Fusion Energy Applications Research Articles

Innovative construction of high-strength tungsten-copper joint through laser texture and nano-activation

Tungsten-copper (W-Cu) joints hold immense promise as plasma-facing materials in fusion reactors. However, the inherent immiscibility of W-Cu poses significant challenges in joint fabrication. Here, we introduce an innovative methodology that incorporates laser texture, W surface nano-activation, and subsequent diffusion bonding to fabricate W-Cu joints. Remarkably, the joints achieved exhibit unparalleled mechanical properties, with a peak tensile strength of 201 MPa and a shear strength of 141 MPa, surpassing previously reported W-Cu joints. To gain insights into the underlying mechanisms, we conducted a multiscale analysis utilizing scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and density-functional theory (DFT) calculations. Our findings reveal a unique embedded structure and a metallurgically bonded interface at the W-Cu junction. Furthermore, the diffusion zone at the interface exhibits a fascinating hybrid crystal structure, maintaining a body-centered cubic (BCC) structure in certain regions while displaying a tetragonal crystal structure (with lattice parameters a=b=2.8617, c=3.44) in others. This tetragonal crystal structure formation within the W-Cu diffusion zone remains unexplored in previous literature. In summary, this novel W-Cu bonding approach not only offers a cutting-edge solution for modern manufacturing and fusion energy applications but also lays a solid theoretical foundation for understanding the intricate microstructure-property relationships in W-Cu systems.

Experimental investigation of the cooling performance of an additively manufactured prototype for nuclear fusion energy application

Metal Additive Manufacturing (MAM) is a non-traditional technology recently introduced to manufacture multifunctional mechanical components. In fact, recent developments in Laser Powder Bed Fusion (LPBF) process have enabled the production of materials characterized by high density and high thermal conductivity properties, such as copper alloys and pure copper, making the technology attractive for thermal science. In nuclear fusion energy applications, mechanical components often encounter extremely high heat fluxes. An innovative solution using unconventional integrated cooling channels is therefore required to safely manage the components. However, the high surface roughness in 3D-printed parts represents an intrinsic limitation of the LPBF technology: the cooling channels show high-pressure drops due to the high viscous dissipation generated by the rough surface. To address this challenge, a lab-scale prototype of an e-beam extraction grid for a fusion experiment with an original integrated cooling system was designed and manufactured. Additionally, a novel heating mask was designed and manufactured to reproduce the realistic heat load distribution on the grid during the experimental tests. The prototype was built using additive manufacturing with a CuCrZr copper alloy. The grid underwent heat treatment via solution annealing and age hardening, to increase thermal conductivity from about 100 W m−1 K−1 to almost 300 W m−1 K−1. The prototype was tested at three different constant heat fluxes by varying the water flow rate while measuring the pressure and the maximum temperatures of the grid. A CFD numerical model was also calibrated to estimate the thermo-hydraulic performance of the prototype under test conditions. The experimental and numerical results are presented in terms of overall thermal performance, maximum temperature, and pressure drop.

Alternative coatings to cyanide silver coatings with low infrared emissivity for tokamak components

Alternative to cyanide silver coatings with low infrared emissivity for tokamak components were prepared by cyanide-free silver plating on nuclear-grade 304L stainless steel. The surface morphology, coating adhesion and emissivity were characterized and optimized. The experimental results indicate that the developed cyanide-free silver coating is a viable alternative to cyanide silver coating. The adhesion of silver coating was enhanced by the reactivating of nickel plating. The emissivity was reduced by decreasing surface roughness and increasing coating thickness. Both optimizations reach limit at 0.4 μm and 0.6 μm, respectively. Further decrease in surface roughness and increase in coating thickness did not contribute to decreasing emissivity. To sum up, the obtained cyanide-free silver coatings showed sufficient adhesion with emissivity as low as 0.02, thus providing an eco-friendly alternative solution to fusion energy application.

