NSTX-U, the Princeton Plasma Physics Laboratory's spherical tokamak, is nearing a return to operations. The device will investigate the strengths of the spherical design as a viable route to scalable fusion power plants and test important energetic particle physics for burning plasmas including ITER. As construction began on the "Big Tokamak" projects of the 1970s and 1980s, there was a sense of optimism that fusion power was just around the corner. Princeton's Tokamak Fusion Test Reactor (TFTR) and the Joint European Torus (JET) had net-energy gain as stated project goals. A 1978 Popular Science article breezily claimed that "the basic scientific questions about fusion have largely been answered." However, as the larger devices came online plasma turbulence was far greater than expected, preventing them from reaching their breakeven goals. A helpful analogy for plasma turbulence is that of boiling water in a pot: a similar "bubbling" motion releases the heat within a plasma through "turbulent eddies" less than one centimetre wide. If heat is not confined long enough in a plasma, fusion power is not feasible. In the wake of these discoveries, fusion research in the late 1980s and 1990s focused on designing tokamaks that could tame this turbulence. Born out of this research was the design of the spherical tokamak, which has a few key magnetic field structural advantages that may allow for increased plasma stability and energy confinement at reactor scale. More compact than traditional tokamaks, these devices have reduced aspect ratios¹, which gives the plasma the shape of a "cored apple" rather than a "donut." This design increases the plasma beta; in other words, the more efficient use of magnetic fields increases plasma pressure to enhance global plasma stability. Other factors—such as subtle changes in magnetic fields, which lead to natural suppression of plasma turbulence, contribute to greater energy confinement capabilities. An image showing glowing plasma around the central magnet that runs down the middle of NSTX-U as the backbone of the machine. Once installed, the component will perform a dual role: it will start and maintain the current that generates the plasma that fuels fusion reactions and complete the magnetic field that keeps the plasma bottled up. Credit: Elle Starkman / PPPL Office of Communications After promising initial results at the prototype spherical tokamak START, the Princeton Plasma Physics Laboratory (PPPL) launched the National Spherical Torus Experiment, or NSTX, in 1999. While the device did not have the capability to work at fusion-relevant conditions, plasma behaviour could be studied inside the device to test the hypothesis of increased stability and confinement within spherical tokamaks. To this end, the device's eleven-year run was a clear success. NSTX set plasma beta records and showed enhanced confinement times in line with theoretical predictions. In a wide-ranging study of NSTX published in Nuclear Fusion, the authors summed up the results quite simply:"Overall, these NSTX plasmas have many characteristics required for next-step spherical tokamak devices." That initial NSTX run was paused in 2010 to upgrade the device. NSTX-U was designed to  access plasma conditions closer to those expected in future spherical tokamak devices and to test whether the enhanced stability and confinement would extend to these conditions. The upgrades included a new central magnet, which doubled the toroidal field strength from 0.5 Tesla to 1.0 Tesla, doubled the plasma current from 1 to 2 mega amperes, and increased the pulse duration by a factor of five. A second neutral beam injector doubled the heating power and heat flux on the divertor to enable important tests of plasma current sustainment. After three years of construction, NSTX-U began operations in 2016, but one of the smaller plasma-shaping magnetic field coils failed after ten weeks of operation. To ensure the reliable operation of all magnetic field coils and other critical components on the device, the NSTX-U Recovery Project has been working to enhance the NSTX-U user facility ever since. Brought in to lead PPPL in 2018, PPPL Director Steven Cowley understands the urgency in getting NSTX-U back online. "People need the data from this machine," he says. Recent progress on the recovery has been steady: the new coils have passed their tests, and operations are scheduled to resume in the autumn of 2025. In the meantime, the PPPL team has not been idle. "Over the last five to ten years, the simulations have really caught up. Our team has been using some really brilliant techniques, known as gyrokinetics, to be able to simulate turbulence with some veracity," Cowley explains. "By using data from the original NSTX run, these simulations indicate that a spherical tokamak at reactor scale will have reduced turbulence, and therefore enhanced confinement." A computer-generated image showing plasma turbulence, the unwanted bubbling and roiling that moves heat from the plasma core to the edge and interferes with the crucial fusion reactions. Image: NSTX-U render, Walter Guttenfelder, Filippo Scotti NSTX-U's results will also be directly relevant to burning plasmas such as those expected in ITER and a tokamak-based fusion power plant. In a burning plasma, the plasma self-sustains through "alpha heating"—a term that describes when the newly created helium atoms (alpha particles) continue the heating process by colliding into the deuterium and tritium particles in the plasma. These energetic alpha particles, however, can excite what are known as Alfvén eigenmodes through resonant interaction, which can then eject the alpha particles from the plasma before they can provide heating. For ITER to achieve a burning plasma, it must find fusion regimes (specific plasma conditions) in which these eigenmodes do not eject alpha particles at unsustainably rapid rates. In NSTX-U, the injected neutral beam particle population will occupy similar parameter space to that expected for the alpha population in a burning plasma, allowing the PPPL team to study the possible Alfven eigenmode activity that might occur in ITER. Understanding the NSTX-U results will help ITER optimize the plasma conditions for plasma self-heating by the alphas. NSTX-U's experimental data may also have profound commercial implications. "In a practical world," Cowley explains, "fusion is not just about turbulence and confinement." The reduced size of a spherical tokamak may de-risk investment in initial reactors, due to a lower weighted cost of capital. The compact nature of the device is also a long-term advantage. "When you're developing a new technology, you want lots of steps to optimize. With a smaller reactor, you ultimately bring construction costs down by learningas you make lots of them. Just like cars, when you make thousands of them, you develop processes to drive down costs. Engineering doesn't work very well if you only make a few." Of course, reduced size does come with tradeoffs. A smaller aspect ratio may allow for increased confinement, but it also increases the neutron flux on the surrounding materials. Below a certain critical size, a fusion device becomes too small to properly shield superconducting magnets, which Cowley freely admits. "The spherical tokamak pushes up against that limit. Shielding the superconductors, especially in the center of the device, is a challenge." Ultimately, whether these potential benefits are borne out will depend on the results of the NSTX-U campaign. The data generated from NSTX-U will be a key step in determining the ideal aspect ratio of future commercial reactors. Cowley does not profess to know exactly what that number is. Instead, NSTX-U will contribute to the iterative process of optimizing the design of future fusion power plants. "The idea that we design a fusion reactor, and it stays the same for the next 30 million years, is ridiculous," Cowley explains. "We're going to learn as we go." Related: See this recent story about NSTX-U from PPPL. ¹The aspect ratio of a tokamak is defined as the major radius (the distance between the centre of the tokamak device and the centre of the plasma) divided by the minor radius (the distance between the centre of the plasma and the edge of the plasma chamber).

