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We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

The Tritium Science and Engineering (AET-3) Group at Los Alamos National Laboratory (LANL) performs a variety of activities to support Inertial Fusion (IF) research - both to further fundamental fusion science and to develop technologies in support of Inertial Fusion Energy (IFE) power generation. Inertial fusion ignition target designs have a smooth spherical shell of cryogenic Deuterium-Tritium (DT) solid contained within a metal or plastic shell that is a few mm in diameter. Fusion is attained by imploding these shells under the symmetric application of energy beams. For IFE targets the DT solid must also survive the process of injecting it into the power plant reactor. Non-ignition IF targets often require a non-cryogenic DT gas fill of a glass or polymeric shell. In this paper an overview will be given of recent LANL activities to study cryogenic DT layering, observe tritium exposure effects on IF relevant materials, and fill targets in support of IF implosion experiments. (authors)

Solid deuterium-tritium (the symbol DT is used here to represent the equilibrium mixture of 50% deuterium and 50% tritium, having the molecular composition: 25% D 2 , 50% deuterium tritide molecules, and 25% T 2 ) (DT) is nucleated from DT-wettedfoam and subsequently forms a uniform layer by the beta-layering phenomenon. Compared to DT frozen on smooth metal surfaces, the surface roughness of the inner-lying pure DT solid-vapor interface is substantially lower at all modal values higher than ∼10, possibly due to the small-grain-size polycrystalline nature of the solid. For thick layers, deleterious effects are observed, notably the formation of DT-rich vapor voids in the foam matrix and the subsequent propagation of these voids into the pure solid DT layer.

Beta-layering, the process of beta-decay heat-driven mass redistribution, has been demonstrated in a deuterium-tritium (D-T)-filled polymer sphere of the type required for fusion ignition experiments at the National Ignition Facility. This is the first report, to the best of the authors' knowledge, of a D-T layer formed in a permeation-filled sphere. The 2-mm-diam sphere was filled with D-T by permeation; cooled to cryogenic temperatures while in the high-pressure permeation vessel; and, while cold, removed to an optical axis where the D-T was frozen, melted, and beta-layered in a series of experiments over several weeks' time. This work was performed in the Los Alamos National Laboratory cryogenic pressure loader system. The beta-layering time constant was 24.0 {+-} 2.5 min, less than the theoretical value of 26.8 min, and not showing the significant increase due to build-up of {sup 3}He often observed in beta-layered samples. Supercooling of the liquid D-T was observed. Neither the polymer target nor its tenting material showed visual signs of degradation after 5 weeks of exposure to D-T. Small external thermal gradients were used to shift the D-T material back and forth within the sphere.

A central feature of an Inertial Fusion Energy (IFE) power plant is a target that has been compressed and heated to fusion conditions by the energy input of the driver. The technology to economical...

Estimates of the time-to-melt for cryogenic DT inertial fusion targets in the presence of thermal radiation are presented. This time is defined as that required for thermal radiation in a hypothetical reactor to raise the temperature of small polymer capsules containing solid DT by 1 K and to fully liquefy the contents. The time estimates are in turn based on estimates of the infrared absorption spectra of both solid DT and the polymer capsule material. Assuming typical target dimensions and rapid equilibration of the target temperature, the estimates show that the absorption of thermal radiation and subsequent heating of likely capsule materials will dominate the corresponding quantities of DT ice and thus that the former effect largely determines the time-to-melt of the target. Specific estimates are made for capsules fabricated from KaptonTM polyimide. Comparisons are also made for capsules coated with reflective metal coatings, and the potential benefit of these coatings is discussed.

A coordinated, focused effort is underway to develop Laser Inertial Fusion Energy. The key components are developed in concert with one another and the science and engineering issues are addressed concurrently. Recent advances include: target designs have been evaluated that show it could be possible to achieve the high gains (>100) needed for a practical fusion system.These designs feature a low-density CH foam that is wicked with solid DT and over-coated with a thin high-Z layer. These results have been verified with three independent one-dimensional codes, and are now being evaluated with two- and three-dimensional codes. Two types of lasers are under development: Krypton Fluoride (KrF) gas lasers and Diode Pumped Solid State Lasers (DPSSL). Both have recently achieved repetitive 'first light', and both have made progress in meeting the fusion energy requirements for durability, efficiency, and cost. This paper also presents the advances in development of chamber operating windows (target survival plus no wall erosion), final optics (aluminium at grazing incidence has high reflectivity and exceeds the required laser damage threshold), target fabrication (demonstration of smooth DT ice layers grown over foams, batch production of foam shells, and appropriate high-Z overcoats), and target injection (new facility for target injection and tracking studies).