Thermal runaway criterion as a basis for the protection of high-temperature superconductor magnets

High-temperature superconductor (HTS) based high-field magnet systems are essential for particle accelerators and fusion energy applications. Quench protection of such magnets is difficult owing to a slow quench propagation velocity in HTS. While in conventional NbTi and Nb3Sn-based magnets, a normal zone expands typically quickly, and the stored energy is dissipated across a large volume of the windings, a normal zone in an HTS magnet propagates slowly and, thus, can heat up quickly to high temperatures destroying the conductor. At the same time, growing experimental evidence suggests that HTS conductors can operate in a stable dissipative flux flow regime for a substantial range of operational currents before entering an irreversible thermal runaway. Therefore, a new protection paradigm for HTS magnets has emerged, aiming to prevent quenching, using advanced diagnostics to detect the dissipative regime onset. In the present paper, we propose a simple criterion for the thermal runaway in HTS conductors and calculate allowable temperature margins within which an HTS magnet can be operated safely. Outside of those temperature margins, a common quench integral approach may be used to estimate the upper boundary of the time margin for activating the protection system. We verify the applicability of our approach by comparing the calculated runaway conditions for a Bi-2223 conductor with the experimentally measured values. The thermal and time margins can define the quench protection system’s requirements for implementing the quench-avoiding protection paradigm.

Energy gain of wetted-foam implosions with auxiliary heating for inertial fusion studies

Low convergence ratio implosions (where wetted-foam layers are used to limit capsule convergence, achieving improved robustness to instability growth) and auxiliary heating (where electron beams are used to provide collisionless heating of a hotspot) are two promising techniques that are being explored for inertial fusion energy applications. In this paper, a new analytic study is presented to understand and predict the performance of these implosions. Firstly, conventional gain models are adapted to produce gain curves for fixed convergence ratios, which are shown to well-describe previously simulated results. Secondly, auxiliary heating is demonstrated to be well understood and interpreted through the burn-up fraction of the deuterium-tritium fuel, with the gradient of burn-up with respect to burn-averaged temperature shown to provide good qualitative predictions of the effectiveness of this technique for a given implosion. Simulations of auxiliary heating for a range of implosions are presented in support of this and demonstrate that this heating can have significant benefit for high gain implosions, being most effective when the burn-averaged temperature is between 5 and 20 keV.

Optical roughness calculation for material structural analysis of energy structure applications under dc plasma processes

Surface qualities make aluminium a low-DC plasma interaction candidate. Aluminum for energy system structure building is studied experimentally, with observations obtained. Aluminum is cheap and frequently utilized in aerospace applications. The selection of materials for new applications of thermonuclear fusion energy, such as Tokamak reactor walls and fusion-based spaceship thrust structures, is important to decide in the design phase. In this study, an experimental setup application is created with low DC-type He plasma ions processed on aluminium pellet surfaces. The physical changes of the aluminium pellet material as an example of an energy structure surface are analysed under a scanned array microscope and 3D surface plots to detect optical roughness attributes.

Optical roughness calculation for material structural analysis of energy structure applications under dc plasma processes

Surface qualities make aluminium a low-DC plasma interaction candidate. Aluminum for energy system structure building is studied experimentally, with observations obtained. Aluminum is cheap and frequently utilized in aerospace applications. The selection of materials for new applications of thermonuclear fusion energy, such as Tokamak reactor walls and fusion-based spaceship thrust structures, is important to decide in the design phase. In this study, an experimental setup application is created with low DC-type He plasma ions processed on aluminium pellet surfaces. The physical changes of the aluminium pellet material as an example of an energy structure surface are analysed under a scanned array microscope and 3D surface plots to detect optical roughness attributes.

Sintered nanostructured alloys for advanced fusion energy applications

We describe our recent efforts demonstrating direct current sintering parameters appropriate to mimic near-identical microstructure to optimize reduced activation ferritic martensitic “castable nanostructured alloy”. The fabrication process is presented, and through a combination of computational thermodynamics, multimodal characterization, and mechanical testing we confirm that sintering may be used to produce relevant castable nanostructured alloy (CNA). Our success in demonstrating the applicability of sintering to CNA fabrication opens the opportunity to fabricate functionally graded first wall tile structures or other complicated structures with demanding high-temperature performance, as example fusion high heat flux components.

