Cryogenic Target Research Articles

Improved Inertial Fusion Energy Direct-Drive Target Survival in Chamber Gas through Validated Simulation

Commercial inertial fusion energy power plants will require 5-20Hz fusion target injection rates for utility-scale power production. To mitigate damage from target emission, some designs include a buffer gas in the chamber to reduce heat and particle fluxes to the chamber wall. The evolution of chamber environment between shots is an important issue as residual heat and eddies in the gas pose a serious threat on target survival during injection and target trajectory.We have simulated the evolution of a direct-drive IFE chamber with helium, deuterium, and xenon buffer gases at several densities. To evaluate the link between these simulations and the risk posed to a direct-drive target, we modify an analytical expression of the free-molecular heat flux on a surface element to account for the possibility of chamber gas condensation on the target. We show this expression compares favorably with Monte Carlo simulations in the same gas regime. These results are used to estimate risk for target survival based on several target heating failure modes. Though lower density chamber gas would improve target survival, experimental quantification of several key gas-surface interaction coefficients for cryogenic targets could open the chamber gas design window.

The National Ignition Facility and the Promise of Inertial Fusion Energy

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) in Livermore, CA, is now operational. The NIF is the world’s most energetic laser system capable of producing 1.8 MJ and 500 TW of ultraviolet light. By concentrating the energy from its 192 extremely energetic laser beams into a mm3-sized target, NIF can produce temperatures above 100 million K, densities of 1,000 g/cm3, and pressures 100 billion times atmospheric pressure - conditions that have never been created in a laboratory and emulate those in planetary interiors and stellar environments. On September 29, 2010, the first integrated ignition experiment was conducted, demonstrating the successful coordination of the laser, cryogenic target system, array of diagnostics and infrastructure required for ignition demonstration. In light of this strong progress, the U.S. and international communities are examining the implication of NIF ignition for inertial fusion energy (IFE). A laser-based IFE power plant will require a repetition rate of 10–20 Hz and a laser with 10% electrical-optical efficiency, as well as further development and advances in large-scale target fabrication, target injection, and other supporting technologies. These capabilities could lead to a prototype IFE demonstration plant in the 10- to 15-year time frame. LLNL, in partnership with other institutions, is developing a Laser Inertial Fusion Engine (LIFE) concept and examining in detail various technology choices. This paper will describe the unprecedented experimental capabilities of the NIF and the results achieved so far on the path toward ignition. The paper will conclude with a discussion about the need to build on the progress on NIF to develop an implementable and effective plan to achieve the promise of LIFE as a source of carbon-free energy.

Fibronectin layers by matrix-assisted pulsed laser evaporation from saline buffer-based cryogenic targets

The deposition of fibronectin (FN) from saline buffer-based cryogenic targets by matrix-assisted pulsed laser evaporation (MAPLE) onto silicon substrates is reported. A uniform distribution of FN was revealed by Ponceau staining after control experiments on nitrocellulose paper. Well-organized particulates with heights from hundreds of nanometers up to more than 1 μm packed in homogeneous layers were evidenced by optical microscopy and profilometry on Si substrates. Atomic force microscopy images showed regions composed of buffer and FN aggregates forming a compact film. Comparison of infrared spectra of drop-cast and MAPLE-deposited FN confirmed the preservation of composition and showed no degradation of the protein. The protein deposition on Si was confirmed by antibody staining. Small aggregates and fluorescent fibrils were visualized by fluorescence microscopy. Superior attachment of human osteoprogenitor cells cultivated for 3 h proved the presence of stable and intact FN molecules after transfer.

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about the assembled fuel either by imaging the photons emitted by the hot central plasma, or by active probing of the dense shell by a separate high energy short pulse flash. The planned use of these targets and diagnostics to assess and optimize the assembly of the fuel and how this relates to the predicted performance of DT targets is described. It is found that a good predictor of DT target performance is the THD measurable parameter, Experimental Ignition Threshold Factor, ITFX ∼ Y × dsf 2.3, where Y is the measured neutron yield between 13 and 15 MeV, and dsf is the down scattered neutron fraction defined as the ratio of neutrons between 10 and 12 MeV and those between 13 and 15 MeV.

