Inertial Fusion Energy Chamber Research Articles

Laser-induced fluorescence of C2 and C3 in colliding carbon plasma

The formation of transient molecular species, C2 and C3, was studied in the collision of two laser-produced carbon plasmas using laser-induced fluorescence. In vacuum, two closely neighboring laser-produced plasmas will collide, as they expand into one another or are forced into occupying the same space. This so-called colliding plasma produces a highly collisional composite plasma plume from the two individual plasmas which persists longer with enhanced prevalence of certain plasma processes such as recombination. In an inertial fusion energy (IFE) chamber, successive shots will ablate first wall material (e.g., graphitic carbon). Inwardly collapsing plasma will attain such a colliding plasma state as it converges inwards toward the chamber center. The presented results elucidate the early formation of C2 and C3, precursors to larger carbon structures which may contaminate an IFE chamber.

Neutronics and activation analysis of lithium-based ternary alloys in IFE blankets

An attractive feature of using liquid lithium as the breeder and coolant in fusion blankets is that it has very high tritium solubility and results in very low levels of tritium permeation throughout the facility infrastructure. However, lithium metal vigorously reacts with air and water and presents plant safety concerns. The Lawrence Livermore National Laboratory is carrying an effort to develop a lithium-based ternary alloy that maintains the beneficial properties of lithium (e.g. high tritium breeding and solubility) and at the same time reduces overall flammability concerns. This study evaluates the neutronics performance of lithium-based alloys in the blanket of an inertial fusion energy chamber in order to inform such development. 3-D Monte Carlo calculations were performed to evaluate two main neutronics performance parameters for the blanket: tritium breeding ratio (TBR), and the fusion energy multiplication factor (EMF). It was found that elements that exhibit low absorption cross sections and higher q-values such as Pb, Sn, and Sr, perform well with those that have high neutron multiplication such as Pb and Bi. These elements meet TBR constrains ranging from 1.02 to 1.1. However, most alloys do not reach EMFs greater than 1.15. Additionally, it was found that enriching lithium with 6Li significantly increases the TBR and decreases the minimum lithium concentration by more than 60%. The amount of enrichment depends on how much total lithium is in the alloy to begin with. Alloys that performed well in the TBR and EMF calculations were considered for activation analysis. Activation simulations were executed with 50 years of irradiation and 300 years of cooling. It was discovered that bismuth is a poor choice due to achieving the highest decay heat, contact dose rates, and accident doses. In addition, it does not meet the waste disposal ratings (WDR). Some of the activation results for alloys with Sn, Zn, and Ga were in the higher end and should be considered secondary to elements such as Sr and Ba that had overall better results. The results of this study along with other considerations such as thermodynamics, and chemical reactivity will help down select a preferred lithium ternary alloy.

Integrated inertial fusion energy chamber dynamics and response

This paper presents results of three-dimensional hydrodynamics simulations of the flow inside a model inertial fusion energy (IFE) fusion chamber. Turbulence modeling employing the large-eddy simulation approach is used to estimate the gas dynamics, state, and mixing after a sufficiently large number of target ignitions. The rich radiation-flow physics that takes place immediately after the lasers hit the hohlraum is modeled separately using a high-fidelity one-dimensional model, which provides reference conditions for the complex geometry three-dimensional turbulence simulations. The IFE geometry includes optical ports and recirculation openings as well as a duct to evacuate the debris produced after each energy deposition (as a model of a laser shot). Furthermore, a selected set of sensitivity studies are conducted to estimate the effect of uncertainty in radiative properties of the Xenon gas at the prevalent conditions in the chamber. The results provide guidance regarding the turbulence conditions in the chamber, which seem to have entered a decay state immediately before a new shot takes place. Computational estimates of the density variability within the chamber as well as pressure history at the approximate location of the laser optical ports is presented among other turbulence statistics.

Parameter Study of an Inertial Fusion Energy Chamber Response Using the 1-D BUCKY Radiation Hydrodynamics Code

A parameter study of a proposed inertial fusion energy chamber is performed. A baseline case of a 6-m-radius chamber filled with 6 μg/cm3 of xenon is studied in detail. The maximum first-wall temperature is shown to be 1136 K with an overpressure of 5.83 + 10−3 MPa. A parameter sweep is conducted for the chamber by adjusting the first-wall radius from 4 to 14 m, changing the gas density and changing the fill gas from xenon to argon. The results set limits on the first-wall radius for different gases and densities. Analytic fits to simulation results allow their use in overall engine design trade-off studies.

Thermal and Structural Issues of Target Injection into a Laser-Driven Inertial Fusion Energy Chamber

Inertial fusion energy (IFE) targets injected into fusion chambers must withstand the demanding acceleration forces and the intense thermal environment of the fusion chamber. For indirect targets, the ultrathin capsule support membrane is the target component that is most sensitive to acceleration forces. Maintaining the deuterium-tritium (DT) temperature, to prevent a significant increase in DT vapor pressure, is the most critical thermal requirement. Secondarily, material selection of the high-temperature laser entrance hole window is required. This paper briefly describes how these requirements are satisfied for a laser-driven IFE plant design.

Experimental investigation of aerosol formation in laser fusion reactor chamber by discharge method

Liquid wall chambers are one of the most critical technologies in inertial fusion energy (IFE) plants. The critical problem for the liquid wall IFE chamber is the chamber clearance after laser shots. This paper presents preliminary results from an experimental simulation of ablation processes induced by alpha-particle heating.

An Upper Bound for Stress Waves Induced by Volumetric Heating in IFE Chamber Walls

Rapid heating by x-rays and ions in Inertial Fusion Energy (IFE) chambers will produce stress waves in dry chamber walls, in some cases leading to damage that will ultimately fail the structure. These waves can affect the surface or propagate to the substrate and produce delamination. Hence, it is important that these waves be understood. Models exist for thermally induced stress waves resulting from surface heating, but models with volumetric heating have not been presented for IFE conditions. In this paper we develop models for elastic stresses caused by rapid volumetric heating in a half-space. The stress wave models are obtained analytically for heating distributions which are both uniform over a finite region and exponentially decaying over the entire depth. These two cases cover the relevant heating for a typical IFE threat. Results are given for both x-ray and ion heating using threats from a direct drive target developed for the High Average Power Laser (HAPL) target.

Rosseland and Planck mean and multi-group opacities of weakly non-ideal Flibe plasma

In this article we investigate the physics and develop a theoretical model to perform computations of the Rosseland and Planck mean and multi-group opacities of weakly non-ideal Flibe plasma generated in the inertial fusion energy chamber. Ionization equilibrium and partition functions of all Flibe plasma species are modelled considering the plasma environmental influence (non-ideal effects) on the electronic structure in a static way within the chemical picture. A recently developed reduced formulation and efficient algorithm is used to solve the resulting set of equations and to determine the detailed plasma composition and partition functions for non-ideal Flibe plasma systems. The algorithm considerably reduces the computational efforts required to determine the plasma composition and allows, with considerable simplicity, the determination of all population densities of all plasma species (neutrals, ionized and excited) up to maximum ionization states equal to the atomic numbers of the involved chemical elements and considers an extensive database of energy levels of the excited states and line spectra. The calculated detailed composition is used along with the above mentioned theoretical opacity model to calculate the radiative properties of the Flibe plasma. Optical characteristics such as multi-frequency absorption coefficient, Planck's and Rosseland's mean and muli-group opacities of weakly non-ideal Flibe plasmas are calculated over a wide range of the density-temperature phase space and are presented as a set of isochors.

Transient thermo-mechanical behavior of a direct-drive target during injection in an inertial fusion energy chamber

During injection in an inertial fusion energy (IFE) chamber, a direct-drive target is subject to heat loads from chamber wall radiation and energy exchange from the chamber gas constituents. These heat loads can lead to the deuterium–tritium (DT) reaching its triple point temperature and even undergoing phase change, leading to unacceptable non-uniformity based on target physics requirements for compression and ignition of the DT fuel pellets using multiple laser beams. A two-dimensional bubble nucleation mode was added to the previously presented thermo-mechanical model to help better define the design margin for direct-drive IFE targets. The new model was validated by comparison with analytical results for controlled cases. It was then used to simulate heating experiments on DT targets conducted at the Los Alamos National Laboratory (LANL), where the 3He present in the DT due to tritium decay was found to affect the nucleation process. The previous requirement for target survival was for the temperature of the DT to remain below triple point of DT (19.79 K). If the existence of a melt layer does not violate the symmetry requirements on the target for successful implosion, the constraint could be relaxed by assuming a limit based on the avoidance of bubble nucleation. This study shows that the thresholds for melting and bubble nucleation are significantly different, allowing for extra margin in target survival under this assumption.

Thermodynamic Properties of High-Temperature Weakly Nonideal Flinabe (LiF-NaF-BeF2) Gas

The set of thermodynamic properties of high-temperature, weakly nonideal Flinabe (LiF-NaF-BeF2) gas is calculated and presented. High-temperature Flinabe gases (plasmas) appear in the inertial fusion energy chamber over a wide range of temperatures and pressures due to the absorption of X-rays and debris, emitted from the target microexplosion, within a very thin surface layer of the Flinabe liquid wall. The equation-of-state (EOS) and ionization equilibrium data of the resulting high-temperature gas were computed and are presented in another paper. In this paper, the set of thermodynamic properties (specific enthalpy, specific heats, adiabatic exponent, and sound speed) that are required, in conjunction with the Flinabe EOS, to perform gas dynamics calculations and the required assessments of many research and development issues in nuclear fusion is modeled and computed consistently with the previously presented EOS and ionization equilibrium data. This set of Flinabe thermodynamic properties is missed in the literature, and the need to model and estimate these properties seems to be immediate rather than justifiable. Computational results for Flinabe thermodynamic properties are presented and discussed. These properties have been presented as a set of isobars that have been validated by obtaining the limiting conditions at very high temperatures for a fully dissociated/fully ionized gas.

Equation of State of High-Temperature Weakly Nonideal Flinabe (LiF-NaF-BeF2) Gas

The molten salt Flinabe (LiF-NaF-BeF2) is proposed as a liquid wall material for future fusion reactors because of its many attractive aspects. High-temperature Flinabe gases (plasmas) appear in the inertial fusion energy chamber over a wide range of temperatures and pressures because of the absorption of X-rays and debris, emitted from the target microexplosion, within a very thin surface layer of the Flinabe liquid wall. The deposited energy heats the surface edge of the Flinabe wall to very high temperatures where vaporization, dissociation, and ionization take place and high-temperature plasma is generated. Equation-of-state (EOS) and ionization equilibrium data of the resulting high-temperature gas are needed to perform gas dynamics calculations and the required assessments of many research and development issues in nuclear fusion. Nevertheless, data for Flinabe EOS or ionization states are missing in the literature, and there is an immediate need to model and estimate these properties. In this paper, a self-consistent model for the ionization equilibrium and EOS of weakly nonideal high-temperature Flinabe gas is presented and used to compute the ionization equilibrium data and EOS of such an important material. Nonideality effects have been taken into account in terms of depression of the ionization potentials, coulombic correction to plasma kinetic pressure, and truncated partition functions. A reduced formulation and efficient algorithm to solve the set of nonlinear Saha equations subjected to the constraints of electroneutrality and conservation of nuclei have been used to generate ionization equilibrium and Flinabe EOS over a wide range of temperatures and pressures. A criterion for the validity of the assumption of local thermodynamic equilibrium (LTE) is applied to the results showing the regions of pressure-temperature phase-space over which the LTE assumption can be justified and accepted. Estimates of high-temperature Flinabe EOS and ionization states are presented and discussed.

An embedded boundary method for viscous, conducting compressible flow

The evolution of an inertial fusion energy (IFE) chamber involves a repetition of short, intense depositions of energy (from target ignition) into a reaction chamber, followed by the turbulent relaxation of that energy through shock waves and thermal conduction to the vessel walls. We present an algorithm for 2D simulations of the fluid inside an IFE chamber between fueling repetitions. Our finite-volume discretization for the Navier–Stokes equations incorporates a Cartesian grid treatment for irregularly-shaped domain boundaries. The discrete conservative update is based on a time-explicit Godunov method for advection, and a two-stage Runge–Kutta update for diffusion accommodating state-dependent transport properties. Conservation is enforced on cut cells along the embedded boundary interface using a local redistribution scheme so that the explicit time step for the combined approach is governed by the mesh spacing in the uniform grid. The test problems demonstrate second-order convergence of the algorithm on smooth solution profiles, and the robust treatment of discontinuous initial data in an IFE-relevant vessel geometry.

Dynamics of Liquid-Protected Fusion Chambers

This paper presents an update of the work done at University of California, Berkeley (UCB) on thick-liquid protection of inertial fusion energy (IFE) chambers. UCB is focusing on microsecond, millisecond, and quasi-steady phenomena. Over microsecond time scales, numerical simulations, performed with the code TSUNAMI permit modeling of IFE chambers gas dynamics. For the millisecond range, the liquid jets response to the fusion reaction impulse loading is being studied for both Z-Pinch and HYLIFE-II-type chambers. A new mineral oil has been identified that allows scaled molten salt experiments with low distortion. Vortex tube flow, a key liquid structure of the 2002 Robust Point Design has been investigated in scaled experiments using the mineral oil, while a new design for thick liquid wall protection is under development. In quasi-steady phenomena, recent work has measured the Flibe vapor pressure and composition at near melting point temperature using mass spectrometry.

Views on Neutronics and Activation Issues Facing Liquid-Protected IFE Chambers

Thin and thick liquid walls provide an attractive solution to the challenging material issues facing the heavy-ion applications of the inertial fusion energy (IFE) concept. Given the many advantages of liquid-protected chambers, there are several nuclear-related concerns that are discussed in detail for the thick liquid wall option in particular. These are the ability to protect the steel-based structure from radiation damage and high activation, the feasibility of rewelding the structure, and the pulse-related problems. These issues have a profound impact on the ARIES-IFE thick-liquid protected chamber design.

Simulation of plasma afterglow phase in an inertial fusion energy chamber

A zero-dimensional model that describes the evolution of plasma and gas parameters during the afterglow phase of cyclic operation of an inertial fusion energy chamber is developed. The afterglow phase determines the conditions for next fuel-target injection and therefore its thorough analysis is very important for conceptual design of inertial fusion power plants. The model incorporates important effects of intrinsic impurity ion radiation on plasma cooling as well as of chamber opacity with respect to resonance radiation on plasma recombination in the working chamber. The decay times for the plasma and gas energy and, in particular, their dependence on initially stored energy, chamber size, and background gas concentration are analyzed. The heat power load onto the cryogenic fuel target due to residual plasma, gas, and radiation field is calculated. It is found that the higher the background gas density, the longer is the time into the afterglow phase necessary to reduce the heat flux on the target to an acceptable level.

IFE Chamber Technology – Status and Future Challenges

ABSTRACTSignificant progress has been made on addressing critical issues for inertial fusion energy (IFE) chambers for heavy-ion, laser and Z-pinch drivers. A variety of chamber concepts are being investigated including drywall (currently favored for laser IFE), wetted-wall (applicable to both laser and ion drivers), and thick-liquid-wall (favored by heavy ion and z-pinch drivers). Recent progress and remaining challenges in developing IFE chambers are reviewed.

