When the plasma inside a fusion system starts to misbehave, it needs to be quickly cooled to prevent damage to the device. Researchers at Commonwealth Fusion Systems believe the best bet is a massive gas injection: essentially, a well-timed, rapid blast of cooling gas inside their fusion system, which is known as SPARC. But how many gas valves does it take to quickly tame a plasma that is hotter than the sun? The team has to strike the perfect balance: with too few valves, some parts of SPARC might overheat. With too many, valuable space inside the vessel would be wasted.
To answer this question, researchers turned to a computer code known as M3D-C1, which is developed and maintained by scientists at the U.S. Department of Energy 's (DOE) Princeton Plasma Physics Laboratory (PPPL). The code was used to model different valve configurations, and the results show that spacing six gas valves around the fusion vessel, with three on the top and three on the bottom, provides optimal protection.
The research, conducted by a team from PPPL, the Massachusetts Institute of Technology (MIT), General Atomics and Commonwealth Fusion Systems, is featured in a paper in Nuclear Fusion. The project was partially funded by the Innovation Network for Fusion Energy (INFUSE) program, an initiative designed to accelerate collaboration between national laboratories, universities, and private fusion companies. By advancing disruption mitigation strategies, the research team is helping bring fusion power one step closer to reality.
"This work also demonstrates that M3D-C1 can model a rapid shutdown via a massive gas injection using narrow and more realistic gas jets than in previous simulations," said Andreas Kleiner , a staff research scientist at PPPL and the study's lead author. "Our research also had a direct influence on the design of SPARC," he said, noting that design plans for SPARC now incorporate six gas valves based in large part on this research.
Once completed, SPARC will use powerful magnetic fields to hold plasma in a shape that looks much like a doughnut. While SPARC will be an experimental fusion system, the hope is that one day similar devices will be refined enough to generate power for the electrical grid. A key part of this refinement is creating a system to prevent jets of ultrahot particles from damaging the inner walls of the fusion vessel. This problem is amplified in fusion systems such as SPARC, which use particularly strong magnetic fields to hold the plasma.
"Massive gas injection mitigation is needed to make sure that we can rapidly restart SPARC after a disruption," said Ryan Sweeney, a co-author of the paper and disruption scientist at Commonwealth Fusion Systems.
Instabilities also need to be managed to ensure a long life for the fusion vessel.
"We don't currently have any material that can withstand the power per area that may be deposited during such an event," said Kleiner. That's why it is critical to get the details right for the massive gas injection system that's intended to cool the plasma down rapidly. "If there is no management of these events, the heat that is ejected toward the first wall can melt it."
The most comprehensive disruption simulations to date
The simulations considered symmetric configurations with six, four and two gas valves spaced evenly around the fusion vessel, with half the valves on the top and half on the bottom. The simulations also considered asymmetric configurations with one injector and five valves. Each simulation is extremely time intensive — taking weeks to run — even though the team used extremely powerful, exascale computers .
"These are the most comprehensive disruption simulations that had been done to that point," said Nate Ferraro , deputy head of theory at PPPL and a co-author of the study.
M3D-C1 has been a cornerstone of fusion research, and Ferraro played a central role in its development. Ferraro, who built the initial code as a graduate student alongside PPPL Principal Research Physicist Stephen Jardin , has spent years refining its capabilities. "Our ability to model the interaction between injected gas and plasma instabilities has grown significantly, making this study possible," he said.
Specifically, the version of M3D-C1 that was used incorporates a more realistic representation of features like gas valves. It also offers a new approach to creating simulations called non-equidistant meshing, which enables finer resolution where it matters most. The M3D-C1 mesh divides the tokamak into slices. But the gas jets are small –– roughly a centimeter wide –– compared to roughly 10 meters of distance around the tokamak, so resolving these jets accurately would take hundreds of slices if they were uniformly spaced. But with non-equidistant meshing, scientists can slice the plasma unevenly. For the SPARC models, more slices were made closer to the gas valves because the scientists expected that's where the most important changes would happen. The researchers say the approach made for a more realistic simulation overall.
"We could have modeled this before, but not with this level of accuracy," Kleiner said.
This study highlights the importance of public-private partnerships in advancing fusion technology. PPPL worked closely with Commonwealth Fusion Systems, General Atomics and MIT to provide the high-fidelity simulations necessary for SPARC's design optimization. This, in turn, will help with designs for Commonwealth Fusion System's ARC power plant that will be built in Chesterfield County, Virginia.
"Running a code like M3D-C1 is very complicated. It's a very niche skill," said Sweeney. "PPPL has very unique expertise in being able to develop and run these types of codes, so it's fantastic to be able to interface with the Lab."
Ferraro said working with private partners also benefits PPPL because working on new machines allows Lab scientists the opportunity to apply their knowledge to new systems and learn new techniques. "This project is a good example of how when it comes to fusion, it's not public versus private research. We are working together. We both have a role to fill to get to fusion."
Alongside Kleiner, Ferraro and Sweeney, co-authors of the study include Brendan Lyons of General Atomics and Matthew Reinke of Commonwealth Fusion Systems. This work was supported by the INFUSE program and the DOE under grant numbers DE-AC02-09CH11466 and DE-AC02-05CH11231.
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