In the worldwide initiative to utilize power from plasma fusion, the Princeton Plasma Physics Laboratory (PPPL) and the University of Seville’s Plasma Science and Fusion Technology Lab have collaborated to develop computer codes, engineering structures, and physics principles for a new innovative fusion reactor: the SMall Aspect Ratio Tokamak.
As atoms unite to unleash energy, fusion scientists globally are collaborating to tackle the energy crisis threatening our planet. Transforming fusing plasma into a dependable power source for the energy grid is highly complex, necessitating contributions from various nations.
The PPPL, a national laboratory in the U.S. supported by the Department of Energy (DOE), is spearheading several significant projects, including a partnership on a novel fusion device at the University of Seville in Spain. The design of the SMall Aspect Ratio Tokamak (SMART) heavily relies on PPPL’s computer codes and their expertise in magnetics and sensor technologies.
“The SMART initiative exemplifies our collective effort to address the challenges of fusion while educating future generations based on our acquired knowledge,” stated Jack Berkery, PPPL’s deputy director of research for the National Spherical Torus Experiment-Upgrade (NSTX-U) and principal investigator for the collaboration with SMART. “We must work together to make this a reality; otherwise, it won’t materialize.”
Professors Manuel Garcia-Munoz and Eleonora Viezzer from the University of Seville, who co-lead the Plasma Science and Fusion Technology Lab along with the SMART project, noted that PPPL was their top choice for their first tokamak experiment. The next decision was determining the type of tokamak they wanted to construct. “It had to be both affordable for a university and capable of contributing uniquely to the field of fusion at the university level,” Garcia-Munoz explained. “We aimed to integrate established technologies: a spherical tokamak with negative triangularity, creating a one-of-a-kind reactor – it truly was an excellent idea.”
SMART aims to facilitate manageable fusion plasma
Triangularity describes the plasma’s shape in relation to the tokamak. Typically, the plasma’s cross-section in a tokamak resembles the letter D. If the flat side of the D faces the center, it’s said to possess positive triangularity. Conversely, if the curved side faces inwards, it has negative triangularity.
According to Garcia-Munoz, negative triangularity should enhance performance by reducing instabilities that can push particles and energy out of the plasma, safeguarding the tokamak walls from damage. “This approach could revolutionize fusion performance and power management for future compact reactors,” he remarked. “Negative triangularity results in fewer fluctuations in the plasma while also providing a larger area to manage heat exhaust.”
SMART’s spherical configuration is expected to improve plasma confinement compared to a traditional doughnut shape. The shape is crucial for effective plasma retention. This is why NSTX-U, the main fusion experiment at PPPL, features a more rounded design than some other tokamaks; the rounder the shape, the easier it is to maintain plasma confinement. In fact, SMART will be the first spherical tokamak to fully investigate a specific plasma configuration known as negative triangularity.
PPPL’s expertise in computer codes is invaluable
PPPL has a rich history in spherical tokamak research. The fusion team at the University of Seville initially reached out to PPPL for assistance in implementing SMART within TRANSP, a simulation software that the Lab developed and updates regularly. TRANSP is utilized by numerous facilities, including private entities like Tokamak Energy in England.
“PPPL is a world leader in many areas, including fusion simulation; TRANSP exemplifies their achievements,” Garcia-Munoz noted.
Mario Podesta, a former PPPL staff member, played a critical role in helping the University of Seville configure the neutral beams that will be used to heat the plasma. This effort culminated in a paper published in the journal Plasma Physics and Controlled Fusion.
Stanley Kaye, NSTX-U’s research director, is currently collaborating with Diego Jose Cruz-Zabala, a EUROfusion Bernard Bigot Researcher Fellow from the SMART team, to utilize TRANSP “to calculate the shaping coil currents required to achieve their design plasma configurations of positive and negative triangularity throughout different operational phases.” Kaye explained that the initial phase would involve a “very basic” plasma setup, while phase two would incorporate neutral beams to heat the plasma.
In addition, other computational tools were employed to analyze the stability of future SMART plasmas, with contributions from Berkery, former undergraduate intern John Labbate (who is now a graduate student at Columbia University), and former University of Seville student Jesús Domínguez-Palacios, who has transitioned to a U.S. company. A recent article by Domínguez-Palacios in Nuclear Fusion outlines this research.
