Article Source: Chemical & Engineering News
The Large Hadron Collider discovered the Higgs boson. Now the facility’s researchers are searching for more environmentally friendly gases to run its detectors.
Scientists turned off the world’s largest particle accelerator in December. This planned, 3-year shutdown at the Large Hadron Collider (LHC) will let staff make repairs and upgrade the accelerator so it can collect more detailed data about fundamental particles in our universe.
Researchers at CERN (the European Organization for Nuclear Research), which operates the accelerator, are also using the shutdown as an opportunity to improve the facility’s environmental footprint. To perform correctly, some of the LHC’s key equipment relies on gases known to contribute significantly to global warming. These gases are either vented to the atmosphere purposely, or they leak from instruments, making it to the outside air accidentally. CERN scientists are working to reduce the lab’s contributions to climate change by plugging up leaks and, in some equipment, replacing the most pernicious greenhouse gases with less problematic alternatives. They hope their efforts can serve as a blueprint for other large scientific facilities to examine their own on-site choices and work toward a more sustainable future.
The LHC’s 1,200 magnets accelerate particles to nearly the speed of light in opposite directions through a 27 km long circular tunnel. Where the beams intersect and the particles collide, detectors record the resulting particles and their decay products. CERN physicists in 2012 used this equipment to detect a Higgs boson, a fundamental particle whose existence lends support to the standard model of particle physics.
To detect these types of particles, CERN uses greenhouse gases in a few ways. Gases inside the facility’s detectors enable the equipment to function properly and protect the machinery. Much of the LHC’s equipment operates at low temperatures. For instance, scientists keep the computers and sensors below 0 °C with refrigerants like those found in a refrigerator or air conditioner.
The connection between refrigerant gases and global warming is a bit of an unlucky coincidence, says Mark McLinden, who studies refrigerants at the National Institute of Standards and Technology in Boulder, Colorado. A good refrigerant, he says, is a nonflammable, nontoxic small molecule with a boiling point between about –20 and –50 °C that’s stable in a refrigerator for 20 years or so. Fluorinated alkanes fit that bill nicely, he says. Unfortunately, the carbon-fluorine bonds in those molecules also absorb infrared light, creating a greenhouse effect.
To be clear, CERN’s emissions are a mere blip in the global accounting of gases that contribute to climate change. In 2014, the United Nations’ Intergovernmental Panel on Climate Change estimated the world’s 2010 CO2 emissions at about 38 metric gigatons. CERN estimates it releases 200 metric kilotons of CO2 per year—only 0.000005% of the global total. By way of comparison, CERN’s director for accelerators and technology, Frédérick Bordry, says, “A day of operation at CERN is a day of a cruise ship [operating].”
Perhaps the easiest way to reduce the LHC’s emissions is to reduce how much gas leaves the facility. During the facility’s first long shutdown, which was dubbed LS1 and spanned 2013 to 2015, CERN installed gas recirculators on several detectors to reduce gas consumption and limit the amount that could be vented to the environment. “All the systems where we use greenhouse gases are using lossless recirculation,” says Roberto Guida, a CERN scientist who works on gas systems and detectors. In 2016, Guida and his colleague Beatrice Mandelli reported a 90% emission reduction after the recirculators’ installation.
Technicians will add other systems during the current shutdown. Matthew F. Herndon, a physicist at the University of Wisconsin–Madison who runs a research project using the LHC’s Compact Muon Solenoid (CMS) detector, says an abatement system being added to this part of the facility will capture any vented gas and then burn it to break it down into less harmful compounds such as water and CO2. “In principle such a system can be 100% effective,” he says.
Leaks remain a problem, though, Guida says. In some detectors, 10% of the gas is lost to leaks and scientists are adding new gas to replace what’s being lost. “But during LS2 we’ll do a huge search for leaks,” he says, referring to the current LHC shutdown.
The other avenue CERN is exploring is replacing refrigerants in its detectors and other equipment with more environmentally friendly versions—ones that climate models indicate have low global warming potential (GWP). An irony of these efforts is that back when scientists chose the refrigerants currently in use, they were the environmentally friendly alternatives. In the 1990s, during the LHC’s design, refrigerant pollution was thought of mostly in terms of the gases’ potential to destroy ozone molecules, CERN’s Bordry says. “The gases were chosen to minimize ozone-layer influence,” he says.
At that time, CERN decided to eschew chlorofluorocarbons (CFCs), which are now widely banned after being identified as a main culprit in ozone-layer damage. Instead, decision-makers chose to use hydrofluorocarbons (HFCs). Today, scientists know that although HFCs aren’t a threat to the ozone layer, they are greenhouse gases with GWPs that can be thousands or tens of thousands of times as high as carbon dioxide’s.
Considering carbon dioxide’s role in the general conversation about global warming, it might seem counterintuitive that it’s one of the leading contenders to replace HFC refrigerants at the LHC. CERN physicist Paolo Petagna is leading a team developing pumped-loop CO2 cooling systems for a variety of the LHC’s detectors.
He says he’s been pushing for CO2 cooling for a long time. In the early 2000s, he suggested CO2 as an alternative to other refrigerants. At that time, Petagna says, the advantage he saw in CO2 wasn’t its lower GWP but the small size of CO2 molecules. Smaller refrigerants would allow physicists to make cooling systems with thinner pipes. “When you talk about cooling, smaller is better,” Petagna says because reducing the amount of material inside the detector that needs to be cooled improves detector performance.
