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      in Energy

      AI method radically speeds predictions of materials’ thermal properties

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      It is estimated that about 70 percent of the energy generated worldwide ends up as waste heat.If scientists could better predict how heat moves through semiconductors and insulators, they could design more efficient power generation systems. However, the thermal properties of materials can be exceedingly difficult to model.The trouble comes from phonons, which are subatomic particles that carry heat. Some of a material’s thermal properties depend on a measurement called the phonon dispersion relation, which can be incredibly hard to obtain, let alone utilize in the design of a system.A team of researchers from MIT and elsewhere tackled this challenge by rethinking the problem from the ground up. The result of their work is a new machine-learning framework that can predict phonon dispersion relations up to 1,000 times faster than other AI-based techniques, with comparable or even better accuracy. Compared to more traditional, non-AI-based approaches, it could be 1 million times faster.This method could help engineers design energy generation systems that produce more power, more efficiently. It could also be used to develop more efficient microelectronics, since managing heat remains a major bottleneck to speeding up electronics.“Phonons are the culprit for the thermal loss, yet obtaining their properties is notoriously challenging, either computationally or experimentally,” says Mingda Li, associate professor of nuclear science and engineering and senior author of a paper on this technique.Li is joined on the paper by co-lead authors Ryotaro Okabe, a chemistry graduate student; and Abhijatmedhi Chotrattanapituk, an electrical engineering and computer science graduate student; Tommi Jaakkola, the Thomas Siebel Professor of Electrical Engineering and Computer Science at MIT; as well as others at MIT, Argonne National Laboratory, Harvard University, the University of South Carolina, Emory University, the University of California at Santa Barbara, and Oak Ridge National Laboratory. The research appears in Nature Computational Science.Predicting phononsHeat-carrying phonons are tricky to predict because they have an extremely wide frequency range, and the particles interact and travel at different speeds.A material’s phonon dispersion relation is the relationship between energy and momentum of phonons in its crystal structure. For years, researchers have tried to predict phonon dispersion relations using machine learning, but there are so many high-precision calculations involved that models get bogged down.“If you have 100 CPUs and a few weeks, you could probably calculate the phonon dispersion relation for one material. The whole community really wants a more efficient way to do this,” says Okabe.The machine-learning models scientists often use for these calculations are known as graph neural networks (GNN). A GNN converts a material’s atomic structure into a crystal graph comprising multiple nodes, which represent atoms, connected by edges, which represent the interatomic bonding between atoms.While GNNs work well for calculating many quantities, like magnetization or electrical polarization, they are not flexible enough to efficiently predict an extremely high-dimensional quantity like the phonon dispersion relation. Because phonons can travel around atoms on X, Y, and Z axes, their momentum space is hard to model with a fixed graph structure.To gain the flexibility they needed, Li and his collaborators devised virtual nodes.They create what they call a virtual node graph neural network (VGNN) by adding a series of flexible virtual nodes to the fixed crystal structure to represent phonons. The virtual nodes enable the output of the neural network to vary in size, so it is not restricted by the fixed crystal structure.Virtual nodes are connected to the graph in such a way that they can only receive messages from real nodes. While virtual nodes will be updated as the model updates real nodes during computation, they do not affect the accuracy of the model.“The way we do this is very efficient in coding. You just generate a few more nodes in your GNN. The physical location doesn’t matter, and the real nodes don’t even know the virtual nodes are there,” says Chotrattanapituk.Cutting out complexitySince it has virtual nodes to represent phonons, the VGNN can skip many complex calculations when estimating phonon dispersion relations, which makes the method more efficient than a standard GNN. The researchers proposed three different versions of VGNNs with increasing complexity. Each can be used to predict phonons directly from a material’s atomic coordinates.Because their approach has the flexibility to rapidly model high-dimensional properties, they can use it to estimate phonon dispersion relations in alloy systems. These complex combinations of metals and nonmetals are especially challenging for traditional approaches to model.The researchers also found that VGNNs offered slightly greater accuracy when predicting a material’s heat capacity. In some instances, prediction errors were two orders of magnitude lower with their technique.A VGNN could be used to calculate phonon dispersion relations for a few thousand materials in just a few seconds with a personal computer, Li says.This efficiency could enable scientists to search a larger space when seeking materials with certain thermal properties, such as superior thermal storage, energy conversion, or superconductivity.Moreover, the virtual node technique is not exclusive to phonons, and could also be used to predict challenging optical and magnetic properties.In the future, the researchers want to refine the technique so virtual nodes have greater sensitivity to capture small changes that can affect phonon structure.“Researchers got too comfortable using graph nodes to represent atoms, but we can rethink that. Graph nodes can be anything. And virtual nodes are a very generic approach you could use to predict a lot of high-dimensional quantities,” Li says.“The authors’ innovative approach significantly augments the graph neural network description of solids by incorporating key physics-informed elements through virtual nodes, for instance, informing wave-vector dependent band-structures and dynamical matrices,” says Olivier Delaire, associate professor in the Thomas Lord Department of Mechanical Engineering and Materials Science at Duke University, who was not involved with this work. “I find that the level of acceleration in predicting complex phonon properties is amazing, several orders of magnitude faster than a state-of-the-art universal machine-learning interatomic potential. Impressively, the advanced neural net captures fine features and obeys physical rules. There is great potential to expand the model to describe other important material properties: Electronic, optical, and magnetic spectra and band structures come to mind.”This work is supported by the U.S. Department of Energy, National Science Foundation, a Mathworks Fellowship, a Sow-Hsin Chen Fellowship, the Harvard Quantum Initiative, and the Oak Ridge National Laboratory. More