Addressing the issues associated with target fabrication and injection is a major part of an international program to establish the feasibility of inertial fusion energy (IFE), both for laser-driven and heavy-ion driven concepts. A summary of the unique materials science and chemistry research programs associated with supplying targets for an IFE power plant is presented. The cost of manufacturing targets for commercial power applications is a significant perceived feasibility issue for IFE, and preliminary estimates of Target Fabrication Facility costs are discussed for both direct and indirect drive systems.

Targets for inertial fusion research and ignition at OMEGA, the National Ignition Facility, LMJ, and future facilities rely on beta-radiation-driven layering of spherical cryogenic DT ice layers contained within plastic or metal shells. Plastic shells will be permeation filled at room temperature then cooled to cryogenic temperatures before removal of the overpressure. The cryogenic pressure loader (CPL) was recently developed at Los Alamos National Laboratory as a testbed for studying the filling and layering of plastic target shells with DT. A technical description of the CPL is provided. The CPL consists of a cryostat, which contains a high-pressure permeation cell, and has optical access for investigating beta layering. The cryostat is housed within a tritium glovebox that contains manifolds for supplying high-pressure DT. The CPL shares some design elements with the cryogenic target handling system at the OMEGA facility to allow testing of tritium issues related to that system. The CPL has the capability to fill plastic targets by permeation to pressures up to 100 MPa and to cool them to 15K. The CPL will accommodate a range of targets and may be modified for future experiments.

The first observation of the beta-layering phenomenon showed that it was possible to fabricate inertial confinement fusion (ICF) targets having an outer ablating shell surrounding a symmetric solid...

Beta-layering of deuterium-tritium (D-T) ice in spherical shell geometries is numerically and analytically considered to investigate the relationship between temperature differences that arise because of inner-surface perturbations and the absolute shell thickness. The calculations use dimensions based on a proposed design of an inertial confinement fusion target for use at the National Ignition Facility. The temperature differences are calculated within D-T ice shells of varying total thicknesses, and the temperature differences calculated in three dimensions are compared both to the one-dimensional results and to the expected limits in three dimensions for long- and short-wavelength surface perturbations. The three-dimensional numeric results agree well with both the long- and short-wavelength limits; the region of crossover from short- to long-wavelength behavior is mapped out. Temperature differences due to surface perturbations are proportional to D-T layer thickness in one-dimensional systems but not in three-dimensional spherical shells. In spherical shells, surface perturbations of long wavelength give rise to temperature perturbations that are approximately proportional to the total shell thickness, while for short-wavelength perturbations, the temperature differences are inversely related to total shell thickness. In contrast to the one-dimensional result, we find that in three dimensions there is not a general relationship between shell thickness andmore » surface temperature differences.« less

Solid D-T layers are equilibrated inside a 2 mm diameter beryllium toroidal cell at temperatures ranging from 19.0 K to 19.6 K, using the beta-layering process. Each experimental run consisted of multiple cycles of rapid- or slow-freezing of the initially liquid D-T charge. Each of these freeze cycles was followed by a lengthy period of beta-layering equilibration, which was terminated by melting the layer. The temperature was changed in discrete steps at the end of some equilibration cycles in an attempt to simulate actual ICF target conditions. High-precision images of the D-T solid-vapor interface were analyzed to yield the surface roughness {sigma}{sub rms} as a sum of modal contributions. Results show an average {sigma}{sub rms} of 1.3 {+-} 0.3 {micro}m for layers equilibrated at 19.0 K and show an inverse dependence of {sigma}{sub rms} on equilibration temperature up to 19.525 K. Inducing sudden temperature perturbations lowered {sigma}{sub rms} to 1.0 {+-} 0.05 {micro}m.

Following the successful pressure loading with DT of a thin-walled plastic inertial fusion target shell (such as those designed for use at the OMEGA facility at the University of Rochester`s Laboratory for Laser Energetics (UR/LLE)), continual care must be taken to safeguard the shell from being exposed to unacceptable pressure differentials across its wall. In particular, once the DT has been condensed into a liquid or solid phase and the outside pressure has been reduced, the target must be maintained below some upper cutoff temperature such that the vapor pressure of the DT is below the bursting pressure for the shell. Through the process of {beta}-decay the DT self-heats, but while the shell is in a high vacuum environment (P {much_lt} 0.8 Pa (6 mtorr) for the OMEGA layering sphere) there is only a negligible heat loss mechanism. This will cause the temperature to increase. A calculation has been done to estimate the rate of temperature increase of the loaded target under high vacuum conditions. A functional form for calculating the target`s temperature increase of the loaded target under high vacuum conditions. A functional form for calculating the target`s temperature increase given its starting temperature is presented. An overall resultmore » is that under high vacuum conditions the DT changes from a solid at 10 K to a liquid at 37 K ({Tc} = 39.4 K) in about 19 minutes. This holding time is significantly less if the initial temperature is higher, the initial state is liquid, or the upper allowed temperature is lower. Simplifying assumptions which were made and their impact on interpreting the results of this calculation are discussed.« less