The difficulty of using CO2 is that it must be held at least at double the pressure of halocarbon refrigerants. Petagna says this pressure requirement made it hard to sell CO2 refrigeration originally. Decision makers at CERN wanted to avoid extra problems, he recalls.
Petagna’s group has already retrofitted parts of the CMS and a few other detectors with CO2 cooling systems. By the end of 2026, he says, enough detectors at the facility will be cooled this way so that the LHC’s CO2 refrigeration will have increased by an order of magnitude.
CO2 cooling systems use a closed loop of CO2 flowing through the machinery. But they still use HFC refrigerant gases like R404A—a mixture of 1,1,1,2-tetrafluoroethane, pentafluoroethane, and 1,3,3,3-tetrafluoro-1-propene—to help cool the CO2. Now Petagna’s group is collaborating with researchers at the Norwegian University of Science and Technology to develop so-called transcritical CO2 refrigerant systems for the LHC, making high-GWP refrigerants unnecessary. Unlike traditional refrigeration systems in which a phase change from liquid to gas is part of the cooling process, transcritical systems cool the CO2 in the gaseous state. These systems operate at higher pressures than standard CO2 refrigeration and cool more efficiently in CERN’s climate. And, Petagna adds, they may eventually find their way onto fishing boats to help keep the day’s catch fresh for the market.
CO2 won’t work for the LHC’s so-called beauty experiment (dubbed LHCb), which tracks particles to better understand matter and antimatter. A new detector that is being added to this experiment during the current shutdown aims to improve the spatial resolution of the particle tracking. Petagna says the detectors require temperatures that CO2 can’t cool them to. In that case, he says, CERN plans to use a hydrofluoroolefin (HFO) refrigerant. HFOs are recent entrants in the refrigerant arena designed to have much lower GWP than HFC refrigerants. Some have lower GWP than even CO2.
Greenhouse gases play roles at CERN that go beyond just cooling. Some of the LHC’s detectors are filled with gas to help particle detection. When protons collide, they ionize those gas molecules, creating ions and electrons that can be measured as electrical current. Herndon explains that most detectors use a three-gas mixture. The first is the ionizing gas, which is often argon. Another gas commonly chosen for this purpose is R134a—1,1,1,2-tetrafluoroethane—CERN’s Guida says. Second is a quenching gas that has lots of vibrational or rotational modes to absorb the energy of scattered photons, Herndon says. For his experiments, the CMS’s cathode strip chamber detector uses CO2 for quenching. Last is an “anti-aging” gas to prevent the quenching gas from polymerizing on wires inside the detector. In the cathode strip chamber detector, that’s tetrafluoromethane, a greenhouse gas.
Scientists choose these detector gases because of their stability, CERN physicist Petagna says. Detecting particles depends on breaking some of the gas molecules in the detector to generate ions, Guida explains. But ionizing too many molecules isn’t good. “The more we break, the more impurities we’re going to create in the chamber,” which can reduce the detector’s effectiveness.
That’s why researchers often pick their detector gases from the ranks of refrigerants. Refrigerant gases also need to be stable so they can survive repeated compression and heating cycles and friction effects inside a refrigeration system.
Guida and Mandelli have been studying potential replacements for detector gases to see how they would hold up in experiments. In some cases, gases need to survive in a detector for decades, Guida says.
But balancing stability and environmental friendliness is difficult. Guida explains that many low-GWP gases are intentionally less stable because they are designed to break down in the atmosphere before they can trap too much heat. Worse still, the same conditions—humidity and radiation—that degrade these molecules in the atmosphere are found in the LHC’s detectors.
Working with collaborators at universities around Italy, Guida and Mandelli have made some progress. A leading candidate right now is 1,3,3,3-tetrafluoropropene, also known as HFO-1234ze. Tests are ongoing to make sure a detector could operate with that molecule for 20 years without repair.
Changes in the size and charge of the gas molecules used in detectors can interfere with the equipment’s ability to sense particles efficiently, Guida says. In particular, ionization of fluorinated gases—which many low-GWP gases are—can produce fluorine radicals that etch surfaces in the detector or damage them in other ways. “We need long-term study,” he says.
Herndon has been testing alternatives to tetrafluoromethane, currently used in the CMS’s detector. For 2 years he’s been operating a simulated detector in another part of the facility that has a muon source. The search hasn’t yet been fruitful. “In general we have found nothing that works anywhere near as well,” he says.
But researchers have another idea. Herndon’s group members have experimented with simply using less CF4 in their mixtures. He says they’ve reduced the amount by four-fifths and found in the tests they’ve run so far that the detectors still work well and don’t display aging effects beyond those observed with regular gas mixtures.
Bordry says people shouldn’t expect a miracle from this round of improvements to the LHC’s environmental footprint. The facility’s detectors were designed as early as the 1990s, and because of their age, only so much can be done to reduce their climate impact. He’s hopeful that the CMS will achieve a 10% reduction in greenhouse gas emissions after the current shutdown and perhaps as much as 50% after the next shutdown, planned for 2023.
He’s thinking ahead to the next generation of detectors and how CERN’s efforts could inform their design. For him, that’s the underlying purpose of this work, to develop new technologies that can reduce greenhouse gas use. “The mandate of CERN is not just for research but also for developing technology,” Bordry points out. He says he’s thinking not just about the problems of emissions at CERN but about what solutions he and his collaborators can offer in the global fight against climate change.