Hohlraum Temperature Research Articles

Establishment and analysis of numerical model of deuterium fuel ice migration in indirect-drive cryogenic target

A cryogenic target is the core component in inertial confinement fusion. For meeting the experimental requirements, the uniformity of the fuel ice layer in the capsule needs to reach 99% at least. The capsule has a nonuniform temperature distribution natively resulting from the columnar hohlraum geometry, leading to a nonuniform fuel ice layer profile. A proper temperature distribution along the hohlraum can be used to improve the uniformity of the fuel ice layer. A numerical model that can predict the kinetics of the fuel solid-vapor interface during the migration process is proposed by coupling the heat transfer and flow equations with the Stefan condition. This numerical model has good calculational accuracy verified with the pure-ice sublimation experiment. By studying different hohlraum temperatures and fuel ice layer initial eccentricities, the basic characteristic of ice immigration has been figured out. For the manufacture process, a temperature control scheme has been proposed by which the uniformity of the fuel ice layer can reach 99.57%. For the retention process, a perfectly uniform fuel ice layer can be maintained for 2000s before it cannot meet the ignition standard. These results are of significant guidance on cryogenic target experiments.

Investigation of various methods for wall loss reduction in Inertial Confinement Fusion hohlraums

In this paper, different techniques are explored to reduce wall loss in Inertial Confinement Fusion (ICF) hohlraums. Along with the traditional high-Z element gold, uranium has also been used as the hohlraum wall material as it has higher opacity compared to Au at higher temperatures. Further, opacity is found to increase on adding other high-Z elements like gold, lead and tungsten to uranium in some specific proportions. For the densities and temperatures encountered in reemission zones, cocktails (U0.7Au0.3, U0.8Pb0.2 and U0.7W0.3) show the highest opacity enhancement. Radiation hydrodynamic (RHD) simulations are performed on the planar foils made up of above mentioned hohlraum wall materials by using tabular Equation of State (EOS) and opacity data. Significant reduction in wall loss occurs for the cocktails compared to pure Au on using a constant, time dependent temperature drive (obtained from constant 1 ns laser pulse) and a generic NIF ignition drive. Our work also quantitatively estimates the reduction in wall loss for both elements and cocktails on reducing their initial densities as a result of lower hydrodynamic losses. By combining the effects of both material mixing and density reduction, we have reported wall loss lowering as much as ∼ 24%. Wall loss scaling relations have also been obtained for high-Z elements and cocktails for constant and time dependent temperature drives. Resulting scaling relations are further used to obtain the temperature in empty hohlraum driven by constant power 1 ns laser pulse using hohlraum energy balance model. Hohlraum temperatures are found to increase by ∼ 9 eV for uranium and ∼ 11 eV for the cocktails compared to pure gold for 60 TW laser power. These results will prove to be useful in choosing the ICF hohlraum wall material.

Implosion shape control of high-velocity, large case-to-capsule ratio beryllium ablators at the National Ignition Facility

Experiments at the National Ignition Facility (NIF) show that the implosion shape of inertial confinement fusion ablators is a key factor limiting performance. To achieve more predictable, shape tunable implosions, we have designed and fielded a large 4.2 case-to-capsule ratio target at the NIF using 6.72 mm diameter Au hohlraums and 1.6 mm diameter Cu-doped Be capsules. Simulations show that at these dimensions during a 10 ns 3-shock laser pulse reaching 275 eV hohlraum temperatures, the plasma flow from the hohlraum wall and ablator is not significant enough to impede beam propagation. Experiments measuring the shock symmetry and in-flight shell symmetry closely matched the simulations. Most notably, in two experiments, we demonstrated symmetry control from negative to positive Legendre P2 space by varying the inner to total laser power cone fraction by 5% below and above the predicted symmetric value. Some discrepancies found in 1st shock arrival times that could affect agreement in late time implosion symmetry suggest hohlraum and capsule modeling uncertainties do remain, but this target design reduces sensitivities to them.

Experimental study on improving hohlraum wall reemission ratio by low density gold foam

It is important to improve the hohlraum radiation temperature for the research of high energy density physics, especially for study of inertial confinement fusion. Increasing the wall reemission ratio is an effective way to improve the temperature. It is found in theory that low density foam could reduce hohlraum wall energy loss, and then increase hohlraum temperature. In previous studies, experiments have shown that laser-to-X-ray conversion is enhanced by Au foam. However, improving reemission ratio is more important to increase hohlraum radiation temperature, because most of energy is lost in the wall.In this paper, we report our experiments carried out on SGⅢ prototype to compare the X-ray flux reemitted by Au foam and that by Au. For the experimental design, Au solid and Au foam are irradiated symmetrically along the axis by hohlraum radiation source Tr(t), which is assessed by broadband X-ray spectrometer flat-response X-ray diodes. The measured peak temperature is about 190 eV. Reemission flux from sample is measured by transmission grating spectrometer (TGS). The space-resolved image for pure Au sample shows that the hohlraum radiation is asymmetrical along the axis in the experimental conditions, temperature of top is higher than that at the bottom, which is consistent with simulation results obtained by using IRAD3D code. In order to compare the reemission flux from Au solid sample and that from Au foam sample in same conditions, we need to correct the symmetry of hohlraum radiation. By multiplying the ratio of top flux to bottom flux in pure Au target by the bottom flux in Au-Au foam target, where Au foam is on, we make sure that they are ablated by the same radiation source. The calculated results show that X-ray flux is increased by 20% by Au foam of 0.4 g/cc density when the hohlraum temperature is 190 eV. The typical observed time-integrated X-ray reemission spectra for Au solid and Au foam by TGS are also shown. We see that N-band and O-band reemission are clearly enhanced by Au foam, and the O-band reemission is almost the same as M-band reemission. The increased flux concentrates below 1 keV of the soft X-ray emission.The self-similar solution results and MULTI 1D simulation results show that the wall loss energy fraction is saved by Au foam, whose relation to reemission flux can be described by a simple expression. The theoretical solution shows that the emission flux increases about 10%, and the MULTI simulation indicates that the emission flux increases about 6.8%. They are in qualitative agreement with the experiments results. These results show an alluring prospect for Au foam to be used as hohlraum wall.

Numerical investigation on target implosions driven by radiation ablation and shock compression in dynamic hohlraums

In a dynamic hohlraum driven inertial confinement fusion (ICF) configuration, the target may experience two different kinds of implosions. One is driven by hohlraum radiation ablation, which is approximately symmetric at the equator and poles. The second is caused by the radiating shock produced in Z-pinch dynamic hohlraums, only taking place at the equator. To gain a symmetrical target implosion driven by radiation ablation and avoid asymmetric shock compression is a crucial issue in driving ICF using dynamic hohlraums. It is known that when the target is heated by hohlraum radiation, the ablated plasma will expand outward. The pressure in the shocked converter plasma qualitatively varies linearly with the material temperature. However, the ablation pressure in the ablated plasma varies with 3.5 power of the hohlraum radiation temperature. Therefore, as the hohlraum temperature increases, the ablation pressure will eventually exceed the shock pressure, and the expansion of the ablated plasma will obviously weaken the shock propagation and decrease its velocity after propagating into the ablator plasma. Consequently, longer time duration is provided for the symmetrical target implosion driven by radiation ablation. In this paper these processes are numerically investigated by changing drive currents or varying load parameters. The simulation results show that a critical hohlraum radiation temperature is needed to provide a high enough ablation pressure to decelerate the shock, thus providing long enough time duration for the symmetric fuel compression driven by radiation ablation.

Study of implosion dynamics of Z-pinch dynamic hohlraum on the Angara-5-1 facility

The Z-pinch dynamic hohlraum (ZPDH) is one of high-power X-ray sources that has been used in a variety of high energy-density experiments including inertial confinement fusion (ICF) studies. Dynamic hohlraums driven by a 12-mm and a 18-mm-diameter single tungsten wire arrays embedded with a C16H20O6 foam, respectively, exhibit no visible differences in radiation from the axial exit, although the radial radiation is a little higher in a large array. The analysis of the images suggests that the implosion of a large array is quasi-continuous and has a faster imploding velocity, indicating that the large array is matched to the embedded foam and, oppositely, the small array is mismatched. The analysis also shows that the Rayleigh-Taylor instability develops much harder in implosions of a large array, and this leads to a lower hohlraum temperature. The conclusion was drawn that, for the purpose of enhancing the hohlraum temperature, increasing the conversion efficiency of kinetic energy into thermal energy is more important than increasing the kinetic energy from wire plasma.