Designing long-term diagnostic tools
The partnership between SMART and PPPL has also delved into one of the Lab’s fundamental specializations: diagnostics. These are sensor-equipped devices designed to evaluate the plasma. PPPL researchers are working on several of these diagnostic tools. For instance, PPPL physicists Manjit Kaur and Ahmed Diallo, along with Viezzer, are at the forefront of designing SMART’s Thomson scattering diagnostic. This tool will accurately gauge the plasma’s electron temperature and density during fusion processes, as outlined in a recent study published in Review of Scientific Instruments. These assessments will be complemented by ion temperature, rotation, and density measurements provided by a charge exchange recombination spectroscopy suite developed by Alonso Rodriguez-Gonzalez, a graduate student at the University of Seville, along with Cruz-Zabala and Viezzer.
“These diagnostic systems are designed to be operational for decades, so we consider longevity in our designs,” remarked Kaur. It was essential for the diagnostic to withstand the temperature extremes SMART might encounter in the coming decades, beyond just the initial lower values.
From the project’s inception, Kaur has been involved in designing the Thomson scattering diagnostic, selecting and sourcing its various components, including the laser she deemed most suitable for the task. She expressed her excitement over the successful initial laser tests conducted by Gonzalo Jimenez and Viezzer in Spain. The tests involved directing the laser onto a special type of paper referred to as “burn paper.” If the laser is appropriately designed, it will create circular burn marks with smooth edges. “The first results from the laser tests were stunning,” she exclaimed. “We are now eagerly awaiting the arrival of additional components to get the diagnostic operational.”
James Clark, a PPPL research engineer whose doctoral work focused on Thomson scattering systems, was brought on board to collaborate with Kaur. “I’ve been in charge of designing the laser path and the associated optics,” Clark elaborated. Alongside the engineering aspects of the project, he has also facilitated logistics by organizing delivery, installation, and calibration timelines.
PPPL’s Head of Advanced Projects, Luis Delgado-Aparicio, in collaboration with Marie Skłodowska-Curie fellow Joaquin Galdon-Quiroga and University of Seville graduate student Jesus Salas-Barcenas, is spearheading efforts to incorporate two additional types of diagnostics into SMART: a multi-energy, soft X-ray (ME-SXR).
The ME-SXR diagnostic tool is designed to assess the plasma’s electron temperature and density, employing a different technique compared to the Thomson scattering system. It will utilize small electronic components known as diodes to detect X-rays. When used alongside the Thomson scattering diagnostic, ME-SXR will provide a thorough evaluation of the plasma’s electron temperature and density.
By examining the various light frequencies within the tokamak, the spectrometers can reveal information about impurities present in the plasma, including elements like oxygen, carbon, and nitrogen. “We are using commercially available spectrometers and are also developing some devices to integrate them into the machine with fiber optics,” noted Delgado-Aparicio. There is a new publication in the Review of Scientific Instruments that outlines the design of this diagnostic system.
Stefano Munaretto, a Research Physicist at PPPL, contributed to the magnetic diagnostic system for SMART, with fieldwork led by University of Seville graduate student Fernando Puentes del Pozo. “The diagnostic itself is quite straightforward,” Munaretto explained. “It consists of a wire wrapped around a material. The main effort lies in optimizing the sensor’s design by determining the right size, shape, and length, choosing its placement, and handling all subsequent signal processing and data analysis.” The specifics of SMART’s magnetic design are detailed in a new paper.
Munaretto expressed how fulfilling this project has been, highlighting that the magnetic diagnostics team includes many young students with limited experience in the field. “They are eager to learn and hardworking. I certainly see a bright future ahead for them,” he remarked.
Delgado-Aparicio shared similar sentiments. “I greatly enjoyed collaborating with Manuel Garcia-Munoz, Eleonora Viezzer, and other experienced scientists and professors at the University of Seville. However, what I valued the most was working with the energetic group of students there,” he said. “They are extremely talented and have significantly assisted me in understanding the challenges we face and how we can progress towards achieving our first plasmas.”
Researchers at the University of Seville have conducted a test in the tokamak, producing the pink glow of argon when microwaves were applied, which helps prepare the inner walls for a significantly denser plasma at higher pressure. Although this pink glow originates from plasma, it is at such low pressure that the researchers do not regard it as their actual first tokamak plasma. Garcia-Munoz anticipates that the true first plasma will likely be achieved in the fall of 2024.
This research is supported by the DOE under contract number DE-AC02-09CH11466, along with European Research Council Grant Agreements 101142810 and 805162, the Euratom Research and Training Programme Grant Agreement 101052200 — EUROfusion, and the Junta de Andalucía’s aid for R&D+i infrastructure and equipment IE17-5670 and R&D+i projects FEDER Andalucía 2014-2020, US-15570.