Capsules with beryllium ablators have long been considered as alternatives to plastic for the National Ignition Facility laser ; now the superior performance of beryllium is becoming well substantiated. Beryllium capsules have the advantages of relative insensitivity to instability growth, low opacity, high tensile strength, and high thermal conductivity. 3-D calculation with the HYDRA code NTIS Document No. DE-96004569 (M. M. Marinak et.al. in UCRL-LR-105821-95-3) confirm 2-D LASNEX U. B. Zimmerman and W. L. Kruer, Comments Plasmas Phys. Controlled Thermonucl. Fusion, 2, 51(2975) results that particular beryllium capsule designs are several times less sensitive than the CH point design to instability growth from DT ice roughness. These capsule designs contain more ablator mass and leave some beryllium unablated at ignition. By adjusting the level of copper dopant, the unablated mass can increase or decrease, with a corresponding decrease or increase in sensitivity to perturbations. A plastic capsule with the same ablator mass as the beryllium and leaving the same unablated mass also shows this reduced perturbation sensitivity. Beryllium`s low opacity permits the creation of 250 eV capsule designs. Its high tensile strength allows it to contain DT fuel at room temperature. Its high thermal conductivity simplifies cryogenic fielding.

Inertial confinement fusion (ICF) targets designed to achieve ignition must meet strict surface smoothness and sphericity requirements. One potentially valuable method for evaluating the quality of...

Los Alamos National Laboratory is contributing to the core science and technology of the inertial confinement fusion program leading to the National Ignition Facility. Short summaries of a sample o...

A high-resolution optical imaging system and custom-designed image analysis software are used to make surface roughness measurements for deuterium-tritium (D-T) solid layers, equilibrated inside a ...

Prototype inertial confinement fusion targets for the proposed National Ignition Facility are metallic or plastic spherical shells (2 mm o.d., ≊150‐mm thickness) with an ≊80‐mm‐thick layer of solid deuterium–tritium (50/50 mixture) deposited on the inner surface. Ignition will occur only if the D–T fuel layer meets strict sphericity and surface roughness criteria (typically ≊1 mm). Symmetric layering of solid D–T occurs due to the phenomenon of ‘‘beta layering’’ in which tritium‐induced self‐heating drives the redistribution of material. In contrast to the optical techniques usually employed, this work discusses methods in which resonant ultrasound spectroscopy (RUS) and related techniques can be used to help determine the uniformity of the fuel layer inside opaque targets. A tetrahedral array of pinducers in a cryogenic apparatus is used to both mount and probe the sample. Preliminary efforts have focused on the characterization of solid spheres and both aluminum and beryllium shells. Sufficiently high Q...

We have examined two of the variables that affect the beta-layering process in which nonuniform layers of solid deuterium-tritium (DT) are driven toward uniformity by beta-decay induced sublimation...

New experiments in a NIF-scale toroidal cylinder have resulted in true shadowgraphs of the DT ice surface. The spectral analysis of the images summed over l-modes 2 through 256 reveal that the surface roughness reaches values just below 1.0 {micro}m at temperatures of 19 K and above. Summing only modes l {ge} 10, the partial surface roughness is below 0.7 {micro}m at 19.5 K. These results indicate that native beta-layering will be sufficient to meet the NIF requirements for DT ice surface finish for both Be and CH ablating shells. The toroidal cylinder incorporates a linear heater along the cylindrical axis to test the concept of surface enhancement due to heat assisted beta-layering in DT. Additionally, with the use of this heater it is possible to symmetrize a pure D{sub 2} layer.

An optical method is developed for the study of beta‐layering of solid deuterium‐tritium target capsules for the National Inertial Confinement Program. High‐resolution image data show that the surface roughness do not exceed 2 micrometers 50 hours after first target freezing. (AIP)

It has recently been shown that when solid tritium is confined in an isothermal enclosure, self-heating due to beta decay drives a net sublimation of material from thick, warmer layers to thin, cooler ones, ultimately resulting in layer thickness uniformity. We have observed this process of beta-layering'' in a 50--50 D-T mixture in both cylindrical and spherical enclosures at temperatures from 19.6 K, down to 11.6 K. The measured time constants are found to depend on the {sup 3}He content as suggested by recent theoretical predictions. When using an enclosure having low thermal conductivity, the ultimate layer uniformity is found to be a strong function of the exchange gas pressure. This is due to the presence of thermal convection in the exchange gas and consequent temperature anisotropy at the solid layer surface. 6 refs., 2 figs., 1 tab.

Spherical shells are used for a variety of applications ranging from fundamental gas properties measurement in large resonators to studies involving millimeter‐size prototype intertial‐confinement‐fusion (ICF) targets. Most applications demand that either or both shell surfaces have a high degree of sphericity. Fabrication from hemishells inevitably leads to geometric imperfections and leaves the interior surface unavailable to metrological inspection. However, many common asphericities are predicted to break the degeneracy of resonator modes to first order in a boundary perturbation expansion [J. B. Mehl, J. Acoust. Soc. Am. 79, 278–285 (1985)]. Various centimeter size aluminum and beryllium shells were manufactured with artificially enhanced common fabrication errors. The resonant modes of a completely closed shell are excited and detected using a swept‐sine heterodyne technique [A. Miglioriet al., Physica B 183, 1–24 (1993)] with four pinducers in contact with the exterior at the tetrahedral angles. Bo...

Millimeter to centimeter size light‐metallic shells are of interest as investigative tools in the inertial‐confinement‐fusion community. Such shells are typically filled at room temperature with pure or mixed hydrogen isotopes at high pressure. At low temperatures, the hydrogens condense forming a solid or liquid in equilibrium with the vapor. The sound speed of gaseous deuterium (1% He‐3) in a closed aluminum shell has been investigated using a cavity/shell resonance method. An anomalous decrease in the measured sound speed is observed as the saturation pressure Ps (about 15 atm at 38 K) is approached during a nearly constant‐volume reduction in temperature. This effect is explained qualitatively in terms of a reduced acoustic admittance at the shell wall due to a precondensed liquid layer at pressures below Ps [J. B. Mehl and M. R. Moldover, J. Acoust. Soc. Am. 77, 455–465 (1982)]. Comparison is made with precondensation phenomena observed by various authors for single‐component gases in experiments per...

A horizontal cylindrical cavity bounded by isothermal walls was partially filled with liquid tritium which was then frozen by reduction of the temperature to 1.0 K below the triple point. Visual observations revealed that the solid subsequently redistributed itself into a layer of uniform thickness covering the complete interior of the cavity. The time constant for this effect depends on the age (or $^{3}\mathrm{He}$ content) of the tritium and not on the initial filling fraction. Time constants of 14.9, 92, 219, and 234 min were measured for tritium 0.04, 7, 16, and 17 days old, respectively.

Binary phase-separation dynamics in liquid mixtures of He-3 and He-4 has been investigated near the tricritical point with laser-light scattering techniques. Rapid decompression of the mixtures results in quenches into the miscibility gap so that both the metastable and unstable (spinodal) regions can be probed. Quenches into the unstable region allowed measurements of the normalized dynamic structure factor S(k,t) that confirm the dynamical scaling hypotheses for spinodal decomposition. Measurements made for concentrations well away from the tricritical value show different behavior and suggest the presence of a spinodal boundary. Forward scattering intensities for shallow quenches probe nucleation phenomena and permit quantitative measurements of anomalous super-cooling as a function of quench rate. Comparisons with data in organic binary mixtures are given.

Measurements of the dynamics of phase separation in homogeneous mixtures of /sup 3/He-/sup 4/He have been performed at temperatures ranging from 17 mK to within 3 mK of the tricritical temperature. The results are the first to show the expected critical slowing down of the dynamics proportional to epsilon as the tricritical point is approached.

Experimental techniques to study dynamical light scattering phenomena in phase separating 3He–4He liquid mixtures are described. Commercially available self‐scanning 512‐element linear photodiode arrays are used to record ‘‘snapshots’’ of small‐angle light scattering patterns with high angular resolution at time intervals as short as 2 ms. Logic circuitry developed for interfacing the photodiode arrays with a digital oscilloscope recorder is described in detail.

The so-called miscibility gap that exists below the critical point in liquid He-3 - H-4 mixtures makes it possible to study binary phase composition, and the ensuing dispersions, in a system possessing an additional order parameter in one of the components. The physical behavior of a superfluid dispersion produced by pressure quenching an He-3 - He-4 mixture into the miscibility gap is described. The description applies both to quenches of homogeneous and phase-separated initial states in various regions of the miscibility gap.

Measurements are presented of the angular distribution of light scattering by decomposing 3 He 4 He mixtures. It is found that the structure factor has scaling form S( κ , t) = κ −3 m F( κ / κ m ) where κ m −1 is related to the size of a growing domain and F (x) is time independent during the early stages of phase separation.