The NIF: An international high energy density science and inertial fusion user facility

The National Ignition Facility (NIF), a 1.8-MJ/500-TW Nd:Glass laser facility designed to study inertial confinement fusion (ICF) and high-energy-density science (HEDS), is operational at Lawrence Livermore National Laboratory (LLNL). A primary goal of NIF is to create the conditions necessary to demonstrate laboratory-scale thermonuclear ignition and burn. NIF experiments in support of indirect-drive ignition began late in FY2009 as part of the National Ignition Campaign (NIC), an international effort to achieve fusion ignition in the laboratory. To date, all of the capabilities to conduct implosion experiments are in place with the goal of demonstrating ignition and developing a predictable fusion experimental platform in 2012. The results from experiments completed are encouraging for the near-term achievement of ignition. Capsule implosion experiments at energies up to 1.6 MJ have demonstrated laser energetics, radiation temperatures, and symmetry control that scale to ignition conditions. Of particular importance is the demonstration of peak hohlraum temperatures near 300 eV with overall backscatter less than 15%. Important national security and basic science experiments have also been conducted on NIF. Successful demonstration of ignition and net energy gain on NIF will be a major step towards demonstrating the feasibility of laser-driven Inertial Fusion Energy (IFE). This paper will describe the results achieved so far on the path toward ignition, the beginning of fundamental science experiments and the plans to transition NIF to an international user facility providing access to HEDS and fusion energy researchers around the world.

Progress in the indirect-drive National Ignition Campaign

We have carried out precision optimization of inertial confinement fusion ignition scale implosions. We have achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs. Using slower rise pulses to peak power and extended pulses resulted in lower hot-spot adiabat and higher main fuel areal density at about 80% of the ignition goal. Yields are within a factor of 5–6 of that required to initiate alpha dominated burn. It is likely we will require thicker shells (+15–20%) to reach ignition velocity without mixing of ablator material into the hot spot.

The National Ignition Facility: Status and Progress Towards Fusion Ignition

The National Ignition Facility (NIF), the world’s largest and most energetic laser system, built for studying inertial confinement fusion (ICF) and high-energy-density (HED) science, is operational at Lawrence Livermore National Laboratory (LLNL). A primary goal of the early experimental campaign on NIF is to create the conditions necessary to demonstrate laboratory-scale thermonuclear ignition and burn with gain. NIF experiments in support of indirect-drive ignition began late in FY2009 as part of the National Ignition Campaign (NIC) effort to achieve fusion ignition. NIC is a multi-institution partnership between LLNL, General Atomics, Los Alamos National Laboratory, Sandia National Laboratory, and the University of Rochester Laboratory for Energetics (LLE). NIC also includes a variety of collaborators from universities, national laboratories as well as international collaborators. To date, all of the capabilities to conduct implosion experiments are in place with the goal of demonstrating ignition in the laboratory and developing a predictable fusion experimental platform. The results from experiments completed so far are encouraging and show promise for the achievement of ignition. Capsule implosion experiments at energies up to 1.3 MJ have demonstrated laser energetics, radiation temperatures, and symmetry control that scale to ignition conditions. Of particular importance is the demonstration of peak hohlraum temperatures near 300 eV with overall backscatter less than 15%. Important national security and basic science experiments have also been conducted on NIF. Successful demonstration of ignition and net energy gain will be a major step towards demonstrating the feasibility of Inertial Fusion Energy (IFE) and will focus the world’s attention on the possibility of IFE as a carbon-free, practically limitless energy option. This paper describes the unprecedented experimental capabilities of NIF and the results achieved so far on the path toward ignition, for stockpile stewardship, and the beginning of frontier science experiments. The paper will also address plans to transition NIF to a national user facility, providing access for researchers in the international high energy density science field.

New results of Sino-Russian joint Z-pinch experiments

Dynamics of double tungsten-wire array and dynamic hohlraum are investigated on the Angara-5 facility. X-ray power and hohlraum temperature are obtained to be as high as 5.6 TW and 63 eV respectively. Main experimental results together with the design idea are also given in this paper.

Rugby-like hohlraum experimental designs for demonstrating x-ray drive enhancement

A suite of experimental designs for the Omega laser facility [Boehly et al., Opt. Commun. 133, 495 (1997)] using rugby and cylindrical hohlraums is proposed to confirm the energetics benefits of rugby-shaped hohlraums over cylinders under optimal implosion symmetry conditions. Postprocessed Dante x-ray drive measurements predict a 12–17eV (23%–36%) peak hohlraum temperature (x-ray flux) enhancement for a 1ns flattop laser drive history. Simulated core self-emission x-ray histories also show earlier implosion times by 200–400ps, depending on the hohlraum case-to-capsule ratio and laser-entrance-hole size. Capsules filled with 10 or 50atm of deuterium (DD) are predicted to give in excess of 1010 neutrons in two-dimensional hohlraum simulations in the absence of mix, enabling DD burn history measurements for the first time in indirect-drive on Omega. Capsule designs with 50atm of DHe3 are also proposed to make use of proton slowing for independently verifying the drive benefits of rugby hohlraums. Scale-5/4 hohlraum designs are also introduced to provide further margin to potential laser-plasma-induced backscatter and hot-electron production.