Artificial intelligence performs well in predicting weld defects during a novel trial designed by the European Domestic Agency (Fusion for Energy) vacuum vessel team. The vacuum vessel team in Europe charged with the procurement of five sectors of the ITER vacuum vessel has been working for two years to explore the benefits of using artificial intelligence (AI) in nuclear engineering, with the full support of Fusion for Energy and its governance. As reported in a recent article on Fusion for Energy's website, Senior Technical Officer María Ortiz de Zúñiga and Cristian Casanova, Vacuum Vessel Programme Manager at the time, conducted a pilot study using data previously collected from vacuum vessel welds with defects to train an AI model to predict which welds in progress would present defects. "Big Science projects like ITER offer a wealth of data which is ideal for AI. They provide us with a one-of-a-kind opportunity to learn, train, extrapolate and apply these skills in other fields of manufacturing. There are very few attempts in bringing AI in nuclear engineering, so we decided to give it a try and break new ground. We are the first ITER party to test the waters and since this pilot project, there has been an unprecedented interest and enthusiasm harnessing its potential," explains María Ortiz de Zúñiga. The results were conclusive. After data collected from 2,000 electron beam welds was entered into the model, it predicted with 100% accuracy how many of the 100 welds in progress would pass or fail conformity checks. Similarly, the model was able to predict the results of PAUT testing (phased array ultrasonic testing) with 96% accuracy. Read the full article on the Fusion for Energy website here.

Reducing energy consumption is one of the major challenges of our time. It can be addressed at an individual level, like deciding to replace an old oil boiler with an efficient heat pump system or switching from incandescent light bulbs to LED lamps. It can also be addressed at a much larger scale by industry; modifying a production process, limiting energy losses through insulation or installing heat recovery devices are among the many ways industries can optimize consumption, cut back on spending and, ultimately, alleviate pressure on the planet's resources. Twenty years ago, France devised a system of incitement to promote such energy-saving initiatives. Under its terms, large companies such as electricity providers and fuel distributors have an obligation to financially support their clients' efforts to limit energy consumption by compensating part of their investment. A heat recovery system installed on the compressors of the ITER cryoplant was a recent beneficiary of this measure. In order to provide cooling fluids to the machine, ITER operates one the largest cryoplants in the world. Inside the 6,000 m² facility, half of the available space is occupied by 18 megawatt-class compressors, arranged in trains of six, that feed cold boxes with gaseous helium at a pressure of 21 bars. Due to the monoatomic nature of helium, the compression process requires a lot of power and, being largely exothermic, generates a considerable quantity of heat. At full power, the electrical consumption of the cryoplant's helium compressors is equivalent to that of a town of 20,000 people. But only half of that power is consumed by the compression process proper, the rest being dissipated as heat by way of a water-cooling circuit. Heat is a precious commodity, especially when a heat-producing device is located amid buildings and installations that need to be kept at room temperature during the cold season. "Notwithstanding France's incitement policy, it was clear from the beginning, in fact as early as the design phase, that we couldn't let such a massive quantity of heat be lost in the cooling circuit," explains David Grillot, the deputy head of the ITER Plant System Program, who oversaw the cryoplant's design as an engineer in 2012-2013 at Air Liquide. Twelve additional heat exchangers are installed in the liquid helium plant to recover the heat generated by the helium compression process. Three are pictured here. Under the provisions of an incitement system to promote energy savings, the installation received an important financial subsidy from the French utility EDF. When the liquid helium plant is fully operational, cooling water will come out of the compressors at a temperature of 100 °C and at a flow rate of approximately 2,500 m³ per hour. In the cold season, this flow will be diverted to the hot water boiler building, contributing 12 MW of heat to the ITER "central heating" system. And 12 MW is about all that is needed to keep the main facilities on the ITER platform at room temperature¹. In the hot water boiler building, twin 13 MW electrical boilers operate as a redundant contingency system. One of them will kick in as soon as the hot water input from the compressors falls below 12 MW, as for example during maintenance phases. The boilers' contribution, and hence their electrical consumption, will be minimal as long as the liquid helium plant is operating. "As a main actor in the French incitement scheme, the French utility EDF supported our project, which perfectly fit into the 'heat recovery in industrial systems' category," says David. "However EDF had never dealt with such a colossal case." ITER's contribution to cutting back on electricity spending was valued at EUR 5.5 million and the check arrived at the end of last year. "The sum is spectacular but it more or less amounts to what we spent installing the system." Now that the investment has been covered, massive savings are ahead: the buildings of the ITER scientific installation will be heated at very little energy cost and with significant benefits for the planet's resources. ¹Offices and secondary buildings have their own local heating systems, mostly in the form of heat pumps.

In December 2021 the JET tokamak broke the world record for fusion power. Pulse #99971 achieved total fusion energy of 59 MJ—more than doubling JET's 1997 record. STARMAKERS: The Energy Of Tomorrow tells the story of this achievement. It takes the viewer behind the scenes of the landmark European fusion experiment to show how scientists and engineers overcame incredible obstacles to move the world one step closer to a future of clean, safe, cheap and unlimited energy. The documentary is available on Prime Video to rent or buy. Depending on your region, try this link or this link.

The European Master of Science in Nuclear Fusion and Engineering Physics (FUSION-EP) is a two-year Erasmus Mundus Joint Master Degree (EMJMD) funded by the European Union. Focusing on the physics and technology of nuclear fusion as an energy source, it offers research-oriented education in engineering physics, offered by a consortium of eight European universities in Belgium, Czech Republic, France, Germany and Spain. The curriculum is organized in two periods of increasing specialization, each in two different countries, along two distinct tracks: Fusion Sciences and Fusion Technology. Students from all countries can apply. The selection procedure does not make any distinction between EU students and students coming from outside the EU. Scholarships are available to top-ranked students that cover the participation fee and offer a monthly allowance for the two years. The campaign for the program beginning autumn 2024 is underway now. Apply before 15 February 2024 on the FUSION EP website.

The 15th ITER Neutronics Meeting and Fusion Neutronics Workshop 2024 will be held at ITER Headquarters from 8 to 10 April 2024. The workshop will provide the opportunity for the fusion neutronics community to discuss recent advancements, issues and successes in the field. Considering work underway in startups on alternative fusion concepts, progress on other technologies such as inertial fusion, and recent advancements (codes, shielding materials, etc.), the workshop seeks to broaden participation beyond ITER/DEMO scope. We encourage young researchers and fusion start-ups to participate in this event and present their work. Planned topics include: Neutronics related to ITER Nuclear-radiation-related safety issues (safety margins/uncertainties, code qualification) Computational tools and nuclear data relevant for nuclear fusion (transport codes, activation codes, CAD/MCNP converting tools ...) Dose reduction methods and optimization, ALARA Shielding materials Material damage, nuclear heating, radioactive waste production and management  Accelerator-driven neutron sources for fusion applications Activated corrosion products Nuclear licensing for fusion reactors  Neutronics beyond ITER (DEMO, fusion power plants, alternate fusion schemes) This meeting will be held primarily in-person but a few online presentations/participations are also possible. Subject to conditions, this workshop also includes ITER construction site visit. See important dates and registration information on this page. 

On Friday 2 February, the Royal Institution in London is hosting a public discussion on "The latest developments in fusion energy." At this in-person and live-streamed event, co-hosted by the UK Atomic Energy Authority (UKAEA), distinguished experts Fernanda Rimini (UKAEA), Pietro Barabaschi (Director-General of the ITER Project), and Tammy Ma (Lawrence Livermore National Laboratory) will take participants on a journey into the heart of fusion research and the most recent and groundbreaking discoveries in this leading-edge field. See all information for how to attend in person or online here.