Radio Frequency-Based Diagnostics for Superconducting Magnets

Operation of high-field superconducting magnets relies on diagnostic instrumentation for measuring strain, temperature, and magnetic field variations, detecting quenching, and identifying performance problems. Discrete sensor implementation is costly, requires multi-channel data acquisition systems, and sensor density is often insufficient to resolve problematic locations spatially. Distributed sensing is a viable alternative approach providing location-specific diagnostic information over single terminal output. In particular, it can be the key to quench protection of high-temperature superconductor (HTS)-based magnets where hot spots are known to form and persist before a quench. Fiber-optic technology offers distributed sensing solution for magnets but suffers from drawbacks such as fiber fragility, high costs of optical interrogators, and difficulty in differentiating between physical quantities such as temperature and strain. We propose an alternative, robust, and easily integrable way of implementing distributed sensing in magnets using radio-frequency (RF) technologies. RF Time Domain Reflectometry (TDR) technique has been around for nearly 60 years, and it is an essential diagnostic tool used in multiple areas of technology and applied research. We discuss operational principles and practical implementation of RF TDR sensors capable of detecting local variations of strain, temperature, and magnetic field through changes in RF impedance and wave propagation time. Results of cryogenic testing of our TDR sensors with HTS tape conductors are presented. The perspective of enabling a new diagnostics paradigm for high-energy physics and fusion energy applications based on distributed RF sensing is discussed.

Li6BaLa2Ta2O12 Solid-State Probe for Studying Li Activity in Molten Sn-Li Alloys

Metals or alloys that are liquid at or near room temperature are of interest of the scientific community due to the versatility of their applications. Considering the nuclear fusion energy applications, Sn-Li alloys appear as a great candidate for plasma facing materials. However, plasma interactions can induce alterations on the lithium concentration and, as a consequence, modify the properties of the LM. Therefore, in order to prevent these situations, thermodynamic data such as activity coefficients and analytical devices capable of monitoring the lithium content in these alloys are of great interest. In this work, a lithium sensor based on the Li6BaLa2Ta2O12 solid-state electrolyte was used to determine lithium activity correlations for Sn-Li alloys at 400 and 500 °C. It was observed that the activity coefficient was constant for lithium concentrations lower than 8.5 at% with values, at the infinite dilution condition, of 8.4·10−5 at 400 °C and 2.7·10−4 at 500 °C, respectively. Above this concentration (from 8.5 to 16.5 at% Li), the activity coefficient varied with lithium concentration. In this case, correlations between lithium activity and the lithium atomic fraction were obtained.

Unsupervised discovery of nonlinear plasma physics using differentiable kinetic simulations

Plasma supports collective modes and particle–wave interactions that lead to complex behaviour in, for example, inertial fusion energy applications. While plasma can sometimes be modelled as a charged fluid, a kinetic description is often crucial for studying nonlinear effects in the higher-dimensional momentum–position phase space that describes the full complexity of the plasma dynamics. We create a differentiable solver for the three-dimensional partial-differential equation describing the plasma kinetics and introduce a domain-specific objective function. Using this framework, we perform gradient-based optimization of neural networks that provide forcing function parameters to the differentiable solver given a set of initial conditions. We apply this to an inertial-fusion-relevant configuration and find that the optimization process exploits a novel physical effect.

A conservative implicit-PIC scheme for the hybrid kinetic-ion fluid-electron plasma model on curvilinear meshes

The hybrid kinetic-ion fluid-electron plasma model is widely used to study challenging multi-scale problems in space and laboratory plasma physics. Here, a novel conservative scheme for this model employing implicit particle-in-cell techniques [1] is extended to arbitrary coordinate systems via curvilinear maps from logical to physical space. The scheme features a fully non-linear electromagnetic formulation with a multi-rate time advance – including sub-cycling and orbit-averaging for the kinetic ions. By careful choice of compatible particle-based kinetic-ion and mesh-based fluid-electron discretizations in curvilinear coordinates, as well as particle-mesh interpolations and implicit midpoint time advance, the scheme is proven to conserve total energy for arbitrary curvilinear meshes. In the electrostatic limit, the method is also proven to conserve total momentum for arbitrary curvilinear meshes. Although momentum is not conserved for arbitrary curvilinear meshes in the electromagnetic case, it is for an important subset of Cartesian tensor-packed meshes. The scheme and its novel conservation properties are demonstrated for several challenging numerical problems using different curvilinear meshes, including a merging flux-rope simulation for a space weather application, and a helical m=1 mode simulation for magnetic fusion energy application.

Advanced Processing and Machining of Tungsten and Its Alloys

Tungsten is a refractory metal with the highest melting temperature and density of all metals in this group. These properties, together with the high thermal conductivity and strength, make tungsten the ideal material for high-temperature structural use in fusion energy and other applications. It is widely agreed that the manufacture of components with complex geometries is crucial for scaling and optimizing power plant designs. However, there are challenges associated with the large-scale processing and manufacturing of parts made from tungsten and its alloys which limit the production of these complex geometries. These challenges stem from the high ductile-to-brittle transition temperature (DBTT), as well as the strength and hardness of these parts. Processing methods, such as powder metallurgy and additive manufacturing, can generate near-net-shaped components. However, subtractive post-processing techniques are required to complement these methods. This paper provides an in-depth exploration and discussion of different processing and manufacturing methods for tungsten and identifies the challenges and gaps associated with each approach. It includes conventional and unconventional machining processes, as well as research on improving the ductility of tungsten using various methods, such as alloying, thermomechanical treatment, and grain structure refinement.