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      in Energy

      Nuno Loureiro named director of MIT’s Plasma Science and Fusion Center

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      Nuno Loureiro, professor of nuclear science and engineering and of physics, has been appointed the new director of the MIT Plasma Science and Fusion Center, effective May 1.Loureiro is taking the helm of one of MIT’s largest labs: more than 250 full-time researchers, staff members, and students work and study in seven buildings with 250,000 square feet of lab space. A theoretical physicist and fusion scientist, Loureiro joined MIT as a faculty member in 2016, and was appointed deputy director of the Plasma Science and Fusion Center (PSFC) in 2022. Loureiro succeeds Dennis Whyte, who stepped down at the end of 2023 to return to teaching and research.Stepping into his new role as director, Loureiro says, “The PSFC has an impressive tradition of discovery and leadership in plasma and fusion science and engineering. Becoming director of the PSFC is an incredible opportunity to shape the future of these fields. We have a world-class team, and it’s an honor to be chosen as its leader.”Loureiro’s own research ranges widely. He is recognized for advancing the understanding of multiple aspects of plasma behavior, particularly turbulence and the physics underpinning solar flares and other astronomical phenomena. In the fusion domain, his work enables the design of fusion devices that can more efficiently control and harness the energy of fusing plasmas, bringing the dream of clean, near-limitless fusion power that much closer. Plasma physics is foundational to advancing fusion science, a fact Loureiro has embraced and that is relevant as he considers the direction of the PSFC’s multidisciplinary research. “But plasma physics is only one aspect of our focus. Building a scientific agenda that continues and expands on the PSFC’s history of innovation in all aspects of fusion science and engineering is vital, and a key facet of that work is facilitating our researchers’ efforts to produce the breakthroughs that are necessary for the realization of fusion energy.”As the climate crisis accelerates, fusion power continues to grow in appeal: It produces no carbon emissions, its fuel is plentiful, and dangerous “meltdowns” are impossible. The sooner that fusion power is commercially available, the greater impact it can have on reducing greenhouse gas emissions and meeting global climate goals. While technical challenges remain, “the PSFC is well poised to meet them, and continue to show leadership. We are a mission-driven lab, and our students and staff are incredibly motivated,” Loureiro comments.“As MIT continues to lead the way toward the delivery of clean fusion power onto the grid, I have no doubt that Nuno is the right person to step into this key position at this critical time,” says Maria T. Zuber, MIT’s presidential advisor for science and technology policy. “I look forward to the steady advance of plasma physics and fusion science at MIT under Nuno’s leadership.”Over the last decade, there have been massive leaps forward in the field of fusion energy, driven in part by innovations like high-temperature superconducting magnets developed at the PSFC. Further progress is guaranteed: Loureiro believes that “The next few years are certain to be an exciting time for us, and for fusion as a whole. It’s the dawn of a new era with burning plasma experiments” — a reference to the collaboration between the PSFC and Commonwealth Fusion Systems, a startup company spun out of the PSFC, to build SPARC, a fusion device that is slated to turn on in 2026 and produce a burning plasma that yields more energy than it consumes. “It’s going to be a watershed moment,” says Loureiro.He continues, “In addition, we have strong connections to inertial confinement fusion experiments, including those at Lawrence Livermore National Lab, and we’re looking forward to expanding our research into stellarators, which are another kind of magnetic fusion device.” Over recent years, the PSFC has significantly increased its collaboration with industrial partners such Eni, IBM, and others. Loureiro sees great value in this: “These collaborations are mutually beneficial: they allow us to grow our research portfolio while advancing companies’ R&D efforts. It’s very dynamic and exciting.”Loureiro’s directorship begins as the PSFC is launching key tech development projects like LIBRA, a “blanket” of molten salt that can be wrapped around fusion vessels and perform double duty as a neutron energy absorber and a breeder for tritium (the fuel for fusion). Researchers at the PSFC have also developed a way to rapidly test the durability of materials being considered for use in a fusion power plant environment, and are now creating an experiment that will utilize a powerful particle accelerator called a gyrotron to irradiate candidate materials.Interest in fusion is at an all-time high; the demand for researchers and engineers, particularly in the nascent commercial fusion industry, is reflected by the record number of graduate students that are studying at the PSFC — more than 90 across seven affiliated MIT departments. The PSFC’s classrooms are full, and Loureiro notes a palpable sense of excitement. “Students are our greatest strength,” says Loureiro. “They come here to do world-class research but also to grow as individuals, and I want to give them a great place to do that. Supporting those experiences, making sure they can be as successful as possible is one of my top priorities.” Loureiro plans to continue teaching and advising students after his appointment begins.MIT President Sally Kornbluth’s recently announced Climate Project is a clarion call for Loureiro: “It’s not hyperbole to say MIT is where you go to find solutions to humanity’s biggest problems,” he says. “Fusion is a hard problem, but it can be solved with resolve and ingenuity — characteristics that define MIT. Fusion energy will change the course of human history. It’s both humbling and exciting to be leading a research center that will play a key role in enabling that change.”  More

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      in Environment

      Two MIT teams selected for NSF sustainable materials grants

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      Two teams led by MIT researchers were selected in December 2023 by the U.S. National Science Foundation (NSF) Convergence Accelerator, a part of the TIP Directorate, to receive awards of $5 million each over three years, to pursue research aimed at helping to bring cutting-edge new sustainable materials and processes from the lab into practical, full-scale industrial production. The selection was made after 16 teams from around the country were chosen last year for one-year grants to develop detailed plans for further research aimed at solving problems of sustainability and scalability for advanced electronic products.

      Of the two MIT-led teams chosen for this current round of funding, one team, Topological Electric, is led by Mingda Li, an associate professor in the Department of Nuclear Science and Engineering. This team will be finding pathways to scale up sustainable topological materials, which have the potential to revolutionize next-generation microelectronics by showing superior electronic performance, such as dissipationless states or high-frequency response. The other team, led by Anuradha Agarwal, a principal research scientist at MIT’s Materials Research Laboratory, will be focusing on developing new materials, devices, and manufacturing processes for microchips that minimize energy consumption using electronic-photonic integration, and that detect and avoid the toxic or scarce materials used in today’s production methods.

      Scaling the use of topological materials

      Li explains that some materials based on quantum effects have achieved successful transitions from lab curiosities to successful mass production, such as blue-light LEDs, and giant magnetorestance (GMR) devices used for magnetic data storage. But he says there are a variety of equally promising materials that have shown promise but have yet to make it into real-world applications.

      “What we really wanted to achieve is to bring newer-generation quantum materials into technology and mass production, for the benefit of broader society,” he says. In particular, he says, “topological materials are really promising to do many different things.”

      Topological materials are ones whose electronic properties are fundamentally protected against disturbance. For example, Li points to the fact that just in the last two years, it has been shown that some topological materials are even better electrical conductors than copper, which are typically used for the wires interconnecting electronic components. But unlike the blue-light LEDs or the GMR devices, which have been widely produced and deployed, when it comes to topological materials, “there’s no company, no startup, there’s really no business out there,” adds Tomas Palacios, the Clarence J. Lebel Professor in Electrical Engineering at MIT and co-principal investigator on Li’s team. Part of the reason is that many versions of such materials are studied “with a focus on fundamental exotic physical properties with little or no consideration on the sustainability aspects,” says Liang Fu, an MIT professor of physics and also a co-PI. Their team will be looking for alternative formulations that are more amenable to mass production.

      One possible application of these topological materials is for detecting terahertz radiation, explains Keith Nelson, an MIT professor of chemistry and co-PI. This extremely high-frequency electronics can carry far more information than conventional radio or microwaves, but at present there are no mature electronic devices available that are scalable at this frequency range. “There’s a whole range of possibilities for topological materials” that could work at these frequencies, he says. In addition, he says, “we hope to demonstrate an entire prototype system like this in a single, very compact solid-state platform.”

      Li says that among the many possible applications of topological devices for microelectronics devices of various kinds, “we don’t know which, exactly, will end up as a product, or will reach real industrial scaleup. That’s why this opportunity from NSF is like a bridge, which is precious, to allow us to dig deeper to unleash the true potential.”

      In addition to Li, Palacios, Fu, and Nelson, the Topological Electric team includes Qiong Ma, assistant professor of physics in Boston College; Farnaz Niroui, assistant professor of electrical engineering and computer science at MIT; Susanne Stemmer, professor of materials at the University of California at Santa Barbara; Judy Cha, professor of materials science and engineering at Cornell University; industrial partners including IBM, Analog Devices, and Raytheon; and professional consultants. “We are taking this opportunity seriously,” Li says. “We really want to see if the topological materials are as good as we show in the lab when being scaled up, and how far we can push to broadly industrialize them.”