Time-Dependent Neutronics in Structural Materials of Inertial Fusion Reactors and Simulation of Defect Accumulation in Pulsed Fe and SiC

New results are presented on the time-dependent neutron intensities and energy spectra from compressed inertial fusion energy (IFE) targets and in structural Fe walls behind typical IFE chamber protection schemes. Protection schemes of LiPb and Flibe have been considered with two different thicknesses, and neutron fluxes in the outer Fe layer as a function of the time from target emission are given. Differences between the two solutions are noted and explained, and the effect of thickness is quantitatively shown. Time-dependent defect characterization of the Fe layer under pulse irradiation is presented. A new well-established multiscale modeling procedure injects, at the appropriate dose rate, damage cascades in a kinetic Monte Carlo lattice (microscopic) to study defect diffusion, clustering, and disintegration. The differences with a continuous irradiation for a still low fluence of irradiation are presented. Experimental validation of a multiscale modeling approach has been recognized and proposed in the Spanish VENUS-II project by using Fe ions on pure and ultrapure Fe. To study similar problems in SiC, new tools are needed to quantify the kinetic defects; results leading to the validation of a new tight binding molecular dynamics code for SiC are presented.

IFE chamber dry wall materials response to pulsed X-rays and ions at power-plant level fluences

We have begun a collaborative investigation of the response of candidate first-wall inertial fusion energy (IFE) reactor chamber drywall materials to X-rays on the Z facility, and to ions on RHEPP-1, both located at Sandia National Laboratories. Dose levels are comparable to those anticipated in future direct-drive reactors. Due to the 5–10 Hz repetition rate expected in such reactors, per-pulse effects such as material removal must be negligible. The primary wall materials investigated here are graphite and tungsten in various forms. After exposure on either RHEPP or Z, materials were analyzed for roughening and/or material removal (ablation) as a function of dose. Graphite is observed to undergo significant ablation/sublimation in response to ion exposure at the 3–4 J/cm 2 level, significantly below doses expected in future dry-wall power plants. Evidence of thermomechanical stresses resulting in material loss occurs for both graphite and tungsten, and is probably related to the pulsed nature of the energy delivery. These effects are not seen in typical magnetic fusion energy (MFE) conditions where these same kinds of materials are used. Results are presented for thresholds below which no roughening or ablation occurs. Use of graphite in a ‘velvet’ two-dimensional form may mitigate the effects seen with the flat material, and alloying tungsten with rhenium may reduce its roughening due to the increased ductility of the alloy.

IFE chamber walls: requirements, design options, and synergy with MFE plasma facing components

The wall of inertial fusion energy (IFE) chambers faces demanding conditions. IFE operation is cyclic in nature and following each micro-explosion, the chamber wall is subjected to a large flux of photons, energetic particles and neutrons. Key requirements are that: (i) the chamber wall accommodates the cyclic energy deposition while providing the required lifetime, and (ii) that after each shot the chamber is cleared and returned to a quiescent state in preparation for the target injection and the firing of the driver for the subsequent shot. This paper summarizes the IFE operating conditions, discusses their impact on the choice of chamber wall configuration, and identifies the key issues. Particular attention is given to identifying common issues and operating conditions between IFE and magnetic fusion energy (MFE) with the goal of maximizing the synergy between chamber wall design and R&D for MFE and IFE.

Abatement of shocks during disruption of porous thick-liquid blankets in HYLIFE-type inertial fusion chambers

The dispersion and control of liquid kinetic energy and momentum are a major issue after the explosion of targets in an inertial fusion energy (IFE) thick-liquid chamber. For HYLIFE-type IFE chambers the desired repetition rate for shots varies from 5 to 10 Hz—a rate much too fast for natural gravity clearing of liquid debris. The impulse load delivered by a single shot generates enough kinetic energy in disrupted liquid material to do significant damage to surrounding solid structures. However, carefully designed jet arrays with regular void spacing can diffuse and dissipate kinetic energy that could damage adjacent structures, and rapidly moving, oscillating jets can dynamically clear the chamber center. A brief theoretical and mathematical background is given for this method of shock abatement, using the current Berkeley jet array experiment as a representative geometry.