David Flay holds one of the probes that Muon g-2 scientists will use to map the magnetic field inside the experiment’s storage ring. Photo: Reidar Hahn
Muons are mysterious, and scientists are diving deep into the particle to get a handle on a property that might render it — and the universe — a little less mysterious.
Like electrons – muons’ lighter siblings – they are particles with a sort of natural internal magnet. They also have an angular momentum called spin, kind of like a spinning top. The combination of the spin and internal magnet of a particle is called the gyromagnetic ratio, dubbed “g,” but previous attempts at measuring it for muons have thrown up intriguing surprises.
The goal of the Muon g-2 experiment at Fermilab is to measure it more precisely than ever before.
To reach these remarkable levels of precision, scientists have to keep very careful tabs on a few parts of the experiment, one of which is how strong its magnetic field is. The team has been measuring and tweaking the magnetic field for months and is now very close to achieving a stable field before experiments can properly begin.
“We’re in the experiment’s commissioning period right now, where we’re basically learning how our systems behave and making sure everything works properly before we transition into stable running,” said David Flay, a University of Massachusetts scientist working on the calibration of the magnetic field for Muon g-2.
Muon mystery
Muon g-2 is following up on an intriguing result seen at Brookhaven National Laboratory in New York in the early 2000s, when the experiment made observations of muons that didn’t match with theoretical predictions. The experiment’s 15-meter-diameter circular magnet, called a storage ring, was shipped to Illinois across land and sea in 2013, and the measurement is now being conducted at Fermilab with four times the precision.
When Brookhaven carried out the experiment, the result was surprising: The muon value of g differed significantly from what calculations said it should be, and no one is quite sure why. It’s possible the experiment itself was flawed and the result was false, but it also opens the door to the possibility of exotic new particles and theories. With its four-fold increase in precision, Muon g-2 will shed more light on the situation.
To measure g, beams of muons circulating inside the experiment’s storage ring are subjected to an intense magnetic field – about 30,000 times the strength of Earth’s natural field. This causes the muons to rotate around the magnetic field, or precess, in a particular way. By measuring this precession, it is possible to precisely extract the value of g.
The strength of magnetic field to which the muons are exposed directly affects how they precess, so it’s absolutely crucial to make extremely precise measurements of the field strength and maintain its uniformity throughout the ring – not an easy task.
If Muon g-2 backs up Brookhaven’s result, it would be huge news. The Standard Model would need rethinking and it would open up a whole new chapter of particle physics.
A leading theory to explain the intriguing results are new kinds of virtual particles, quantum phenomena that flit in and out of existence, even in an otherwise empty vacuum. All known particles do this, but their total effect doesn’t quite account for Brookhaven’s results. Scientists are therefore predicting one or more new, undiscovered kinds, whose additional ephemeral presence could be providing the strange muon observations.
“The biggest challenge so far has been dealing with the unexpected,” said Joe Grange, scientist at Argonne National Laboratory working on Muon g-2’s magnetic field. “When a mystery pops up that needs to be solved relatively quickly, things can get hectic. But it’s also one of the more fun parts of our work.”
Probing the field
The magnetic field strength measurements are made using small, sensitive electronic devices called probes. Three types of probes – fixed, trolley and plunging – work together to build up a 3-D map of the magnetic field inside the experiment. The field can drift over time, and things like temperature changes in the experiment’s building can subtly affect the ring’s shape, so roughly 400 fixed probes are positioned just above and below the storage ring to keep a constant eye on the field inside. Because these probes are always watching, the scientists know when and by how much to tweak the field to keep it uniform.
For these measurements, and every few days when the experiments is paused and the muon beam is stopped, a 0.5-meter-long, curved cylindrical trolley on rails containing 17 probes is sent around the ring to take a precise field map in the region where the muons are stored. Each orbit takes a couple of hours. The trolley probes are themselves calibrated by a plunging probe, which can move in and out of its own chamber at a specific location in the ring when needed.
The fixed probes have been installed and working since fall 2016, while the 17 trolley probes have recently been removed, upgraded and reinstalled.
“The probes are inside the ring where we can’t see them,” Flay said. “So matching up their positions to get an accurate calibration between them is not an easy thing to do.”
The team developed some innovative solutions to tackle this problem, including a barcode-style system inside the ring, which the trolley scans to relay where it is as it moves around.
Global g-2
Muon g-2 is an international collaboration hosted by Fermilab. Together with scientists from Fermilab, Argonne, and Brookhaven, several universities across the U.S. work with international collaborators from countries as wide-ranging as South Korea, Italy and the UK. In total, around 30 institutions and 150 people work on the experiment.
“It’s the detailed efforts of the Argonne, University of Washington, University of Massachusetts and University of Michigan teams that have produced these reliable, quality tools that give us a complete picture of the magnetic field,” said Brendan Kiburg, Fermilab scientist working on Muon g-2. “It has taken years of meticulous work.”