      Toward sustainable microchip production and use

      The microchips behind everything from smartphones to medical imaging are associated with a significant percentage of greenhouse gas emissions today, and every year the world produces more than 50 million metric tons of electronic waste, the equivalent of about 5,000 Eiffel Towers. Further, the data centers necessary for complex computations and huge amount of data transfer — think AI and on-demand video — are growing and will require 10 percent of the world’s electricity by 2030.

      “The current microchip manufacturing supply chain, which includes production, distribution, and use, is neither scalable nor sustainable, and cannot continue. We must innovate our way out of this crisis,” says Agarwal.

      The name of Agarwal’s team, FUTUR-IC, is a reference to the future of the integrated circuits, or chips, through a global alliance for sustainable microchip manufacturing. Says Agarwal, “We bring together stakeholders from industry, academia, and government to co-optimize across three dimensions: technology, ecology, and workforce. These were identified as key interrelated areas by some 140 stakeholders. With FUTUR-IC we aim to cut waste and CO2-equivalent emissions associated with electronics by 50 percent every 10 years.”

      The market for microelectronics in the next decade is predicted to be on the order of a trillion dollars, but most of the manufacturing for the industry occurs only in limited geographical pockets around the world. FUTUR-IC aims to diversify and strengthen the supply chain for manufacturing and packaging of electronics. The alliance has 26 collaborators and is growing. Current external collaborators include the International Electronics Manufacturing Initiative (iNEMI), Tyndall National Institute, SEMI, Hewlett Packard Enterprise, Intel, and the Rochester Institute of Technology.

      Agarwal leads FUTUR-IC in close collaboration with others, including, from MIT, Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering; Elsa Olivetti, the Jerry McAfee Professor in Engineering; Randolph Kirchain, principal research scientist in the Materials Research Laboratory; and Greg Norris, director of MIT’s Sustainability and Health Initiative for NetPositive Enterprise (SHINE). All are affiliated with the Materials Research Laboratory. They are joined by Samuel Serna, an MIT visiting professor and assistant professor of physics at Bridgewater State University. Other key personnel include Sajan Saini, education director for the Initiative for Knowledge and Innovation in Manufacturing in MIT’s Department of Materials Science and Engineering; Peter O’Brien, a professor from Tyndall National Institute; and Shekhar Chandrashekhar, CEO of iNEMI.

      “We expect the integration of electronics and photonics to revolutionize microchip manufacturing, enhancing efficiency, reducing energy consumption, and paving the way for unprecedented advancements in computing speed and data-processing capabilities,” says Serna, who is the co-lead on the project’s technology “vector.”

      Common metrics for these efforts are needed, says Norris, co-lead for the ecology vector, adding, “The microchip industry must have transparent and open Life Cycle Assessment (LCA) models and data, which are being developed by FUTUR-IC.” This is especially important given that microelectronics production transcends industries. “Given the scale and scope of microelectronics, it is critical for the industry to lead in the transition to sustainable manufacture and use,” says Kirchain, another co-lead and the co-director of the Concrete Sustainability Hub at MIT. To bring about this cross-fertilization, co-lead Olivetti, also co-director of the MIT Climate and Sustainability Consortium (MCSC), will collaborate with FUTUR-IC to enhance the benefits from microchip recycling, leveraging the learning across industries.

      Saini, the co-lead for the workforce vector, stresses the need for agility. “With a workforce that adapts to a practice of continuous upskilling, we can help increase the robustness of the chip-manufacturing supply chain, and validate a new design for a sustainability curriculum,” he says.

      “We have become accustomed to the benefits forged by the exponential growth of microelectronic technology performance and market size,” says Kimerling, who is also director of MIT’s Materials Research Laboratory and co-director of the MIT Microphotonics Center. “The ecological impact of this growth in terms of materials use, energy consumption and end-of-life disposal has begun to push back against this progress. We believe that concurrently engineered solutions for these three dimensions will build a common learning curve to power the next 40 years of progress in the semiconductor industry.”

      The MIT teams are two of six that received awards addressing sustainable materials for global challenges through phase two of the NSF Convergence Accelerator program. Launched in 2019, the program targets solutions to especially compelling challenges at an accelerated pace by incorporating a multidisciplinary research approach. More

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      in Energy

      Propelling atomically layered magnets toward green computers

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      Globally, computation is booming at an unprecedented rate, fueled by the boons of artificial intelligence. With this, the staggering energy demand of the world’s computing infrastructure has become a major concern, and the development of computing devices that are far more energy-efficient is a leading challenge for the scientific community. 

      Use of magnetic materials to build computing devices like memories and processors has emerged as a promising avenue for creating “beyond-CMOS” computers, which would use far less energy compared to traditional computers. Magnetization switching in magnets can be used in computation the same way that a transistor switches from open or closed to represent the 0s and 1s of binary code. 

      While much of the research along this direction has focused on using bulk magnetic materials, a new class of magnetic materials — called two-dimensional van der Waals magnets — provides superior properties that can improve the scalability and energy efficiency of magnetic devices to make them commercially viable. 

      Although the benefits of shifting to 2D magnetic materials are evident, their practical induction into computers has been hindered by some fundamental challenges. Until recently, 2D magnetic materials could operate only at very low temperatures, much like superconductors. So bringing their operating temperatures above room temperature has remained a primary goal. Additionally, for use in computers, it is important that they can be controlled electrically, without the need for magnetic fields. Bridging this fundamental gap, where 2D magnetic materials can be electrically switched above room temperature without any magnetic fields, could potentially catapult the translation of 2D magnets into the next generation of “green” computers.

      A team of MIT researchers has now achieved this critical milestone by designing a “van der Waals atomically layered heterostructure” device where a 2D van der Waals magnet, iron gallium telluride, is interfaced with another 2D material, tungsten ditelluride. In an open-access paper published March 15 in Science Advances, the team shows that the magnet can be toggled between the 0 and 1 states simply by applying pulses of electrical current across their two-layer device. 

      Play video

      The Future of Spintronics: Manipulating Spins in Atomic Layers without External Magnetic FieldsVideo: Deblina Sarkar

      “Our device enables robust magnetization switching without the need for an external magnetic field, opening up unprecedented opportunities for ultra-low power and environmentally sustainable computing technology for big data and AI,” says lead author Deblina Sarkar, the AT&T Career Development Assistant Professor at the MIT Media Lab and Center for Neurobiological Engineering, and head of the Nano-Cybernetic Biotrek research group. “Moreover, the atomically layered structure of our device provides unique capabilities including improved interface and possibilities of gate voltage tunability, as well as flexible and transparent spintronic technologies.”

      Sarkar is joined on the paper by first author Shivam Kajale, a graduate student in Sarkar’s research group at the Media Lab; Thanh Nguyen, a graduate student in the Department of Nuclear Science and Engineering (NSE); Nguyen Tuan Hung, an MIT visiting scholar in NSE and an assistant professor at Tohoku University in Japan; and Mingda Li, associate professor of NSE.