Inertial Confinement Fusion Using the OMEGA Laser System

The OMEGA laser system is being used to investigate several approaches to inertial confinement fusion: the traditional central-hot-spot (CHS) ignition, fast ignition (FI), and shock ignition (SI). To achieve ignition, CHS requires the highly uniform compression of a solid deuterium-tritium (DT)-layered target on a low adiabat (defined as the ratio of the pressure to the Fermi-degenerate pressure) and with an implosion velocity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">imp</sub> ≥ 3.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s. A laser pulse shape with triple pickets is used to produce this low adiabat by optimally timing multiple shocks launched by the pickets and the main laser. Cryogenic targets that imploded optimally with such pulses have demonstrated near-design compression with an areal density ρ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ~ 290 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">imp</sub> = 3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s. These are, by far, the highest DT areal densities demonstrated in the laboratory. SI experiments, where a shock is launched by a picket at the end of the laser pulse into the compressing capsule, have been performed on low-adiabat warm plastic targets. Both yield and areal density improve significantly when a spike is used at the end of the laser pulse, indicating that the energy from the shock is coupled into the compressing target. Integrated FI experiments have begun on the OMEGA/OMEGA EP laser system.

Evolution and Progress of the Cryogenic Target Shroud Remover Prototypes Developed for the LMJ Facility

The Laser Mégajoule (LMJ) cryogenic target is protected from ambient thermal radiation by a thermal shroud. When the cryotarget, held by the cryotarget positioner, is at the LMJ chamber center, the thermal shroud has to be removed just before the shot to allow the laser beams to reach the laser entrance hole of the cavity.The shroud remover, PET, will have to disconnect the thermal shroud from the cryogenic target base without disturbing the target base temperature regulation (˜18 K ± 2 mK), which guarantees the needed cryogenic target conditions to reach the ignition.The shroud withdrawal is divided into two successive phases: a slow withdrawal for the thermal disconnection between shroud and target base and a fast withdrawal for a quick extraction of the shroud out of the laser beamways pointing onto the cavity. The slow shroud withdrawal must be handled within 30 min to respect laser pointing stability. After the final target alignment at the chamber center, the shroud must be ejected 0.5 m away from the source point in <0.1 s before the shot.To cope with all these issues, a prototype of the shroud remover, PPET, has been first built and developed at CEA-Grenoble, at INAC/SBT, before being tested at CEA-CESTA on the DEMOCRYTE setup, a prototype of the cryogenic target charger and holder.The experimental results mainly obtained at CEA-CESTA in 2008 and 2009 on two generations of target bases and shrouds are presented in this paper.

Numerical Simulations of Removing Anticonvection Baffles Inside a Laser Mégajoule Cryotarget

We have determined the thermal criterion for the Laser Mégajoule cryogenic target that leads to a uniform layer of deuterium-tritium as specified for ignition. Thermal models were created, and computational fluid dynamics software was used to calculate this criterion in a spherical geometry as a function of capsule thermal conductivity. According to the values obtained, we have studied the possibility of removing anticonvection baffles in the cryogenic target. Results are presented in this paper and indicate that anticonvection baffles can be removed only if the gas density inside the cryogenic target is reduced and/or if the hydrogen content is reduced.

Status of the Development of the HiPER Single-Shot Target

The High Power laser Energy Research facility (HiPER) is a European project dedicated to demonstrating the feasibility of producing energy by laser-driven inertial confinement fusion. A first design of the fast ignition cryogenic target has been established. It is composed of a thin-walled microshell with an inserted gold cone and filled with deuterium-tritium (DT) fuel by means of a capillary (conically guided capsule). After assembly, targets must be tight at cryogenic temperatures (16 to 19.6 K).In order to evaluate the manufacturing feasibility of a single-shot target prototype, a program has been adapted from the Laser Mégajoule (LMJ) cryogenic target fabrication know-how. Target component study for HiPER concerns a hollow gold cone (25-deg half-angle and ˜25-μm thickness), a thin polymeric microshell (2-mm diameter and 3- to 10-μm thickness), and a silica capillary (30-μm outer diameter).First gas-tight targets at 77 K have been produced (helium gas leak rate ˜1.4 × 10-11 Pa·m3/s). Major efforts have been focused on thin-walled microshells, robust gold cone fabrication, and target assembly (minimizing of the glue quantity as well as helium gas leak tests) and will be discussed in this paper.

A liquid helium temperature target with a 10 K vapor shield for shock compression experiment in the environment condition of 100 Pa

The shock compression experiment of liquid helium is an available way to gain properties of specimen at high temperatures and pressures. Based on Fluent, a thermal insulation analysis and design of a liquid helium temperature target in the environment condition of 100 Pa for shock compression experiment is performed. Then, a cryogenic target with a 10 K helium vapor shield and a separated vacuum interval is particularly developed. A lowest temperature of 3.63 K and a stable temperature of 3.70 K in the specimen cavity with an accuracy of 0.1 K are obtained by means of continuous flow and vacuum cooling. Both time-consuming and temperature stability are well-suited to the requirements of the shock compression experiment. The results show that the calculated and experimental data well-matched each other. The simulation method may be effective and feasible for the optimal design of the cryogenic target.

