The green revolution is coming for power-hungry particle accelerators
Future accelerators may be able to capture spent energy, or use greener cooling gases.
On July 5, marking the end of a three-year hiatus, CERN’s Large Hadron Collider will start collecting data. It will launch beams of high-energy particles in opposite directions around a 16-mile-long loop to create an explosive crash. Scientists will watch the carnage with high-precision detectors and sift through the debris for particles that reveal the inner workings of our universe.
But to do all of that, LHC needs electricity: enough to power a small city. It’s easy for someone outside CERN to wonder just why one physics facility needs all that power. Particle physicists know these demands are extreme, and many of them are trying to make the colliders of the future more efficient.
“I think there is increased awareness in the community that accelerator facilities need to reduce energy consumption if at all possible,” says Thomas Roser, a physicist formerly at Brookhaven National Laboratory in New York.
Scientists are already drawing up plans for LHC’s proposed successor—the so-called Future Circular Collider (FCC), with a circumference nearly four times as large as LHC’s, quite literally encircling most of the city of Geneva. As they do that, they’re looking at a few, sometimes unexpected, sources of energy use and greenhouse gas emissions—and how to reduce them.
LHC, despite its size and energy demands, isn’t that carbon-intensive to operate. For one, CERN sources its electricity from the French grid, whose portfolio of nuclear power plants make it one of the least carbon-reliant in the world. Put LHC in a place with a fossil-fuel-heavy grid, and its climate footprint would be very different.
“We’re very lucky…if it was in the US, it would be terrible,” says Véronique Boisvert, a particle physicist at Royal Holloway, University of London.
But the collider’s climate impacts spread far beyond a little sector of Geneva’s suburbs. Facilities like CERN generate heaps of raw data. To process and analyze that data, particle physics relies on a global network of supercomputers, computer clusters, and servers—which are notoriously power-hungry. At least 22 are, indeed, in the US.
Scientists can plan to build these networks or use computers in places with low-carbon electricity: California, say, over Florida.
“Maybe we should also think about what’s the carbon emission per CPU cycle and use that as a factor in planning your technology, as much as you do cost or power efficiency,” says Ken Bloom, a particle physicist at the University of Nebraska-Lincoln.
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Even though the accelerator itself is only a small portion of particle physics’ carbon footprint, Bosivert believes that researchers should plan to reduce the facility’s energy consumption. By the time FCC comes online in the 2040s and 2050s, decarbonization means that it will have to compete for resources on the power grid with many more cars and appliances than exist today. She thinks it’s wise to plan ahead for that time.
The goal of reducing power use is the same, says Bosivert. “You still need to minimize power, but for a different reason.”
In the name of efficiency and energy conservation, scientists are studying a few technologies that can help make “green accelerators.”
In 2019, researchers at Cornell University and Brookhaven National Lab unveiled a prototype accelerator called the Cornell-Brookhaven ERL Test Accelerator (CBETA). Remarkably, in demonstrations, CBETA recovered all the energy that scientists put into it.
“We took technology that existed, to some extent, and improved it and broadened its application,” says Georg Hoffstaetter, a physicist at Cornell University.
CBETA launched high-energy electrons through a racetrack-shaped loop that could fit inside a warehouse. With every “lap,” the electrons gained an energy boost. After four laps, the machine could slow down the electrons and store their energy to be used again. CBETA was the first time physicists had recovered energy after that many full laps.
It’s not a new technology, but as particle physicists grow more interested in saving energy, similar technology is in FCC’s plans. “There are options for [FCC] that use energy recovery,” says Hoffstaetter. Particles that aren’t smashed can be recovered.
CBETA also saves energy by using different magnets. Most particle accelerators use electromagnets to guide their particles along the arc. Electromagnets get their magnetic strength from running electricity around them; turn off the switch, and the magnetic field disappears. By replacing electromagnets with permanent magnets that don’t need electricity, CBETA could cut down on energy use.
“These technologies are kind of catching on,” says Hoffstater. “They’re being recognized and they’re being incorporated into new projects to save energy.”
Some of those projects are closer to completion than the FCC. Designers of a new collider at Brookhaven, smashing electrons and ions together, have plotted with energy recovery. At the Jefferson Lab, an accelerator facility in Newport News, Virginia, scientists are building a much larger accelerator that uses permanent magnets.
It isn’t the only way that energy from a particle collider finds new life. Much of the collider’s energy is turned into heat. That warmth can be put to work: CERN has experimented with piping heat to homes in the towns that surround LHC.
But focusing on carbon emissions from these facilities misses part of the picture—in fact, the largest part. “That is not the dominant source of emission,” says Bosivert. “The dominant source is the gases we use in our particle detectors.”
To keep an apparatus at the ideal temperatures for detecting particles, the highly sensitive equipment needs to be chilled by gases—similar to the gases used in some refrigerators. Those gases need to be non-flammable and endure high levels of radiation, even while maintaining refrigerated temperatures.
The gases of choice fall into categories hydrofluorocarbon (HFC) and perfluorocarbons (PFC). Some of them are greenhouse gases, far more potent than carbon dioxide. C2H2F2, CERN’s most common HFC, traps heat 1,300 times more effectively.
LHC already tries to capture these gases, reuse them, and stop them from spewing out into the atmosphere. Still, its process isn’t perfect. “A lot of these are in parts of these experiments that are just really difficult to access,” says Bloom. “Leaks can develop in there. They’re going to be very hard to fix.”
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From the logistician’s point of view, the use of HFCs and PFCs poses a procurement problem. Some jurisdictions—the European Union—are moving to ban them. Bosivert says this has led to wild fluctuations in price.
“When you’re designing future detectors, you can’t use those gases anymore,” says Bosivert. “All this R&D—‘Okay, what gases are we going to use?’—needs to happen now, essentially.”
There are alternatives. One, actually, is carbon dioxide itself. CERN has retrofitted some of LHC’s detectors to chill themselves with that compound. It isn’t perfect, but it’s an improvement.
These are the sorts of choices that many scientists want to see enter any planning discussion for a future accelerator.
“Just as monetary costs are a consideration in the planning of any future facility, future experiment, future physics program,” Bloom says, “we can think about climate costs in the same way.”
Correction August 9, 2022: This article has been updated to include Ken Bloom’s university affiliation.