Acceleration and deceleration model of indirect drive ICF capsules

A general zero-dimensional modelling of implosion without thermonuclear reactions is presented, for standard indirectly driven capsules. It is not substantially a new theory, but new demonstrations and improvements of existing models. The model is derived directly from the gas dynamics conservation equations written in integral form for fluid domains with variable mass, in effect the whole non-ablated capsule, the hot spot and the dense shell. The necessary approximations which involve global or mean quantities are justifed theoretically and checked by comparisons with numerical simulations. Two different sets of approximations are developed, one for each of the acceleration and the deceleration phases of the implosion. An improved—in the sense that the time variation of the hohlraum temperature is fully taken into account as it is required for high gain capsules—rocket model is proposed for the acceleration phase. With further approximations, it gives the maximum implosion velocity and the initial capsule mass corresponding to a given final capsule mass, in terms of the initial outer deuterium–tritium radius and the maximum hohlraum temperature. For the deceleration phase, the present model gives an analytical solution for the time decrease in the implosion velocity up to stagnation. Assuming the invariance of PVγ for the different media considered—a property only approximately verified—this model defines the state of these mediums in deceleration and at stagnation, in terms of the mean entropy parameters, the capsule mass, the mean implosion velocity at the end of acceleration and the initial gas mass filling the shell. A simple ODE, which can be easily integrated numerically, is derived for the hot spot mass which depends on the heat conduction wave ablating the fuel from the inside. All the numerical coefficients presently involved in the model can be calculated from the EOS, opacities and heat conduction parameters, except for the value of the supplementary energy transferred to the non-ablated capsule during deceleration which has been taken here, for simplicity, from simulations. Results obtained with this modelling are shown to be in reasonable agreement with numerical simulations for several different capsules.

Time-resolved characterization of Hohlraum radiation temperature via interferometer measurement of quartz shock velocity

A new technique for time-resolved measurement of Hohlraum radiation temperature has been successfully tested in Hohlraums with radiation temperatures in the range of 90–170eV. In these experiments, Hohlraum radiation fields produced ablatively driven shock waves in quartz samples. A line-imaging velocity interferometer was used to track the quartz shock velocity as a function of time, and an empirical relationship (determined in these experiments) was used to relate the measured shock velocity to the Hohlraum radiation temperature. The test experiments were performed at the Omega facility [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] at the University of Rochester Laboratory for Laser Energetics. The technique should also be useful for Hohlraum temperature measurements at other DOE/NNSA high energy density experimental facilities, such as the Z facility [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories and the National Ignition Facility [E. I. Moses, Fusion Sci. Technol. 44, 11 (2003)] at Lawrence Livermore National Laboratory.

Diagnosed internal temperatures and shock evolution provide insight on dynamic-Hohlraum’s axial radiation production and asymmetry

Measurements and analyses [J. P. Apruzese et al., Phys. Plasmas 12, 012705 (2005)] of Al and MgK-shell lines from tracer layers symmetrically embedded in cylindrical dynamic-Hohlraum (DH) targets, driven by two nested tungsten-wire arrays in a z pinch, suggest that the radiation temperatures near either end of top or bottom radiation exit holes (REHs) of the DHs are similar. Moreover, the measured radii inferred for the shock developed within the targets converge towards the z axis symmetrically when viewed simultaneously through either end of the Hohlraums. These two results support the earlier observation [T. W. L. Sanford et al., Phys. Plasmas 12, 022701 (2005)] that the anticorrelation of the axial power with the magnitude of tungsten-wire material flowing near (or across) the given REH is due to increased tungsten opacity at the REH. This mechanism appears to be dominant in affecting the top-bottom (anode-cathode) symmetry in axial power, rather than there being any significant up-down difference in Hohlraum temperature or shock development. Additionally, we show that the insertion of two thin annular pedestals extending into the anode-cathode gap from either electrode, just radially outside of the REHs, improves the up-down power symmetry, decreases the rise time of the axial radiation, and decreases the shot-to-shot variation in the radiation pulse shape, and shock velocity. These improvements suggest that the quality of the plasma shell, which forms within the central region of the implosion, is superior to that adjacent to either electrode. Finally, enhanced emission on axis is observed, prior to the arrival of the main mass-driven shock from the impact of the wire arrays on the target. This phenomenon is consistent with the existence of a radiation-driven shock in the foam target which calculations indicate forms from radiation generated when the outer wire-array plasma impacts the inner array of the nest.

