The US Department of Energy (DOE) Princeton Plasma Physics Laboratory (PPPL) is collaborating on the design and development of a new fusion device at the University of Seville in Spain. The SMall Aspect Ratio Tokamak (SMART) makes use of PPPL computer codes as well as its expertise in magnetics and sensor systems.
“The SMART project is a great example of us all working together to solve the challenges presented by fusion and teaching the next generation what we have already learned,” said Jack Berkery, PPPL’s Deputy Director of Research for the National Spherical Torus Experiment-Upgrade (NSTX-U) and the principal investigator for the PPPL collaboration with SMART. “We have to all do this together or it’s not going to happen.”
Most fusion reactors are based on a doughnut-shaped tokamak machine which confines plasma using magnetic fields. The cross-section of the plasma in a typical tokamak is shaped like a capital letter D. The straight edge of the D faces the centre of the doughnut. This is called “positive triangularity”. However, if the curved part faces inward, then it has “negative triangularity”.
Manuel Garcia-Munoz and Eleonora Viezzer, professors at the University of Seville Department of Atomic, Molecular & Nuclear Physics and co-leaders of the Plasma Science & Fusion Technology Lab and the SMART tokamak project, said PPPL seemed like the ideal partner for their first tokamak experiment. The next step was deciding what kind of tokamak they should build. “It needed to be one that a university could afford but also one that could make a unique contribution to the fusion landscape at the university scale,” said Garcia-Munoz. “The idea was to put together technologies that were already established: a spherical tokamak and negative triangularity, making SMART the first of its kind.”
Negative triangularity may offer enhanced performance because it can suppress instabilities in the device which leads to particles escaping the plasma, damaging the tokamak wall. “It’s a potential game changer with attractive fusion performance and power handling for future compact fusion reactors,” Garcia-Muñoz says. “Negative triangularity has a lower level of fluctuations inside the plasma, but it also has a larger divertor area to distribute the heat exhaust.”
A paper by the SMART collaboration, published in Nuclear Fusion, explores computer codes developed at PPPL to assess the stability of plasmas inside SMART. Two papers published in the Review of Scientific Instruments look at the design of diagnostic tools to provide information about impurities in the plasma such as oxygen, carbon and nitrogen.
“The diagnostic itself is pretty simple,” says co-author on one of the papers Stefano Munaretto from PPPL. “It’s just a wire wound around something. Most of the work involves optimising the sensor’s geometry by getting its size, shape and length correct and selecting where it should be located.”
PPPL has a long history of leadership in spherical tokamak research. The University of Seville fusion team first contacted PPPL to implement SMART in TRANSP, a simulation software developed and maintained by the Lab. “PPPL is a world leader in many, many areas, including fusion simulation; TRANSP is a great example of their success,” said Garcia-Munoz.
Mario Podesta, formerly of PPPL, was integral to helping the University of Seville determine the configuration of the neutral beams used for heating the plasma. That work culminated in a paper published in the journal Plasma Physics and Controlled Fusion.
The collaboration between SMART and PPPL also extended into one of the Lab’s core areas of expertise: diagnostics, which are devices with sensors to assess the plasma. Several such diagnostics are being designed by PPPL researchers. PPPL Physicists Manjit Kaur and Ahmed Diallo, together with Viezzer, are leading the design of the SMART’s Thomson scattering diagnostic, which will precisely measure the plasma electron temperature and density during fusion reactions.
PPPL’s Head of Advanced Projects Luis Delgado-Aparicio, together with Marie Skłodowska-Curie fellow Joaquin Galdon-Quiroga(Link is external) and University of Seville graduate student Jesus Salas-Barcenas, are developing two other kinds of diagnostics for SMART: a multi-energy, soft X-ray (ME-SXR) diagnostic and spectrometers.
Researchers at the University of Seville have already run a test in the tokamak, displaying the pink glow of argon when heated with microwaves. This process helps prepare the tokamak’s inner walls for a far denser plasma contained at a higher pressure. While the pink glow is from a plasma, it’s at such a low pressure that the researchers don’t consider it their real first tokamak plasma.
Support for this research comes from the DOE under contract number DE-AC02-09CH11466, European Research Council Grant Agreements 101142810 and 805162, the Euratom Research and Training Programme Grant Agreement 101052200 – EUROfusion, and the Junta de Andalucía Ayuda a Infraestructuras y Equipamiento de I+D+i IE17-5670 and Proyectos I+D+i FEDER Andalucía 2014-2020, US-15570.
