At temperatures hotter than the sun, even a small disruption can interfere with a fusion reaction.
Scientists planning for the operations of ITER, an international fusion plant now under assembly, needed to solve the problem of runaway electrons, negatively charged particles in the soup of matter in the plasma within the tokamak, a kind of magnetic bottle that confines the plasma and fusion reactions.
Simulations performed on Summit, the 200-petaflop supercomputer at the Department of Energy’s Oak Ridge National Laboratory, could point to a solution.
Runaway electrons result when a fusion reaction merges two nuclei to form a single nucleus. The difference in mass between the two fusing nuclei and the one resulting nucleus converts to raw energy. Plans call for ITER to demonstrate 500 megawatts of fusion power while requiring around 50 megawatts to heat the plasma.
The donut-shaped tokamak relies on a set of coils to confine energy within its magnetic field. Temperatures inside the tokamak routinely top 1 million degrees Kelvin (about 1.8 million Fahrenheit) and tend to fluctuate during a fusion reaction. Studies suggest occasional drops in temperature, known as thermal quenches, can trigger drops in plasma current, known as current quenches. These quenches can send concentrated bursts of runaway electrons shooting toward the outer wall, effectively creating a powerful particle beam that could strike plasma-facing surfaces of the reactor.
“These electrons can possess as much as 100,000 times more energy than the bulk electron population, so the beam becomes highly energetic and can cause significant damage,” said Chang Liu, a research scientist at the Princeton Plasma Physics Laboratory and lead author of the study published in Physical Review Letters that used Summit, which has since been decommissioned. “Our study doesn’t solve the problem entirely, but it shows a promising way to diffuse these electrons.”
Calculating the answer required the power of a leadership-class supercomputer. The research team applied for and received an allocation of time on Summit, capable of more than 200 quadrillion calculations per second. Liu and the team, which included researchers from General Atomics and Columbia University, used Summit to simulate the physics phenomena expected to take place within the tokamak that tend to be strongly coupled with runaway electrons.
“These simulations would have taken at least 30 times longer on a regular CPU-based machine,” Liu said. “This was the first model of its kind. Simulating these electrons is extremely difficult because we’re talking about a large number of particles — as many as a quintillion or more — moving near the speed of light.
Those phenomena include the Alfvén wave, a ripple-like fluctuation of the magnetic field inside the plasma. The simulations on Summit included modeling the excitation of those waves, of their interaction with the runaway electrons, and of the plasma and electromagnetic fields surrounding them. Results showed such electromagnetic waves could scatter the runaway electrons produced by the disruption and keep the electrons from concentrating enough to form a beam.
The findings from the simulations align with the results of limited experiments at the DOE’s D-III National Fusion Facility.
“It’s like clearing snow from the mountain slope to prevent an avalanche,” Liu said. “When we have the wave, the electrons disperse and decay before they can accelerate, and we head off this threat. If we have no wave, then the runaway electrons will keep getting accelerated and their population will grow and concentrate to produce the beam. We hope to adapt this phenomenon into a solution that can help ITER.”
Next steps include incorporating other potential scenarios into the model. Liu said he’s working with experts at the Oak Ridge Leadership Computing Facility, home to Summit and its successor machine Frontier, to optimize the code for Frontier’s exascale speeds, which top 1 quintillion calculations per second.
“With the computational power and huge memory available with Frontier, we can include more particles and their interactions in the model and simulate the whole process in a much more realistic way,” Liu said. “We hope our work can help lead the way into a promising future of clean nuclear energy.”
Support for this research came from the DOE Office of Science’s Advanced Scientific Computing Research program and the Office of Fusion Energy Sciences. The OLCF and DIII-D are Office of Science user facilities.
UT-Battelle manages ORNL for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science. — Matt Lakin
This Oak Ridge National Laboratory news article "Reining in runaway electrons: Summit study could help solve fusion dilemma" was originally found on https://www.ornl.gov/news