      Breaking the mirror symmetries 

      When electric current flows through heavy metals like platinum or tantalum, the electrons get segregated in the materials based on their spin component, a phenomenon called the spin Hall effect, says Kajale. The way this segregation happens depends on the material, and particularly its symmetries.

      “The conversion of electric current to spin currents in heavy metals lies at the heart of controlling magnets electrically,” Kajale notes. “The microscopic structure of conventionally used materials, like platinum, have a kind of mirror symmetry, which restricts the spin currents only to in-plane spin polarization.”

      Kajale explains that two mirror symmetries must be broken to produce an “out-of-plane” spin component that can be transferred to a magnetic layer to induce field-free switching. “Electrical current can ‘break’ the mirror symmetry along one plane in platinum, but its crystal structure prevents the mirror symmetry from being broken in a second plane.”

      In their earlier experiments, the researchers used a small magnetic field to break the second mirror plane. To get rid of the need for a magnetic nudge, Kajale and Sarkar and colleagues looked instead for a material with a structure that could break the second mirror plane without outside help. This led them to another 2D material, tungsten ditelluride. The tungsten ditelluride that the researchers used has an orthorhombic crystal structure. The material itself has one broken mirror plane. Thus, by applying current along its low-symmetry axis (parallel to the broken mirror plane), the resulting spin current has an out-of-plane spin component that can directly induce switching in the ultra-thin magnet interfaced with the tungsten ditelluride. 

      “Because it’s also a 2D van der Waals material, it can also ensure that when we stack the two materials together, we get pristine interfaces and a good flow of electron spins between the materials,” says Kajale. 

      Becoming more energy-efficient 

      Computer memory and processors built from magnetic materials use less energy than traditional silicon-based devices. And the van der Waals magnets can offer higher energy efficiency and better scalability compared to bulk magnetic material, the researchers note. 

      The electrical current density used for switching the magnet translates to how much energy is dissipated during switching. A lower density means a much more energy-efficient material. “The new design has one of the lowest current densities in van der Waals magnetic materials,” Kajale says. “This new design has an order of magnitude lower in terms of the switching current required in bulk materials. This translates to something like two orders of magnitude improvement in energy efficiency.”

      The research team is now looking at similar low-symmetry van der Waals materials to see if they can reduce current density even further. They are also hoping to collaborate with other researchers to find ways to manufacture the 2D magnetic switch devices at commercial scale. 

      This work was carried out, in part, using the facilities at MIT.nano. It was funded by the Media Lab, the U.S. National Science Foundation, and the U.S. Department of Energy. More

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      in Energy

      Lessons from Fukushima: Prepare for the unlikely

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      When a devastating earthquake and tsunami overwhelmed the protective systems at the Fukushima Dai’ichi nuclear power plant complex in Japan in March 2011, it triggered a sequence of events leading to one of the worst releases of radioactive materials in the world to date. Although nuclear energy is having a revival as a low-emissions energy source to mitigate climate change, the Fukushima accident is still cited as a reason for hesitancy in adopting it.

      A new study synthesizes information from multidisciplinary sources to understand how the Fukushima Dai’ichi disaster unfolded, and points to the importance of mitigation measures and last lines of defense — even against accidents considered highly unlikely. These procedures have received relatively little attention, but they are critical in determining how severe the consequences of a reactor failure will be, the researchers say.

      The researchers note that their synthesis is one of the few attempts to look at data across disciplinary boundaries, including: the physics and engineering of what took place within the plant’s systems, the plant operators’ actions throughout the emergency, actions by emergency responders, the meteorology of radionuclide releases and transport, and the environmental and health consequences documented since the event.

      The study appears in the journal iScience, in an open-access paper by postdoc Ali Ayoub and Professor Haruko Wainwright at MIT, along with others in Switzerland, Japan, and New Mexico.

      Since 2013, Wainwright has been leading the research to integrate all the radiation monitoring data in the Fukushima region into integrated maps. “I was staring at the contamination map for nearly 10 years, wondering what created the main plume extending in the northwest direction, but I could not find exact information,” Wainwright says. “Our study is unique because we started from the consequence, the contamination map, and tried to identify the key factors for the consequence. Other people study the Fukushima accident from the root cause, the tsunami.”

      One thing they found was that while all the operating reactors, units 1, 2, and 3, suffered core meltdowns as a result of the failure of emergency cooling systems, units 1 and 3 — although they did experience hydrogen explosions — did not release as much radiation to the environment because their venting systems essentially worked to relieve pressure inside the containment vessels as intended. But the same system in unit 2 failed badly.

      “People think that the hydrogen explosion or the core meltdown were the worst things, or the major driver of the radiological consequences of the accident,” Wainright says, “but our analysis found that’s not the case.” Much more significant in terms of the radiological release was the failure of the one venting mechanism.

      “There is a pressure-release mechanism that goes through water where a lot of the radionuclides get filtered out,” she explains. That system was effective in units 1 and 3, filtering out more than 90 percent of the radioactive elements before the gas was vented. However, “in unit 2, that pressure release mechanism got stuck, and the operators could not manually open it.” A hydrogen explosion in unit 1 had damaged the pressure relief mechanism of unit 2. This led to a breach of the containment structure and direct, unfiltered venting to the atmosphere, which, according to the new study, was what produced the greatest amount of contamination from the whole weeks-long event.

      Another factor was the timing of the attempt to vent the pressure buildup in the reactor. Guidelines at the time, and to this day in many reactors, specified that no venting should take place until the pressure inside the reactor containment vessel reached a specified threshold, with no regard to the wind directions at the time. In the case of Fukushima, an earlier venting could have dramatically reduced the impact: Much of the release happened when winds were blowing directly inland, but earlier the wind had been blowing offshore.

      “That pressure-release mechanism has not been a major focus of the engineering community,” she says. While there is appropriate attention to measures that prevent a core meltdown in the first place, “this sort of last line of defense has not been the main focus and should get more attention.”

      Wainwright says the study also underlines several successes in the management of the Fukushima accident. Many of the safety systems did work as they were designed. For example, even though the oldest reactor, unit 1, suffered the greatest internal damage, it released little radioactive material. Most people were able to evacuate from the 20-kilometer (12-mile) zone before the largest release happened. The mitigation measures were “somewhat successful,” Wainwright says. But there was tremendous confusion and anger during and after the accident because there were no preparations in place for such an event.

      Much work has focused on ways to prevent the kind of accidents that happened at Fukushima — for example, in the U.S. reactor operators can deploy portable backup power supplies to maintain proper reactor cooling at any reactor site. But the ongoing situation at the Zaporizhzhia nuclear complex in Ukraine, where nuclear safety is challenged by acts of war, demonstrates that despite engineers’ and operators’ best efforts to prevent it, “the totally unexpected could still happen,” Wainwright says.

      “The big-picture message is that we should have equal attention to both prevention and mitigation of accidents,” she says. “This is the essence of resilience, and it applies beyond nuclear power plants to all essential infrastructure of a functioning society, for example, the electric grid, the food and water supply, the transportation sector, etc.”