The team is working to finish the main field strength measurement part of the commissioning process by early 2018, before going on to analyze exactly how the muons experience the generated field. The experiment is planned to begin in full in February 2018.
A Fermilab team built and tested the first new superconducting accelerator cryomodule for the LCLS-II project, which will be the nation’s only X-ray free-electron laser facility.
The first cryomodule for SLAC’s LCLS-II X-ray laser departed Fermilab on Jan. 16. Photo: Reidar Hahn
Earlier this week, scientists and engineers at the U.S. Department of Energy’s Fermilab in Illinois loaded one of the most advanced superconducting radio-frequency cryomodules ever created onto a truck and sent it heading west.
Today, that cryomodule arrived at the U.S. DOE’s SLAC National Accelerator Laboratory in California, where it will become the first of 37 powering a three-mile-long machine that will revolutionize atomic X-ray imaging. The modules are the product of many years of innovation in accelerator technology, and the first cryomodule Fermilab developed for this project set a world record in energy efficiency.
These modules, when lined up end to end, will make up the bulk of the accelerator that will power a massive upgrade to the capabilities of the Linac Coherent Light Source at SLAC, a unique X-ray microscope that will use the brightest X-ray pulses ever made to provide unprecedented details of the atomic world. Fermilab will provide 22 of the cryomodules, with the rest built and tested at the U.S. DOE’s Thomas Jefferson National Accelerator Facility in Virginia.
The quality factor achieved in these components is unprecedented for superconducting radio-frequency cryomodules. The higher the quality factor, the lower the cryogenic load, and the more efficiently the cavity imparts energy to the particle beam. Fermilab’s record-setting cryomodule doubled the quality factor compared to the previous state-of-the-art.
“LCLS-II represents an important technological step which demonstrates that we can build more efficient and more powerful accelerators,” said Fermilab Director Nigel Lockyer. “This is a major milestone for our accelerator program, for our productive collaboration with SLAC and Jefferson Lab and for the worldwide accelerator community.”
Today’s arrival is merely the first. From now into 2019, the teams at Fermilab and Jefferson Lab will build the remaining cryomodules, including spares, and scrutinize them from top to bottom, sending them to SLAC only after they pass the rigorous review.
“It’s safe to say that this is the most advanced machine of its type,” said Elvin Harms, a Fermilab accelerator physicist working on the project. “This upgrade will boost the power of LCLS, allowing it to deliver X-ray laser beams that are 10,000 times brighter than it can give us right now.”
With short, ultrabright pulses that will arrive up to a million times per second, LCLS-II will further sharpen our view of how nature works at the smallest scales and help advance transformative technologies of the future, including novel electronics, life-saving drugs and innovative energy solutions. Hundreds of scientists use LCLS each year to catch a glimpse of nature’s fundamental processes.
To meet the machine’s standards, each Fermilab-built cryomodule must be tested in nearly identical conditions as in the actual accelerator. Each large metal cylinder — up to 40 feet in length and 4 feet in diameter — contains accelerating cavities through which electrons zip at nearly the speed of light. But the cavities, made of superconducting metal, must be kept at a temperature of 2 Kelvin (minus 456 degrees Fahrenheit).
Thirty-seven cryomodules lined end to end — half from Fermilab and half from Jefferson Lab — will make up the bulk of the LCLS-II accelerator. Photo: Reidar Hahn
To achieve this, ultracold liquid helium flows through pipes in the cryomodule, and keeping that temperature steady is part of the testing process.
“The difference between room temperature and a few Kelvin creates a problem, one that manifests as vibrations in the cryomodule,” said Genfa Wu, a Fermilab scientist working on LCLS-II. “And vibrations are bad for linear accelerator operation.”
In initial tests of the prototype cryomodule, scientists found vibration levels that were higher than specification. To diagnose the problem, they used geophones — the same kind of equipment that can detect earthquakes — to rule out external vibration sources. They determined that the cause was inside the cryomodule and made a number of changes, including adjusting the path of the flow of liquid helium. The changes worked, substantially reducing vibration levels — to a 10th of what they were originally — and have been successfully applied to subsequent cryomodules.
Fermilab scientists and engineers are also ensuring that unwanted magnetic fields in the cryomodule are kept to a minimum, since excessive magnetic fields reduce the operating efficiency.
“At Fermilab, we are building this machine from head to toe,” Lockyer said. “From nanoengineering the cavity surface to the integration of thousands of complex components, we have come a long way to the successful delivery of LCLS-II’s first cryomodule.”
Fermilab has tested seven cryomodules, plus one built and previously tested at Jefferson Lab, with great success. Each of those, along with the modules yet to be built and tested, will get its own cross-country trip in the months and years to come.
Read more about the LCLS-II project in SLAC’s press release.
This project is supported by DOE’s Office of Science. LCLS is a DOE Office of Science user facility.
The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov.