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The artistic representation illustrates a measurement of a beam in a particle accelerator. The measurement is shown in progressively higher dimensions, with each higher dimension revealing information that was previously hidden. (Jill Hemman/ORNL)

A team of researchers led by UT physics professor Sarah Cousineau has made the first-ever 6D measurement of an accelerator beam, an achievement that has eluded scientists for decades.

The new measurement will advance understanding of what goes on inside accelerators and help scientists and engineers build more powerful ones in the future. The findings were published in the journal Physical Review Letters today.

Cousineau co-authored the paper with UT graduate student and lead author Brandon Cathey and Oak Ridge National Laboratory’s Alexander Aleksandrov and Alexander Zhukov.

“Achieving this measurement advanced the field in one big leap — we have information we’ve never had before. There was a key piece missing,” said Cousineau, who holds a joint appointment as a physics professor at UT and a group leader in ORNL’s Research Accelerator Division.

Taking measurements in 6D includes the same dimensions of a 3D measurement, plus additional data points for the velocity in each direction along the x, yand axes. Until now, scientists stitched together three 2D measurements to create a representation of a 6D measurement.

“The 6D space is more complicated than everyone thought, and it’s impossible to see this complexity in low dimensions,” Cousineau said. “For years, we have been making assumptions in our models based on incomplete information, and we now know that those assumptions are wrong.”

After completing an undergraduate internship with the Spallation Neutron Source group, Cathey came onboard this experiment as a graduate student.

“I essentially had my own accelerator for almost a year,” he said. “I’m incredibly grateful to have had this experience.”

It took the team a year of trial and error before they achieved a successful measurement, which itself took 35 hours to complete. The inability to gain access to a fully operational state-of-the-art accelerator for that length of time made it impossible to even attempt the measurements before now.

Instead, they relied on a five-meter replica of the first portion of the Spallation Neutron Source’s linear accelerator, commonly known as a linac. The team also had state-of-the-art instrumentation to do the measurements.

“When we proposed making a 6D measurement 15 years ago, the problems associated with the curse of dimensionality seemed insurmountable,” said ORNL physicist and co-author Alexander Aleksandrov. “Now that we’ve succeeded, we’re sure we can improve the system to make faster, higher resolution measurements, adding an almost ubiquitous technique to the arsenal of accelerator physicists everywhere.”

“With better simulations, we could build more efficient accelerators and test novel accelerator designs without needing to risk huge investments,” Cathey said.

The project was funded in part with a grant from the National Science Foundation.

“This result shows the value of combining the freedom and ingenuity of NSF-funded academic research with facilities available through the broad national laboratory complex,” said Vyacheslav Lukin, the NSF program officer who oversees the grant.

“There is no better way to introduce a new scientist — a graduate student — to the modern scientific enterprise than by allowing them to lead a first-of-a-kind research project at a facility that uniquely can dissect the particles that underpin what we know and understand about matter and energy.”

Accelerator science brings together physics and engineering, and this intersection was a draw for Cathey.

“I get to actually put my hands on the experiment. The beam is physically there and I can see it respond to changes I implement. At the same time, an accelerator is an incredibly complicated machine,” he said. “Even though it’s a technology I’ve used regularly now, there are still plenty of questions left to answer.”

Originally built to test a piece of equipment, the replica will continue to provide a unique hands-on learning experience for students. The opportunity to use the test accelerator on a regular basis will help attract top students as Cousineau and her colleagues continue to build an accelerator science program at UT.

“If you’re going to build an accelerator science program, it makes sense to do it where you have the world’s most powerful accelerator in your backyard,” she said.

The researchers hope to use the new measurements to model the entire beam, which could allow them to solve persistent problems in accelerators like beam loss. More pressing, though, will be finding software capable of analyzing the roughly 5 million data points that their 6D measurement generated.


Karen Dunlap (, 865-974-8674)