Overview on Materials and Technological Developments for the LMJ Cryogenic Target Assembly

The cryogenic target assemblies (CTAs) designed for Laser Mégajoule (LMJ) experiments have many functions and have to meet severe specifications imposed by implosion physics, the CTA thermal environment, and the CTA interfaces with the Mégajoule laser cryogenic target positioner. Therefore, CTA fabrication uses many challenging materials and requires several technological studies. During the last 2 years, many developments have enabled better collection of comprehensive data on target constitutive materials and improvements in the fabrication of the CTA base, hohlraum, and aluminum turret.Studies have been carried out (a) to better characterize thermal properties of materials allowing optimization of the thermal simulation of the hohlraum, (b) to improve the CTA base fabrication process in order to optimize thermal studies of the LMJ experimental filling station (EFS), and (c) to determine coatings on the polyimide membrane that may limit the 300 K thermal effect on the microshell and increase the deuterium-tritium fuel lifetime.CTAs have been produced to evaluate fabrication knowledge, to characterize CTAs, to study air tightness, and to study filling and D2 ice layering on the EFS.An overview of the results that have been obtained during the past 2 years is presented in this paper.

On-Chip Double Emulsion Droplet Assembly Using Electrowetting-on-Dielectric and Dielectrophoresis

The double emulsion (DE) droplets used for fabrication of cryogenic foam targets for inertial confinement fusion experiments require precisely controlled volumes. On-chip electric field actuated microfluidic assembly of DE droplets can be used to achieve such precision. The electrowetting-on-dielectric and dielectrophoresis effects make it possible to manipulate both conductive and dielectric droplets simultaneously on a substrate. Aqueous and nonaqueous liquid droplets precisely dispensed from two reservoirs on a microfluidic chip are transported and combined to form oil-in-water-in-air or water-in-oil-in-air DE droplets. The dispensing reproducibility is studied as a function of a set of operation parameters. Conditions for spontaneous emulsification for DE formation are developed in terms of droplet surface energies.

NIF Target Assembly Metrology Methodology and Results

Inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) require cryogenic targets at the 1-cm scale to be fabricated, assembled, and metrologized to micron-level tolerances. During assembly of these ICF targets, there are physical dimension metrology steps to be made of the components, subassemblies, and completed targets. Metrology is primarily completed using optical coordinate measurement machines that provide repeatable measurements with micron precision, while also allowing in-process data collection for absolute accuracy in assembly. To date, 51 targets have been assembled and metrologized, and 34 targets have been successfully fielded on NIF relying on these metrology data. In the near future, ignition experiments on NIF will require tighter tolerances and more demanding target assembly and metrology capability.Metrology methods, calculations, and uncertainty estimates will be discussed. Target diagnostic port alignment, target position, and capsule location results will be reviewed for the 2009 Energetics Campaign. The information is presented via control charts showing the effect of process improvements that were made during target production. Certain parameters, including capsule position, met the 2009 campaign specifications but will have much tighter requirements in the future. To meet these new requirements assembly process changes and metrology capability upgrades will be necessary.

Four Years of Experimental Results on the Prototypes of the Cryogenic Holders of the Laser Mégajoule Facility

The Laser Mégajoule cryotarget positioner, PCC, will be used to set cryogenic targets in appropriate conditions for the laser shot in order to reach ignition. These conditions can be summarized as a few parameters of the deuterium-tritium (DT) solid layer: sphericity, roughness, and density of DT gas. The DT mixture is confined and held in a target assembly that is handled and cooled by the PCC. Thus, the parameters of the DT solid layer are controlled by the PCC. In particular, roughness depends on the control of the target base temperature (±1 mK), on the temperature slope while crossing the DT triple point (0.5 mK/min), and on the thermodynamic way followed to reach gas density conditions expected at the laser shot [slow cooling (0.5 mK/min), quenching (several kelvins per second), or rapid cooling (several kelvins per minute)]. Moreover, the required gas density needs high cryogenic power performances of the PCC to be fulfilled. As the target is gripped at cryogenic temperature by the PCC, thermal contact resistance added to power load problems must be faced.We have investigated all these cryogenic challenges on DEMOCRYTE, the prototype of the cryogenic holder setup. Experimental results obtained between 2006 and 2009 are described in this paper.