Evidence of lithium mobility under neutron irradiation

Understanding compositional and microstructural changes in functional intermetallic coatings is of great importance for fusion energy and nuclear materials applications. Tritium ( 3 H) and lithium ( 6 Li, 7 Li) transport within a neutron irradiated target rod employing an aluminide-coated austenitic stainless-steel cladding was investigated using state-of-the-art multimodal imaging. Specifically, a scanning electron microscope augmented with focused ion beam (SEM-FIB) was used to prepare lift-out samples of the irradiated coating for microanalysis. Scanning transmission electron microscopy (STEM) was used to acquire atomic-scale information on the coating surface microstructure, morphology, and composition. Atomic force microscopy (AFM) was used to determine lift-out dimensions nondestructively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the presence of carbonaceous species and unexpected lithium isotopic distributions in the irradiated tubing, suggesting light isotope mobility between internal target components during irradiation. SIMS chemical mapping of aluminide coatings at core midplane and lower core locations of the cladding shows that light isotopic (e.g., 3 H, 6 Li, 7 Li) distributions are different in the irradiated coating. Advance correlative imaging results suggest lithium transport during the tritium production process and give new insights into the fundamental transport mechanism within the target during irradiation and non-equilibrium conditions.

Driven resonant current amplification in self-organized plasma configurations with uniform λ and plasma pressure confinement

Driven resonant current amplification in uniform λ=μoJB, self-organized plasma configurations with plasma pressure confinement is studied. This work extends previous analyses of force-free states (J∥B) to equilibria with plasma pressure gradients (∇P=J×B≠0). An analytical treatment shows that in equilibria with uniform λ and a plasma pressure gradient that varies linearly with poloidal magnetic flux, driven resonances are either downshifted or upshifted from their force-free values. Additionally, a modification of off resonance current amplification is observed in pressure confining equilibria. Computational results add support to analytical predictions, and also include driven resonance behavior with equilibrium profiles not considered analytically. These results suggest not only persistence, but also modification of driven resonant current amplification in sustained spheromak configurations with plasma pressure confinement when compared to force-free states. By providing a basis for the existence of driven, high current amplification, pressure confining equilibria, this work provides insight into the accessibility of sustained spheromak states of most interest for potential fusion energy applications.

Evolution of Defects in CVD-W Irradiated by H/He Neutral Beam Using Positron Annihilation Spectroscopy

One of the key problems for the application of nuclear fusion energy is to select the suitable plasma facing materials (PFMs). Among the W-based materials, CVD-W exhibits some unique advantages. In order to estimate the performance of CVD-W under the fusion environment, the vacancy-type defects and their evolution are investigated by the Doppler-broadening slow positron beam analysis (DB-SPBA) combined with SEM (scanning electron microscope). There are two kinds of neutral beam irradiation, the pure H neutral beam and the H + 6 at.% He neutral beam irradiation, which are performed at the neutral beam facility GLADIS (IPP, Germany). The surface temperatures of CVD-W irradiated by H (H + 6 at.% He) are 850 and 1000 (700 and 800 °C). By comparing the samples under different conditions, the defect evolution of CVD-W is obtained. As for the pure H neutral beam irradiated samples, the DB-SPBA results demonstrate that the CVD-W sample at the surface temperature of 1000 °C, compared to the 850 °C sample, shows a decrease in S parameters, which is due to the reduction of vacancy-type defect concentration. The defect damage layer in 1000 °C sample is narrower than that of 850 °C sample and the defect type tends to be consistent in 1000 °C sample. The SEM results suggest that the surface damage of the 1000 °C sample was recovered to some extent. As for the H + 6 at.% He neutral beam irradiated samples, compared with the CVD-W sample at the surface temperature of 700 °C, the 800 °C sample shows an increased S parameters, which can be attributed to the volume increase of vacancy-type defect. The defect damage layer in the 800 °C sample is wider than that of the 700 °C sample. Both the H + 6 at.% He irradiated samples show complex defect types. The surface of the 800 °C sample exhibits more dense pinhole damage structures compared to that of the 700 °C sample.

