The Large Hadron Collider is the largest and most powerful particle accelerator in the world. Image credit: CERN
When training for a marathon, runners must gradually increase the distance of their runs. They know that their runs in the early days of training do not define what they will one day be capable of; You will build a strong foundation that will help them reach their full potential.
The car-length magnets that direct particles around the Large Hadron Collider go through a similar process. Scientists have to keep pushing them to their limits until they can handle enormous amounts of electricity.
“These magnets are great engineering marvels,” says scientist Kathleen Amm, director of the magnet division of the Brookhaven National Laboratory at the US Department of Energy in New York. “But one thing we can’t do is put them straight into an accelerator. They have to be trained.”
Scientists, engineers and technicians in Brookhaven are now training magnets for an even more difficult task: directing and focusing particles in a next-generation accelerator, the powerful high-luminosity LHC at CERN. Fortunately, in addition to being able to withstand exercise, these magnets are given the ability to carry even more electricity than before.
Resists lightning strikes
With a new superconducting wire based on niobium-3-tin, Nb3Sn, the HL-LHC accelerator magnets will be able to conduct around 40% more electrical current than the previous magnet iteration for the LHC. Each carries about 16,500 amps – roughly the same as a small lightning bolt. The average laptop uses less than 5 amps.
LHC magnets are made from materials that differ in one important way from what a laptop is made of: They are superconducting. This means that they can conduct an electrical current without losing energy. They do not generate heat because they have no electrical resistance.
The catch: Both the old and the new LHC magnets only acquire the property of superconductivity when they are cooled to extremely low temperatures. In the LHC they are kept at 1.9 Kelvin (minus 456.25 Fahrenheit) just above absolute zero.
But that’s not always enough: A small mistake can cause a magnet to suddenly lose its superconducting properties – a process known as quenching.
“A quench means that part of the superconductor is becoming normal,” says scientist Sandor Feher, who oversees the testing and training of HL-LHC magnets. “Its temperature starts to rise and that heat spreads to other parts of the magnet.”
A quench can be ruinous. “If a superconductor loses its superconducting properties, the electrical resistance changes from zero to a very high electrical resistance,” says Amm. “Previously [of superconductor development]”Magnets would burn out through this rapid transition.”
But this overheating doesn’t always mean a disaster. During magnet training, controlled quenches induce helpful structural changes at the microscopic level that improve a magnet’s performance.
The anatomy of a magnet
At the age of 12, Martel Walls won a local art competition with a detailed and realistic drawing of a courthouse in Bloomington, Illinois. “My drawing ended up in the courthouse,” he says. “Since then, I’ve known that I wanted to work in an area that uses my eye for detail and a steady hand.”
Walls’ eye for complex shapes eventually led him to his job as senior solenoid development technician at the Fermi National Accelerator Laboratory in Illinois, where teams both manufacture and test magnets for the HL-LHC.
The magnets that Walls and his team put together are 450 meters (approximately 1480 feet) of superconducting Nb3Sn cable wrapped around two interlocking support structures. The coils are about 4.5 meters long. Every inch of cable is checked both before and during the winding process.
The coils are then heated to 665 degrees Celsius (1229 degrees Fahrenheit) in an 11 day heating cycle; a process that transforms the ordinary niobium-tin cable into a superconductor, but also makes it incredibly brittle. “It gets as fragile as uncooked spaghetti,” says Walls.
The technicians treat them as gently as possible and solder other components onto the coils before soaking them in epoxy resin. The final coils are shipped to the Lawrence Berkeley National Laboratory in California, where several coils are assembled and then wrapped in a strong steel case. They will then be shipped to Brookhaven to begin their training program.
When the Brookhaven test team connects the magnets with electricity, the coils push and pull together with enormous forces due to the high magnetic fields.
Even a tiny movement on the order of only 10 to 20 micrometers – roughly the width of a human hair – can be enough to create a quench.
Engineers recognized early on that a well-built magnet could remember these microscopic movements. When an unstable component shifts to a more comfortable position, the component then usually stops. The result is a magnet that is more stable the next time you turn it on.
During training, scientists and engineers gradually increase the electrical current circulating in the magnet. When any part of the magnet moves or releases energy, it does so in a controlled laboratory environment and not in an inaccessible underground accelerator complex.
Magnet training in Brookhaven begins with immersing the magnet in a bath of liquid helium. After cooling down, the test team introduces the electric current and gradually increases it.
As soon as a quench occurs, the current is automatically diverted from the magnet. The liquid helium bath evaporates and carries the heat of the quench with it. After each quench, the helium is collected for reuse and the process starts all over again.
“Our goal is three quenches per magnet and day,” says Feher. “We start at 5 or 6 o’clock in the morning and work in shifts until 6 or 7 o’clock in the evening.”
Little by little, the Brookhaven test team is exposing the magnets to ever higher currents.
“We can see 50 to 60 quenches during magnet research and development,” says Amm. “When we go into production, the goal is to see a minimum number of quenches, like 14 or 15, before we get to the desired field level.”
After completing the training – ie the magnet can be operated with the desired current without quenching – it is sent back to Fermilab for further equipment and testing. The final magnets are then shipped to CERN.
According to Amm, designing, building, and preparing magnets for upgrading the LHC is more than applied physics: it’s a form of craftsmanship.
“This is where art and science come into play,” she says. “You can do so much science and technology, but in the end you have to build and test a lot of magnets before you understand the sweet spot.”
‘Magnet Training’ at LHC Provided by Brookhaven National Laboratory
Quote: How the Large Hadron Collider trains its magnets (2021, October 6), accessed October 6, 2021 from https://phys.org/news/2021-10-large-hadron-collider-magnets.html
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