      One thing the researchers recommend is that in designing evacuation protocols, planners should make more effort to learn from much more frequent disasters such as wildfires and hurricanes. “We think getting more interdisciplinary, transdisciplinary knowledge from other kinds of disasters would be essential,” she says. Most of the emergency response strategies presently in place, she says, were designed in the 1980s and ’90s, and need to be modernized. “Consequences can be mitigated. A nuclear accident does not have to be a catastrophe, as is often portrayed in popular culture,” Wainright says.

      The research team included Giovanni Sansavini at ETH Zurich in Switzerland; Randall Gauntt at Sandia National Laboratories in New Mexico; and Kimiaki Saito at the Japan Atomic Energy Agency. More

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      in Energy

      Future nuclear power reactors could rely on molten salts — but what about corrosion?

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      Most discussions of how to avert climate change focus on solar and wind generation as key to the transition to a future carbon-free power system. But Michael Short, the Class of ’42 Associate Professor of Nuclear Science and Engineering at MIT and associate director of the MIT Plasma Science and Fusion Center (PSFC), is impatient with such talk. “We can say we should have only wind and solar someday. But we don’t have the luxury of ‘someday’ anymore, so we can’t ignore other helpful ways to combat climate change,” he says. “To me, it’s an ‘all-hands-on-deck’ thing. Solar and wind are clearly a big part of the solution. But I think that nuclear power also has a critical role to play.”

      For decades, researchers have been working on designs for both fission and fusion nuclear reactors using molten salts as fuels or coolants. While those designs promise significant safety and performance advantages, there’s a catch: Molten salt and the impurities within it often corrode metals, ultimately causing them to crack, weaken, and fail. Inside a reactor, key metal components will be exposed not only to molten salt but also simultaneously to radiation, which generally has a detrimental effect on materials, making them more brittle and prone to failure. Will irradiation make metal components inside a molten salt-cooled nuclear reactor corrode even more quickly?

      Short and Weiyue Zhou PhD ’21, a postdoc in the PSFC, have been investigating that question for eight years. Their recent experimental findings show that certain alloys will corrode more slowly when they’re irradiated — and identifying them among all the available commercial alloys can be straightforward.

      The first challenge — building a test facility

      When Short and Zhou began investigating the effect of radiation on corrosion, practically no reliable facilities existed to look at the two effects at once. The standard approach was to examine such mechanisms in sequence: first corrode, then irradiate, then examine the impact on the material. That approach greatly simplifies the task for the researchers, but with a major trade-off. “In a reactor, everything is going to be happening at the same time,” says Short. “If you separate the two processes, you’re not simulating a reactor; you’re doing some other experiment that’s not as relevant.”

      So, Short and Zhou took on the challenge of designing and building an experimental setup that could do both at once. Short credits a team at the University of Michigan for paving the way by designing a device that could accomplish that feat in water, rather than molten salts. Even so, Zhou notes, it took them three years to come up with a device that would work with molten salts. Both researchers recall failure after failure, but the persistent Zhou ultimately tried a totally new design, and it worked. Short adds that it also took them three years to precisely replicate the salt mixture used by industry — another factor critical to getting a meaningful result. The hardest part was achieving and ensuring that the purity was correct by removing critical impurities such as moisture, oxygen, and certain other metals.

      As they were developing and testing their setup, Short and Zhou obtained initial results showing that proton irradiation did not always accelerate corrosion but sometimes actually decelerated it. They and others had hypothesized that possibility, but even so, they were surprised. “We thought we must be doing something wrong,” recalls Short. “Maybe we mixed up the samples or something.” But they subsequently made similar observations for a variety of conditions, increasing their confidence that their initial observations were not outliers.

      The successful setup

      Central to their approach is the use of accelerated protons to mimic the impact of the neutrons inside a nuclear reactor. Generating neutrons would be both impractical and prohibitively expensive, and the neutrons would make everything highly radioactive, posing health risks and requiring very long times for an irradiated sample to cool down enough to be examined. Using protons would enable Short and Zhou to examine radiation-altered corrosion both rapidly and safely.

      Key to their experimental setup is a test chamber that they attach to a proton accelerator. To prepare the test chamber for an experiment, they place inside it a thin disc of the metal alloy being tested on top of a a pellet of salt. During the test, the entire foil disc is exposed to a bath of molten salt. At the same time, a beam of protons bombards the sample from the side opposite the salt pellet, but the proton beam is restricted to a circle in the middle of the foil sample. “No one can argue with our results then,” says Short. “In a single experiment, the whole sample is subjected to corrosion, and only a circle in the center of the sample is simultaneously irradiated by protons. We can see the curvature of the proton beam outline in our results, so we know which region is which.”

      The results with that arrangement were unchanged from the initial results. They confirmed the researchers’ preliminary findings, supporting their controversial hypothesis that rather than accelerating corrosion, radiation would actually decelerate corrosion in some materials under some conditions. Fortunately, they just happen to be the same conditions that will be experienced by metals in molten salt-cooled reactors.

      Why is that outcome controversial? A closeup look at the corrosion process will explain. When salt corrodes metal, the salt finds atomic-level openings in the solid, seeps in, and dissolves salt-soluble atoms, pulling them out and leaving a gap in the material — a spot where the material is now weak. “Radiation adds energy to atoms, causing them to be ballistically knocked out of their positions and move very fast,” explains Short. So, it makes sense that irradiating a material would cause atoms to move into the salt more quickly, increasing the rate of corrosion. Yet in some of their tests, the researchers found the opposite to be true.

      Experiments with “model” alloys

      The researchers’ first experiments in their novel setup involved “model” alloys consisting of nickel and chromium, a simple combination that would give them a first look at the corrosion process in action. In addition, they added europium fluoride to the salt, a compound known to speed up corrosion. In our everyday world, we often think of corrosion as taking years or decades, but in the more extreme conditions of a molten salt reactor it can noticeably occur in just hours. The researchers used the europium fluoride to speed up corrosion even more without changing the corrosion process. This allowed for more rapid determination of which materials, under which conditions, experienced more or less corrosion with simultaneous proton irradiation.

      The use of protons to emulate neutron damage to materials meant that the experimental setup had to be carefully designed and the operating conditions carefully selected and controlled. Protons are hydrogen atoms with an electrical charge, and under some conditions the hydrogen could chemically react with atoms in the sample foil, altering the corrosion response, or with ions in the salt, making the salt more corrosive. Therefore, the proton beam had to penetrate the foil sample but then stop in the salt as soon as possible. Under these conditions, the researchers found they could deliver a relatively uniform dose of radiation inside the foil layer while also minimizing chemical reactions in both the foil and the salt.

      Tests showed that a proton beam accelerated to 3 million electron-volts combined with a foil sample between 25 and 30 microns thick would work well for their nickel-chromium alloys. The temperature and duration of the exposure could be adjusted based on the corrosion susceptibility of the specific materials being tested.

      Optical images of samples examined after tests with the model alloys showed a clear boundary between the area that was exposed only to the molten salt and the area that was also exposed to the proton beam. Electron microscope images focusing on that boundary showed that the area that had been exposed only to the molten salt included dark patches where the molten salt had penetrated all the way through the foil, while the area that had also been exposed to the proton beam showed almost no such dark patches.

      To confirm that the dark patches were due to corrosion, the researchers cut through the foil sample to create cross sections. In them, they could see tunnels that the salt had dug into the sample. “For regions not under radiation, we see that the salt tunnels link the one side of the sample to the other side,” says Zhou. “For regions under radiation, we see that the salt tunnels stop more or less halfway and rarely reach the other side. So we verified that they didn’t penetrate the whole way.”

      The results “exceeded our wildest expectations,” says Short. “In every test we ran, the application of radiation slowed corrosion by a factor of two to three times.”

      More experiments, more insights

      In subsequent tests, the researchers more closely replicated commercially available molten salt by omitting the additive (europium fluoride) that they had used to speed up corrosion, and they tweaked the temperature for even more realistic conditions. “In carefully monitored tests, we found that by raising the temperature by 100 degrees Celsius, we could get corrosion to happen about 1,000 times faster than it would in a reactor,” says Short.

      Images from experiments with the nickel-chromium alloy plus the molten salt without the corrosive additive yielded further insights. Electron microscope images of the side of the foil sample facing the molten salt showed that in sections only exposed to the molten salt, the corrosion is clearly focused on the weakest part of the structure — the boundaries between the grains in the metal. In sections that were exposed to both the molten salt and the proton beam, the corrosion isn’t limited to the grain boundaries but is more spread out over the surface. Experimental results showed that these cracks are shallower and less likely to cause a key component to break.

      Short explains the observations. Metals are made up of individual grains inside which atoms are lined up in an orderly fashion. Where the grains come together there are areas — called grain boundaries — where the atoms don’t line up as well. In the corrosion-only images, dark lines track the grain boundaries. Molten salt has seeped into the grain boundaries and pulled out salt-soluble atoms. In the corrosion-plus-irradiation images, the damage is more general. It’s not only the grain boundaries that get attacked but also regions within the grains.

      So, when the material is irradiated, the molten salt also removes material from within the grains. Over time, more material comes out of the grains themselves than from the spaces between them. The removal isn’t focused on the grain boundaries; it’s spread out over the whole surface. As a result, any cracks that form are shallower and more spread out, and the material is less likely to fail.

      Testing commercial alloys

      The experiments described thus far involved model alloys — simple combinations of elements that are good for studying science but would never be used in a reactor. In the next series of experiments, the researchers focused on three commercially available alloys that are composed of nickel, chromium, iron, molybdenum, and other elements in various combinations.

      Results from the experiments with the commercial alloys showed a consistent pattern — one that confirmed an idea that the researchers had going in: the higher the concentration of salt-soluble elements in the alloy, the worse the radiation-induced corrosion damage. Radiation will increase the rate at which salt-soluble atoms such as chromium leave the grain boundaries, hastening the corrosion process. However, if there are more not-soluble elements such as nickel present, those atoms will go into the salt more slowly. Over time, they’ll accumulate at the grain boundary and form a protective coating that blocks the grain boundary — a “self-healing mechanism that decelerates the rate of corrosion,” say the researchers.

      Thus, if an alloy consists mostly of atoms that don’t dissolve in molten salt, irradiation will cause them to form a protective coating that slows the corrosion process. But if an alloy consists mostly of atoms that dissolve in molten salt, irradiation will make them dissolve faster, speeding up corrosion. As Short summarizes, “In terms of corrosion, irradiation makes a good alloy better and a bad alloy worse.”

      Real-world relevance plus practical guidelines

      Short and Zhou find their results encouraging. In a nuclear reactor made of “good” alloys, the slowdown in corrosion will probably be even more pronounced than what they observed in their proton-based experiments because the neutrons that inflict the damage won’t chemically react with the salt to make it more corrosive. As a result, reactor designers could push the envelope more in their operating conditions, allowing them to get more power out of the same nuclear plant without compromising on safety.

      However, the researchers stress that there’s much work to be done. Many more projects are needed to explore and understand the exact corrosion mechanism in specific alloys under different irradiation conditions. In addition, their findings need to be replicated by groups at other institutions using their own facilities. “What needs to happen now is for other labs to build their own facilities and start verifying whether they get the same results as we did,” says Short. To that end, Short and Zhou have made the details of their experimental setup and all of their data freely available online. “We’ve also been actively communicating with researchers at other institutions who have contacted us,” adds Zhou. “When they’re planning to visit, we offer to show them demonstration experiments while they’re here.”

      But already their findings provide practical guidance for other researchers and equipment designers. For example, the standard way to quantify corrosion damage is by “mass loss,” a measure of how much weight the material has lost. But Short and Zhou consider mass loss a flawed measure of corrosion in molten salts. “If you’re a nuclear plant operator, you usually care whether your structural components are going to break,” says Short. “Our experiments show that radiation can change how deep the cracks are, when all other things are held constant. The deeper the cracks, the more likely a structural component is to break, leading to a reactor failure.”

      In addition, the researchers offer a simple rule for identifying good metal alloys for structural components in molten salt reactors. Manufacturers provide extensive lists of available alloys with different compositions, microstructures, and additives. Faced with a list of options for critical structures, the designer of a new nuclear fission or fusion reactor can simply examine the composition of each alloy being offered. The one with the highest content of corrosion-resistant elements such as nickel will be the best choice. Inside a nuclear reactor, that alloy should respond to a bombardment of radiation not by corroding more rapidly but by forming a protective layer that helps block the corrosion process. “That may seem like a trivial result, but the exact threshold where radiation decelerates corrosion depends on the salt chemistry, the density of neutrons in the reactor, their energies, and a few other factors,” says Short. “Therefore, the complete guidelines are a bit more complicated. But they’re presented in a straightforward way that users can understand and utilize to make a good choice for the molten salt–based reactor they’re designing.”

      This research was funded, in part, by Eni S.p.A. through the MIT Plasma Science and Fusion Center’s Laboratory for Innovative Fusion Technologies. Earlier work was funded, in part, by the Transatomic Power Corporation and by the U.S. Department of Energy Nuclear Energy University Program. Equipment development and testing was supported by the Transatomic Power Corporation.

      This article appears in the Winter 2024 issue of Energy Futures, the magazine of the MIT Energy Initiative. More

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      in Energy

      Optimizing nuclear fuels for next-generation reactors

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      In 2010, when Ericmoore Jossou was attending college in northern Nigeria, the lights would flicker in and out all day, sometimes lasting only for a couple of hours at a time. The frustrating experience reaffirmed Jossou’s realization that the country’s sporadic energy supply was a problem. It was the beginning of his path toward nuclear engineering.

      Because of the energy crisis, “I told myself I was going to find myself in a career that allows me to develop energy technologies that can easily be scaled to meet the energy needs of the world, including my own country,” says Jossou, an assistant professor in a shared position between the departments of Nuclear Science and Engineering (NSE), where is the John Clark Hardwick (1986) Professor, and of Electrical Engineering and Computer Science.

      Today, Jossou uses computer simulations for rational materials design, AI-aided purposeful development of cladding materials and fuels for next-generation nuclear reactors. As one of the shared faculty hires between the MIT Schwarzman College of Computing and departments across MIT, his appointment recognizes his commitment to computing for climate and the environment.

      A well-rounded education in Nigeria

      Growing up in Lagos, Jossou knew education was about more than just bookish knowledge, so he was eager to travel and experience other cultures. He would start in his own backyard by traveling across the Niger river and enrolling in Ahmadu Bello University in northern Nigeria. Moving from the south was a cultural education with a different language and different foods. It was here that Jossou got to try and love tuwo shinkafa, a northern Nigerian rice-based specialty, for the first time.

      After his undergraduate studies, armed with a bachelor’s degree in chemistry, Jossou was among a small cohort selected for a specialty master’s training program funded by the World Bank Institute and African Development Bank. The program at the African University of Science and Technology in Abuja, Nigeria, is a pan-African venture dedicated to nurturing homegrown science talent on the continent. Visiting professors from around the world taught intensive three-week courses, an experience which felt like drinking from a fire hose. The program widened Jossou’s views and he set his sights on a doctoral program with an emphasis on clean energy systems.

      A pivot to nuclear science

      While in Nigeria, Jossou learned of Professor Jerzy Szpunar at the University of Saskatchewan in Canada, who was looking for a student researcher to explore fuels and alloys for nuclear reactors. Before then, Jossou was lukewarm on nuclear energy, but the research sounded fascinating. The Fukushima, Japan, incident was recently in the rearview mirror and Jossou remembered his early determination to address his own country’s energy crisis. He was sold on the idea and graduated with a doctoral degree from the University of Saskatchewan on an international dean’s scholarship.

      Jossou’s postdoctoral work registered a brief stint at Brookhaven National Laboratory as staff scientist. He leaped at the opportunity to join MIT NSE as a way of realizing his research interest and teaching future engineers. “I would really like to conduct cutting-edge research in nuclear materials design and to pass on my knowledge to the next generation of scientists and engineers and there’s no better place to do that than at MIT,” Jossou says.

      Merging material science and computational modeling

      Jossou’s doctoral work on designing nuclear fuels for next-generation reactors forms the basis of research his lab is pursuing at MIT NSE. Nuclear reactors that were built in the 1950s and ’60s are getting a makeover in terms of improved accident tolerance. Reactors are not confined to one kind, either: We have micro reactors and are now considering ones using metallic nuclear fuels, Jossou points out. The diversity of options is enough to keep researchers busy testing materials fit for cladding, the lining that prevents corrosion of the fuel and release of radioactive fission products into the surrounding reactor coolant.

      The team is also investigating fuels that improve burn-up efficiencies, so they can last longer in the reactor. An intriguing approach has been to immobilize the gas bubbles that arise from the fission process, so they don’t grow and degrade the fuel.

      Since joining MIT in July 2023, Jossou is setting up a lab that optimizes the composition of accident-tolerant nuclear fuels. He is leaning on his materials science background and looping computer simulations and artificial intelligence in the mix.

      Computer simulations allow the researchers to narrow down the potential field of candidates, optimized for specific parameters, so they can synthesize only the most promising candidates in the lab. And AI’s predictive capabilities guide researchers on which materials composition to consider next. “We no longer depend on serendipity to choose our materials, our lab is based on rational materials design,” Jossou says, “we can rapidly design advanced nuclear fuels.”

      Advancing energy causes in Africa

      Now that he is at MIT, Jossou admits the view from the outside is different. He now harbors a different perspective on what Africa needs to address some of its challenges. “The starting point to solve our problems is not money; it needs to start with ideas,” he says, “we need to find highly skilled people who can actually solve problems.” That job involves adding economic value to the rich arrays of raw materials that the continent is blessed with. It frustrates Jossou that Niger, a country rich in raw material for uranium, has no nuclear reactors of its own. It ships most of its ore to France. “The path forward is to find a way to refine these materials in Africa and to be able to power the industries on that continent as well,” Jossou says.

      Jossou is determined to do his part to eliminate these roadblocks.

      Anchored in mentorship, Jossou’s solution aims to train talent from Africa in his own lab. He has applied for a MIT Global Experiences MISTI grant to facilitate travel and research studies for Ghanaian scientists. “The goal is to conduct research in our facility and perhaps add value to indigenous materials,” Jossou says.

      Adding value has been a consistent theme of Jossou’s career. He remembers wanting to become a neurosurgeon after reading “Gifted Hands,” moved by the personal story of the author, Ben Carson. As Jossou grew older, however, he realized that becoming a doctor wasn’t necessarily what he wanted. Instead, he was looking to add value. “What I wanted was really to take on a career that allows me to solve a societal problem.” The societal problem of clean and safe energy for all is precisely what Jossou is working on today. More

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      Tests show high-temperature superconducting magnets are ready for fusion

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      In the predawn hours of Sept. 5, 2021, engineers achieved a major milestone in the labs of MIT’s Plasma Science and Fusion Center (PSFC), when a new type of magnet, made from high-temperature superconducting material, achieved a world-record magnetic field strength of 20 tesla for a large-scale magnet. That’s the intensity needed to build a fusion power plant that is expected to produce a net output of power and potentially usher in an era of virtually limitless power production.

      The test was immediately declared a success, having met all the criteria established for the design of the new fusion device, dubbed SPARC, for which the magnets are the key enabling technology. Champagne corks popped as the weary team of experimenters, who had labored long and hard to make the achievement possible, celebrated their accomplishment.

      But that was far from the end of the process. Over the ensuing months, the team tore apart and inspected the components of the magnet, pored over and analyzed the data from hundreds of instruments that recorded details of the tests, and performed two additional test runs on the same magnet, ultimately pushing it to its breaking point in order to learn the details of any possible failure modes.

      All of this work has now culminated in a detailed report by researchers at PSFC and MIT spinout company Commonwealth Fusion Systems (CFS), published in a collection of six peer-reviewed papers in a special edition of the March issue of IEEE Transactions on Applied Superconductivity. Together, the papers describe the design and fabrication of the magnet and the diagnostic equipment needed to evaluate its performance, as well as the lessons learned from the process. Overall, the team found, the predictions and computer modeling were spot-on, verifying that the magnet’s unique design elements could serve as the foundation for a fusion power plant.

      Enabling practical fusion power

      The successful test of the magnet, says Hitachi America Professor of Engineering Dennis Whyte, who recently stepped down as director of the PSFC, was “the most important thing, in my opinion, in the last 30 years of fusion research.”

      Before the Sept. 5 demonstration, the best-available superconducting magnets were powerful enough to potentially achieve fusion energy — but only at sizes and costs that could never be practical or economically viable. Then, when the tests showed the practicality of such a strong magnet at a greatly reduced size, “overnight, it basically changed the cost per watt of a fusion reactor by a factor of almost 40 in one day,” Whyte says.

      “Now fusion has a chance,” Whyte adds. Tokamaks, the most widely used design for experimental fusion devices, “have a chance, in my opinion, of being economical because you’ve got a quantum change in your ability, with the known confinement physics rules, about being able to greatly reduce the size and the cost of objects that would make fusion possible.”

      The comprehensive data and analysis from the PSFC’s magnet test, as detailed in the six new papers, has demonstrated that plans for a new generation of fusion devices — the one designed by MIT and CFS, as well as similar designs by other commercial fusion companies — are built on a solid foundation in science.

      The superconducting breakthrough

      Fusion, the process of combining light atoms to form heavier ones, powers the sun and stars, but harnessing that process on Earth has proved to be a daunting challenge, with decades of hard work and many billions of dollars spent on experimental devices. The long-sought, but never yet achieved, goal is to build a fusion power plant that produces more energy than it consumes. Such a power plant could produce electricity without emitting greenhouse gases during operation, and generating very little radioactive waste. Fusion’s fuel, a form of hydrogen that can be derived from seawater, is virtually limitless.

      But to make it work requires compressing the fuel at extraordinarily high temperatures and pressures, and since no known material could withstand such temperatures, the fuel must be held in place by extremely powerful magnetic fields. Producing such strong fields requires superconducting magnets, but all previous fusion magnets have been made with a superconducting material that requires frigid temperatures of about 4 degrees above absolute zero (4 kelvins, or -270 degrees Celsius). In the last few years, a newer material nicknamed REBCO, for rare-earth barium copper oxide, was added to fusion magnets, and allows them to operate at 20 kelvins, a temperature that despite being only 16 kelvins warmer, brings significant advantages in terms of material properties and practical engineering.

      Taking advantage of this new higher-temperature superconducting material was not just a matter of substituting it in existing magnet designs. Instead, “it was a rework from the ground up of almost all the principles that you use to build superconducting magnets,” Whyte says. The new REBCO material is “extraordinarily different than the previous generation of superconductors. You’re not just going to adapt and replace, you’re actually going to innovate from the ground up.” The new papers in Transactions on Applied Superconductivity describe the details of that redesign process, now that patent protection is in place.

      A key innovation: no insulation

      One of the dramatic innovations, which had many others in the field skeptical of its chances of success, was the elimination of insulation around the thin, flat ribbons of superconducting tape that formed the magnet. Like virtually all electrical wires, conventional superconducting magnets are fully protected by insulating material to prevent short-circuits between the wires. But in the new magnet, the tape was left completely bare; the engineers relied on REBCO’s much greater conductivity to keep the current flowing through the material.

      “When we started this project, in let’s say 2018, the technology of using high-temperature superconductors to build large-scale high-field magnets was in its infancy,” says Zach Hartwig, the Robert N. Noyce Career Development Professor in the Department of Nuclear Science and Engineering. Hartwig has a co-appointment at the PSFC and is the head of its engineering group, which led the magnet development project. “The state of the art was small benchtop experiments, not really representative of what it takes to build a full-size thing. Our magnet development project started at benchtop scale and ended up at full scale in a short amount of time,” he adds, noting that the team built a 20,000-pound magnet that produced a steady, even magnetic field of just over 20 tesla — far beyond any such field ever produced at large scale.

      “The standard way to build these magnets is you would wind the conductor and you have insulation between the windings, and you need insulation to deal with the high voltages that are generated during off-normal events such as a shutdown.” Eliminating the layers of insulation, he says, “has the advantage of being a low-voltage system. It greatly simplifies the fabrication processes and schedule.” It also leaves more room for other elements, such as more cooling or more structure for strength.

      The magnet assembly is a slightly smaller-scale version of the ones that will form the donut-shaped chamber of the SPARC fusion device now being built by CFS in Devens, Massachusetts. It consists of 16 plates, called pancakes, each bearing a spiral winding of the superconducting tape on one side and cooling channels for helium gas on the other.

      But the no-insulation design was considered risky, and a lot was riding on the test program. “This was the first magnet at any sufficient scale that really probed what is involved in designing and building and testing a magnet with this so-called no-insulation no-twist technology,” Hartwig says. “It was very much a surprise to the community when we announced that it was a no-insulation coil.”

      Pushing to the limit … and beyond

      The initial test, described in previous papers, proved that the design and manufacturing process not only worked but was highly stable — something that some researchers had doubted. The next two test runs, also performed in late 2021, then pushed the device to the limit by deliberately creating unstable conditions, including a complete shutoff of incoming power that can lead to a catastrophic overheating. Known as quenching, this is considered a worst-case scenario for the operation of such magnets, with the potential to destroy the equipment.

      Part of the mission of the test program, Hartwig says, was “to actually go off and intentionally quench a full-scale magnet, so that we can get the critical data at the right scale and the right conditions to advance the science, to validate the design codes, and then to take the magnet apart and see what went wrong, why did it go wrong, and how do we take the next iteration toward fixing that. … It was a very successful test.”

      That final test, which ended with the melting of one corner of one of the 16 pancakes, produced a wealth of new information, Hartwig says. For one thing, they had been using several different computational models to design and predict the performance of various aspects of the magnet’s performance, and for the most part, the models agreed in their overall predictions and were well-validated by the series of tests and real-world measurements. But in predicting the effect of the quench, the model predictions diverged, so it was necessary to get the experimental data to evaluate the models’ validity.

      “The highest-fidelity models that we had predicted almost exactly how the magnet would warm up, to what degree it would warm up as it started to quench, and where would the resulting damage to the magnet would be,” he says. As described in detail in one of the new reports, “That test actually told us exactly the physics that was going on, and it told us which models were useful going forward and which to leave by the wayside because they’re not right.”

      Whyte says, “Basically we did the worst thing possible to a coil, on purpose, after we had tested all other aspects of the coil performance. And we found that most of the coil survived with no damage,” while one isolated area sustained some melting. “It’s like a few percent of the volume of the coil that got damaged.” And that led to revisions in the design that are expected to prevent such damage in the actual fusion device magnets, even under the most extreme conditions.

      Hartwig emphasizes that a major reason the team was able to accomplish such a radical new record-setting magnet design, and get it right the very first time and on a breakneck schedule, was thanks to the deep level of knowledge, expertise, and equipment accumulated over decades of operation of the Alcator C-Mod tokamak, the Francis Bitter Magnet Laboratory, and other work carried out at PSFC. “This goes to the heart of the institutional capabilities of a place like this,” he says. “We had the capability, the infrastructure, and the space and the people to do these things under one roof.”

      The collaboration with CFS was also key, he says, with MIT and CFS combining the most powerful aspects of an academic institution and private company to do things together that neither could have done on their own. “For example, one of the major contributions from CFS was leveraging the power of a private company to establish and scale up a supply chain at an unprecedented level and timeline for the most critical material in the project: 300 kilometers (186 miles) of high-temperature superconductor, which was procured with rigorous quality control in under a year, and integrated on schedule into the magnet.”

      The integration of the two teams, those from MIT and those from CFS, also was crucial to the success, he says. “We thought of ourselves as one team, and that made it possible to do what we did.” More