Acceleration phase and improved rocket model for indirectly driven capsules

The system of differential equations for the non-ablated mass, the average implosion velocity, and the ablation radius of an indirectly driven capsule in acceleration phase, has been obtained from conservation principles of hydrodynamics. Two phases are distinguished during acceleration, according to the uniformity of the velocity in the non-ablated shell. The results of the integration of this system are well compared with numerical simulation of optimized capsules. Assuming that the ablation pressure depends only on the Hohlraum temperature, the relations between the non-ablated mass, the implosion velocity, and the ablation radius are obtained for optimized temperature shape. These relations provide the maximum implosion velocity and the remaining non-ablated mass in terms of the initial capsule and the maximum temperature (or the initial capsule mass in terms of the remaining non-ablated mass) useful to determine the required ablator thickness for optimized capsules. These results are also compared with numerical simulations of different capsules.

Experimental and Theoretical Characterization of the Actively Controlled Thermal Environment in a Cryogenic Hohlraum

We investigate the actively controlled thermal environment of a National Ignition Facility (NIF) cryogenic hohlraum using measurements and calculations of hydrogen ice equilibrium. Thermal modeling of the hohlraum temperature field is shown to be consistent with measurements of the ice distribution. These results demonstrate the ability to accurately model and control the thermal environment in an NIF hohlraum.

Dynamics and characteristics of a 215-eV dynamic-hohlraum x-ray source on Z

A radiation source has been developed on the 20-MA Z facility that produces a high-power x-ray pulse, generated in the axial direction primarily from the interior of a collapsing dynamic hohlraum (DH). The hohlraum is created from a solid cylindrical CH2 target centered within an imploding tungsten wire-array Z pinch. Analyses and interpretation of measurements made of the x-ray generation within and radiated from the hohlraum target have been done using radiation-magnetohydrodynamic-code simulations in the r-z plane that take account of the magnetic Rayleigh–Taylor (RT) instability. These analyses suggest that a significantly reduced RT seed (relative to that used to explain targetless Z-pinch data on Z) is required to explain the observations. Although some quantitative and qualitative agreement with the measurements is obtained with the reduced RT seed, differences remain. Initial attempts to include into the simulations a precursor plasma, arising from wire material driven ahead of the main implosion, did not ameliorate the differences. Modification of the simulated W/CH2 interface may be required to properly explain the measured axial radiation pulse. This pulse, which exits a 4.5-mm2 hole centered above the target, begins ∼5 ns prior to stagnation (as defined by peak radial radiation power). The 5-ns interval leading to stagnation represents the duration when the imploding tungsten plasma acts as a hohlraum wall, trapping radiation within the interior of the foam target. The hohlraum radiation exiting the hole at 6 degrees to the z-axis reaches a maximum intensity of 3.1±0.6 TW/str (associated with an average hohlraum temperature of 215±10 eV), 1.4±0.4 ns prior to stagnation. (The uncertainties represent rms shot-to-shot variations.) This radiation pulse, characterized here, is useful for performing radiation-transport experiments with drive temperatures in excess of 200 eV.

Double Z-pinch hohlraum drive with excellent temperature balance for symmetric inertial confinement fusion capsule implosions.

A double Z pinch driving a cylindrical secondary hohlraum from each end has been developed which can indirectly drive intertial confinement fusion capsule implosions with time-averaged radiation fields uniform to 2%-4%. 2D time-dependent view factor and 2D radiation hydrodynamic simulations using the measured primary hohlraum temperatures show that capsule convergence ratios of at least 10 with average distortions from sphericity of <Delta r>/r<or=30% are possible on the Z accelerator and may meet radiation symmetry requirements for scaling to fusion yields of >200 MJ.

Indirect-drive noncryogenic double-shell ignition targets for the National Ignition Facility: Design and analysis

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, indicates a possibility of achieving ignition, i.e., fusion yields greater than 1 MJ. Planned experiments on the Omega laser will test current understanding of high-energy radiation flux asymmetry and mix-induced yield degradation in double-shell targets.

Development and characterization of a Z-pinch-driven hohlraum high-yield inertial confinement fusion target concept

Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling (“hohlraum energetics”) is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of &amp;gt;200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)].