Kaon-nucleon strong interaction in kaonic atoms

The SIDDHARTA experiment (SIlicon Drift Detector for Hadronic Atom Research by Timing Application) aims to perform a precise measurement of the X-ray K-series of kaonic hydrogen and the first-ever measurement of kaonic deuterium, to obtain the strong-interaction energy-level shifts and widths of the lowest lying atomic states. The measurements will allow the determination of the isospin-dependent kaon-nucleon (KN) scattering lengths, directly connected to the physics of the KN interaction. The experiment uses the low-energy kaon beam generated at DAΦNE collider, high-density cryogenic targets and fast, large area X-ray spectroscopic detectors. In addition to the kaonic hydrogen and kaonic deuterium, the collaboration performed measurements of the kaonic Helium transitions to the 2p level (L-lines). Kaonic Helium 4 was measured for the first time in a gaseous target while the kaonic Helium 3 was mesured for the first time ever.

A hardened gated x-ray imaging diagnostic for inertial confinement fusion experiments at the National Ignition Facility

A gated x-ray detector is under development for use at the National Ignition Facility that is intended to provide plasma emission images in the presence of neutron yields up to 10(15) expected during inertial confinement fusion experiments with layered cryogenic targets. These images are expected to provide valuable time-resolved measurements of core and fuel symmetries. Additional capabilities of this instrument will include the ability to make spatially resolved electron temperature measurements. A description of this instrument and its operation is given with emphasis on features that differentiate it from previous designs.

Analyses of laser-plasma interactions in NIF ignition emulator designs

The National Ignition Campaign is currently conducting energetics experiments at the National Ignition Facility (NIF). These experiments directly test all aspects of ignition hohlraum performance. An important aspect of good performance is understanding and mitigation of laser-plasma interactions, which enables laser coupling to the target. The hohlraum energetics target and pulse-shape are designed to produce an ignition-hohlraum-like plasma to test laser-plasma interactions. The pre-shot laser-plasma interaction predictions for the first energetics targets (one room temperature target, and one cryogenic target) are presented here. These findings correlate well with the experimental data, namely that the primary concern is stimulated Raman scatter of the inner beams deep in the target. Such scatter is mostly re-absorbed, but modifies the energy deposition profile and thus the symmetry. Stimulated Brillouin scatter is at low levels, both in pre-shot predictions and in the experiments.

Progress in cryogenic target implosions on OMEGA

Cryogenic deuterium-tritium targets are imploded on the OMEGA Laser System in a direct-drive configuration. Areal densities of approximately 200 mg/cm2 have been measured with implosion velocities of 3 × 107 cm/s. These implosions are used to study the dynamics of cryogenic target compression and to develop areal-density diagnostics that will be used as part of the ignition campaign on the National Ignition Facility.

Simple collision operators for direct Vlasov simulations of laser plasma interaction and transport

Non-local electron transport effects have a direct influence on the compression of cryogenic targets in laser driven ICF and target heating in high energy density experiments. There is a growing need for self-consistent models of laser plasma interactions coupled to nonlocal transport. We present a direct Vlasov solver that includes multiple species and a simple collision operator. This BGK model operator – which conserves particle density, energy and momentum – is fully implicit. For collisionless plasmas it has been shown that a double layer may be formed in which an accelerated, kinetic ion population satisfies the zero current condition. Here we extend this result to collisionalities of interest to laser driven ignition to assess the validity of nonlocal electron transport models based on fluid ions.

Target developments program to prepare LMJ campaigns

To carry out laser plasma experiments on CEA laser facilities, a R&D program was set up and is still under way to deliver complex targets. For a decade, specific developments are also dedicated to "Ligne d'Intégration Laser" (LIL) in France and Omega facilities (USA). To prepare the targets intended for the first experiments on the Laser "Mégajoule" (LMJ) facility, new developments are required, such as cocktail hohlraum fabrication, gas barrier coating and foam shells developments. For fusion experiments on LMJ, an important program is also under way to elaborate the Cryogenic Target Assembly (CTA), to fill and transport the CTA and to study the conformation process of the DT layer.

Preheating of a target by laser radiation through plasma and polymer aerogel

Processing of X-ray and optical streak-camera images after brightness amplification 5 proves unambiguously the existence of a preheating of the foil in two-layer targets (polymer aerogel plus Al foil). We observed this effect earlier at the accuracy limit of the experiment. Preheating may be explained by the initial transmission of laser radiation by an inhomogeneous plasma and aerogel remnants. The fraction of laser energy that passed through a microheterogeneous plasma to the foil or shell is small in comparison with the total energy in the pulse, but it comes to the target (foil or shell) at the initial period of the laser pulse and can be significantly affected by the dynamics of plasma and the efficiency of spherical cryogenic thermonuclear targets.