Quantifying uncertainties in neutron- α scattering with chiral nucleon-nucleon and three-nucleon forces

Background: Modern ab initio theory combined with high-quality nucleon-nucleon (NN) and three-nucleon (3N) interactions from chiral effective field theory (EFT) can provide a predictive description of low-energy light-nuclei reactions relevant for astrophysics and fusion-energy applications. However, the high cost of computations has so far impeded a complete analysis of the uncertainty budget of such calculations. Purpose: Starting from NN potentials up to fifth order (N4LO) combined with leading-order 3N forces, we study how the order-by-order convergence of the chiral expansion and confidence intervals for the 3N contact and contact-plus-one-pion-exchange low-energy constants (cE and cD) contribute to the overall uncertainty budget of many-body calculations of neutron-He elastic scattering. Methods: We compute structure and reaction observables for three-, four- and five-nucleon systems within the ab initio frameworks of the no-core shell model an no-core shell model with continuum. Using a small set of design runs, we construct a Gaussian process model (GPM) that acts as a statistical emulator for the theory. With this, we gain insight into how uncertainties in the 3N low-energy constants propagate throughout the calculation and determine the Bayesian posterior distribution of these parameters with Markov-Chain Monte-Carlo.

Superconducting Cu/Nb nanolaminate by coded accumulative roll bonding and its helium damage characteristics

A very broad distribution of microstructural length scales spanning few nm- to the μm-scale has proven effective to achieve exceptional materials properties. Here, we fabricate a Cu/Nb two-phase composite made of a hierarchically layered structure by modifying the conventional accumulative roll bonding (ARB) technique, where fresh Nb sheets are inserted and bonded during a repeated stacking and rolling process. This barcode-like multilayer with a designed hierarchical length scale distribution possesses densely distributed phase boundaries and rich interfacial structures. The composite demonstrates similar superconductivity characteristics as pure Nb, but is 3 × stronger, has theoretically better oxidation resistance, and retains considerable ductility. Under the helium irradiation environment, the unique interfacial structures featuring chemical intermixing zones (3-dimensional) are more immune to the formation of large helium clusters than atomically sharp interfaces (2-dimensional), screening them from radiation damage and improving their long-term mechanical integrity. This work signifies an effective strategy of constructing hierarchical laminates to achieve high-performance materials, which holds promise in fusion and fission energy applications.

Development and experiments of LiPb coolant technologies for fusion blanket in China

The liquid lithium lead (LiPb) blanket concept has become a promising design for fusion reactor, for example, international thermonuclear experimental reactor (ITER) and the demonstration (DEMO) reactor. To promote the successful application of fusion energy, detailed studies on key issues of the LiPb blanket were performed, such as corrosion behavior, thermal hydraulics and coolant safety etc. Some experimental facilities needed for the key issue studies were designed and developed under the research program. The series LiPb experimental loops, named DRAGON, were developed in China, including the thermal convection loops DRAGON-I/II/III and the multi-functional experimental loop DRAGON-IV. To perform the integrated experiments under the multi-physical field conditions for DEMO blanket, a dual-coolant thermal integrated experimental loop DRAGON-V was built in 2017. It is a multi-functional test platform for the research and development (R&D) of key issues of liquid LiPb blanket to provide the necessary database for ITER and DEMO. Up to now, a lot of experiments have been performed to investigate the LiPb technologies, such as corrosion behaviors of structural materials both with and without magnetic field, corrosion resistance technology, magneto-hydrodynamics effect, and interaction between typical coolants during in-box loss of coolant accident etc. The results will support the R&D of the key techniques and the engineering design for ITER test blanket module and further DEMO.

Nuclear Astrophysics with Lasers

A systematic program of investigations of nuclear reactions in laser-produced plasmas is addressed. Such reactions provide an important diagnostic tool for probing the dynamics and thermodynamics in the plasma and understanding laser ion acceleration and neutron production mechanisms. The goal will be to reach the level of knowledge that allows the measurement of fundamental nuclear cross-sections at low and high particle densities. The quantitative measurement of fusion probabilities in hot and dense plasmas will contribute significantly to our comprehension of stellar composition and evolution and will provide important information for development of fusion energy production and applications such as medical isotope production and compact neutron source development. All of these are some of the main goals of the European Extreme Light Infrastructure (ELI), the Shanghai Superintense Ultrafast Laser Facility, and the Station of Extreme Light in China and similar projects in other countries.