Making Magnets for the Large Hadron Collider
The world’s largest particle accelerator, measuring 27 kilometres, is turned off – the second shutdown in a series of efforts to upgrade CERN’s Large Hadron Collider (LHC) since its first run in 2009.
However, Dr Ian Pong (Croucher Scholarship 2005, Croucher Fellowship 2008) is already preparing for Long Shutdown 3 scheduled for 2024, the High Luminosity upgrade, an international collaboration that will take two and a half years to complete.
Key to this upgrade is the replacement of the focusing magnets and correctors, collectively called the “inner triplet magnets”, near the collision points in the LHC. Pong explained that the upgrade is needed because the current magnets have been degraded from high-energy particle debris over the years.
Pong works in California with the Lawrence Berkeley National Laboratory – commonly known as the Berkeley Lab – where he is the cable task leader, overseeing the production of all the superconducting cables for the US magnets. He leads a team of engineers, scientists, technicians and interns to manage the task.
“As task leader, I am ultimately responsible for the product my team delivers,” Pong said. “There is a great responsibility to deliver that comes with the trust – the trust from our funding agency, our international collaborators, and other task leaders in our project team that every cable I sign off meets or exceeds the performance requirements and is delivered on schedule and within budget.”
Pong’s journey towards the LHC began in his undergraduate days at Imperial College London where he studied aerospace materials engineering and developed an interest in compounds with superconducting properties. A PhD at Cambridge followed, when he studied the application of materials characterisation techniques, including x-ray microtomography, on the superconducting wires he was investigating.
Independently, CERN scientists in Switzerland were trying to do something similar. After his PhD, he joined them as an Honorary Croucher Fellow and CERN fellow, to study prototype conductors for future particle accelerators. After a second fellowship – two years at the ITER International Fusion Energy Organisation in France – he moved to California and the Berkeley Lab.
Four “inner triplet magnet” groups are needed for the LHC upgrade he is working on. Within each group are three types of quadrupole focusing magnets, of which two are being made in the United States and one in Europe.
The Department of Energy oversees the US’s roughly $200 million in-kind contribution to the LHC. Three labs are working together to deliver this: Fermi National Accelerator Laboratory, Brookhaven National Laboratory, and the Berkeley Lab.
For the first time in an accelerator, the magnets will use Nb3Sn (niobium-tin) superconductors which will increase the luminosity – the collision rate per unit cross section – of the LHC by a factor of ten. Pong described the resulting change from this upgrade as of a similar scale to entering a dark room with a floor lamp, instead of a candle.
“Since this is the first time these magnets are used in a collider, we are under a lot of scrutiny – a blessing and a challenge,” said Pong. “We have to ensure the magnets will perform while also making them against a reasonably aggressive budget and schedule. Plus since this has never before been done there are a lot of unknowns.”
The team has instituted rigorous quality assurance to meet this challenge. The actual cable forming process takes only three hours of the 60 days to complete, from planning to quality assurance, which includes testing the superconductor at cryogenic temperatures.
Another challenge is the delicate nature of the materials used to form the magnets, so working in Pong’s team requires diligence and dedication. The process begins with forming wires into a flat “Rutherford” superconducting cable. A glass fibre insulation braid is then applied.
Pong likens the cabling process to cooking: preparing the ingredients and washing up are essential parts of a chef’s responsibility. The cooking is just an element, albeit a critical one. The manufacturing process is closely monitored as the margin of error is tiny while the materials used in one cable are expensive – worth twice the average engineer’s annual salary.
“The make-or-break cable run itself takes only a few hours. Just one defect a few millimeters long will render the half-km long cable unusable, with a cost impact that is equivalent to crashing a few sports cars,” said Pong.
Once formed, the cables are sent to Fermilab and Brookhaven to be wound into 4.2-metre-long coils. Four coils are then assembled as a “quadrupole” half-magnet back at Berkeley. (One coil makes one pole, and a “quadrupole” magnet has four poles). After testing, two quadrupole half-magnets are put together in a cryostat as a “cryo-assembly”. Its performance must be verified prior to acceptance and delivery to CERN.
After a thorough research and development programme, the project is now in production phase, and about 30 percent of all the needed cables have already been made. Two prototype half-magnets have been completed and a third one is being tested at Brookhaven. The team is hard at work and looks forward to the day when they have delivered the ten cryo-assemblies - four plus one spare of each type - and the “inner triplet magnets” begin delivering high luminosity particle collisions to the LHC.
Dr Ian Pong received his MEng from Imperial College London in 2005, followed by a PhD as a Croucher Scholar and Honorary Cambridge Trust Scholar from University of Cambridge in 2009. He undertook post-doctoral studies at CERN, Switzerland, as a CERN fellow and Honorary Croucher Fellow, and then at the ITER International Fusion Energy Organisation, France, as ITER-Monaco Fellow. He is currently a project scientist at the Lawrence Berkeley National Laboratory in California, USA.
To view Dr Pong's Croucher profile, please click here.