed laser beams. The experimental findings and supporting calculations indicate that filamentation and gain for stimulated Raman and Brillouin scattering is suppressed in the hohlraum plasma for smoothed laser beams. The scaling of the radiation temperature is well described by integrated radiation hydrodynamic LASNEX modeling [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 85 (1975)] following the Marshak scaling. Peak radiation temperatures are in excess of 230 eV in gas-filled hohlraums in agreement with the detailed LASNEX modeling.

A new method for performing compressible hydrodynamic instability experiments using high-power lasers is presented. A plasma piston is created by supersonically heating a low-density carbon based foam with x-rays from a gold hohlraum heated to ∼200 eV by a ∼1 ns Nova laser pulse [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)]. The piston causes an almost shockless acceleration of a thin, higher-density payload consisting of a layer of gold, initially 1/2 μm thick, supported on 10 μm of solid plastic, at ∼45 μm/ns2. The payload is also heated by hohlraum x-rays to in excess of 150 eV so that the Au layer expands to ∼20 μm prior to the onset of instability growth. The Atwood number between foam and Au is ∼0.7. Rayleigh–Taylor instability, seeded by the random fibrous structure of the foam, causes a turbulent mixing region with a Reynolds number >105 to develop between piston and Au. The macroscopic width of the mixing region was inferred from the change in Au layer width, which was recorded via tim...

The original ignition “point designs” (circa 1992) for the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] 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 [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] and Omega [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] 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, one can credibly explore target enhancements which couple more of NIFs energy to an ignition capsule. These include using optimized mixtures of materials to reduce x-ray wall losses, slightly reduced laser entrance holes, and laser operation strategies which increase the amount of energy one can extract from NIF. It is found that 3–4× increases in absorbe...

The next generation of large, high power lasers, such as the National Ignition Facility in the U.S., Laser Mega Joule in France or Helen Successor in the United Kingdom offers the prospect of x-ray fluorescence based diagnosis of hydrodynamic experiments. We discuss two promising techniques for pumping this fluorescence. One is to use a large fraction of these facilities’ high power to efficiently make multi-kilovolt x rays which, in turn, causes dopants placed in experimental packages to fluoresce. 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. Here, 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.

An improved understanding of strongly driven laser-plasma coupling is important for optimal use of the National Ignition Facility (NIF) for both inertial fusion and for a variety of advanced applications. Such applications range from high-energy x-ray sources and high-temperature hohlraums to fast ignition and laser radiography. We discuss a novel model for the scaling of strongly driven stimulated Brillouin and Raman scattering. This model postulates an intensity-dependent correlation length associated with spatial incoherence due to filamentation and stimulated forward scattering. We first describe the model and then relate it to a variety of experiments. Particular attention is paid to high-temperature hohlraum experiments, which exhibit low to modest stimulated Brillouin scattering even though this instability is strongly driven. We also briefly discuss the strongly nonlinear interaction physics for efficient generation of high-energy electrons either by irradiating a large plasma with near quarter-critical density or by irradiating overdense targets with ultra-intense laser light.

Symmetry experiments have been performed on the Omega laser facility using cylindrical hohlraum targets with as many as 40thinspthinspbeams arranged into multiple beam cones. These experiments constitute a first step in the development of {open_quotes}beam phasing{close_quotes} in which beams are arranged into multiple beam cones, forming multiple rings of beam spots on the inner surface of a cylindrical hohlraum, and demonstrate the ability to model hohlraums incorporating multiple beam cones and to tune the time-integrated capsule flux asymmetry by adjustment of the beam pointing. {copyright} {ital 1998} {ital The American Physical Society}

Current plans for time-dependent control of flux asymmetry in the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, and M. Sorem, “The National Ignition Facility Project,” ICF Quart. 5, 110 (1995)] hohlraums rely on multiple beam cones with different laser power temporal profiles in each cone. Experiments with multiple beam cones have begun on the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. In addition to allowing symmetry experiments similar to those performed on Nova [A. Hauer et al., Rev. Sci. Instrum. 66, 672 (1995)], the Omega facility allows multiple beam cones to be moved independently to confirm our ability to model the resulting implosion image shapes. Results indicate that hohlraum symmetry behaves similarly with multiple rings of beams as with a single ring, but with the weighted beam spot position used to parametrize the beam pointing.

Time resolved hard x-ray images (hv > 9 keV) and time integrated hard x-ray spectra (hv = 18-150 keV from vacuum hohlraums irradiated with four 351 nm wavelength NIF laser beams are presented as a function of hohlraum size and laser power and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (f hot ) shows correlation with both laser intensity and with an analytic plasma filling model.

We present the first Thomson scattering measurements of local plasma conditions in ignition-relevant, gas-filled, inertial-confinement-fusion hohlraums. 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. The data are consistent with modeling using significantly inhibited heat transport at the peak of the drive. Further, we find that stagnating plasma regions on the hohlraum axis are well described by the calculations. {copyright} {ital 1997} {ital The American Physical Society}

A novel method to control lowest-order (P2) flux asymmetry in Nova cylindrical hohlraums [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks of varying radii to partially block the capsule view of the laser-entrance holes. Some advantages in using axial disks include the prospect for added drive on target, the potential for P4 control when used in tandem with laser pointing, and possibly reduced time-dependent P2(t) flux asymmetry swings at early time. Neutron-based diagnostics have provided some suggestion of increased drive, but a more direct measure of drive enhancement is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disk placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock speed that enables easier detection of both average shock motio...

The Saturn pulsed power accelerator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] at Sandia National Laboratories (SNL) and the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at Lawrence Livermore National Laboratory (LLNL) have been used to explore techniques for studying the behavior of ablator material in x-ray radiation environments comparable in magnitude, spectrum, and duration to those that would be experienced in National Ignition Facility (NIF) hohlraums [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)]. The large x-ray outputs available from the Saturn pulsed-power-driven z pinch have enabled us to drive hohlraums of full NIF ignition scale size at radiation temperatures and time scales comparable to those required for the low-power foot pulse of an ignition capsule. The high-intensity drives available in t...

In this paper we report the results of experiments that compare the x-ray emission from a laser spot in a radiation-filled hohlraum to that from a similar laser spot on a simple disk target. The studies were done using the Nova laser facility [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)] in its 0.35 μm wavelength, 1 ns square pulse configuration. Focal spot intensities were 2–3.5×1015 W/cm2. X-ray images measured x-ray conversion in a hohlraum and from an isolated disk simultaneously. A laser spot inside a hohlraum emitted more x rays, after subtracting the background emission from the hohlraum walls, than a spot on a disk. Numerical models suggest the enhanced spot emission inside the hohlraum is due to an increase in lateral transport relative to the disk. Filamentation in the hohlraum will also increase the spot size. The models agree fairly well with the results on spot spreading but do not explain the overall increase in conversion efficiency.

High-Z, laser heated hohlraums can be made thick enough to contain thermal radiation, yet thin enough to let out x rays >∼6 keV produced by hot, relatively dense blow-off plasma. We use such “thin wall hohlraums” to observe the physical location of hot, dense, laser produced hohlraum plasmas. This technique has allowed us to come to some understanding of laser transport/deposition, plasma stagnation, and bulk plasma filling.