A project will simulate a new design for potentially boosting a particle accelerator’s maximum energy
Electrons zip round and round the racetrack-shaped Continuous Electron Beam Accelerator Facility (CEBAF) before being used in nuclear physics experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
Seven years ago, an upgrade doubled the maximum operating energy of CEBAF’s electron beam from 6 GeV to 12 GeV. Since then, researchers have been using this higher-energy beam to learn more about the particles that make up the atomic nucleus. Now, the list of experiments booked for this DOE Office of Science user facility equate to roughly a full decade of running.
But what happens after that?
Two years ago, Alex Bogacz, a senior staff scientist at Jefferson Lab and leader of the accelerator physics group in the lab’s Center for Advanced Studies of Accelerators, started looking for answers to this question.
Bogacz thought the best path forward would be another upgrade to CEBAF. After some review of the possibilities, he suggested a design that would allow the lab to once again double the maximum energy of CEBAF to offer new experimental possibilities — this time from 12 GeV to around 24 GeV. This would capitalize on the investments that the scientific community and the DOE has already made in CEBAF, to take advantage of the experience its operators has gained in running the machine, and to serve the dedicated scientific user community who continued to come up with new ways to exploit its unique capabilities for their research.
“It would be a major revamp, but would save 90 percent of existing CEBAF infrastructure and stay within the existing layout,” Bogacz said.
In October 2021, Bogacz and Ryan Bodenstein, another staff scientist at Jefferson Lab, began a projected two-year Laboratory Directed Research and Development (LDRD) project to probe the feasibility of this preliminary design. The LDRD program supports small-scale research projects that expand the lab’s core scientific capabilities.
“Jefferson Lab has to find a path forward in the future and one of those is higher energy,” Bodenstein said. “Right now, we’re painting the picture, but we’re not sure we’re using the right paint.”
To see if they are using the right paint, Bodenstein and Bogacz, co-principal investigators of the LDRD project, will simulate elements of the design piece by piece, starting with the arcs.
More laps for more energy
CEBAF is shaped like a big racetrack. The straight sections on either side are linear accelerators, which impart electron beams with energy as they zip through. These linear accelerators are connected by semicircles known as arcs.
The arcs have magnets inside of them that steer the beam from the linear accelerator on one side to the linear accelerator on the other side. Currently, the arcs in CEBAF allow electron beams to take up to 5.5 laps through the machine. With each pass, the beam gains more energy from the linear accelerators until it reaches its 12 GeV maximum.
This LDRD will test a design that could double CEBAF’s maximum energy to 24 GeV by recirculating the beam more times.
Right now, CEBAF has five electromagnetic arcs on either side. Each electromagnetic arc contains magnets that can control one specific beam energy at a time. But the electron beam itself is actually made up of beams of different energies, with up to four separate beams destined for each of four experimental halls. These beams are all moving at about the same speed and can travel together through the linear accelerators. But as the beams approach these five arcs, each beam is separated out to travel through the electromagnetic arc designed to control the specific energy it has at that time.
The new design will replace one electromagnetic arc on each side with a fixed field alternating gradient (FFA) arc.
“FFA arcs are purposely designed with multiple energies in mind,” Bodenstein said. “They allow beams of several different energies to co-exist and be controlled in the same beam pipe.”
After the FFA arc substitution, CEBAF’s beam will take four laps through the remaining electromagnetic arcs and then six additional laps through the two FFA arcs, which would increase the total number of passes from 5.5 to 10.5.
“That would do the trick of doubling the energy,” Bogacz said.
Ten and a half passes would bump the maximum beam energy up to somewhere between 20 and 24 GeV. The exact number of FFA passes needed for experiments would be determined by experimental requirements.
“The number of FFA passes depends upon the needs of the user community and what energy gives the most useful physics results,” Bodenstein said.
Recently, DOE’s Brookhaven National Laboratory and Cornell University used FFA arcs to transport beams of four different energies through the same arc in the Cornell-Brookhaven ERL Test Accelerator (CBETA).
“We were inspired by the pioneering results of CBETA,” said Bogacz.
He and other Jefferson Lab scientists are collaborating on this new design with members of CBETA from Brookhaven and Cornell, as well as scientists from DOE’s Oak Ridge National Laboratory.
More energy for more physics (using less power)
Doug Higinbotham, a senior staff scientist and experimentalist at Jefferson Lab, said a higher-energy beam will allow unprecedented precision in understanding how quarks and gluons make up protons and neutrons.
“The study Ryan, Alex and others are doing is truly amazing,” Higinbotham said. “Not so long ago, I didn't think such an energy upgrade would be feasible within the current accelerator tunnel. But it now looks very feasible and like a very exciting future upgrade for Jefferson Lab.”
But before experiments can benefit from a renovated CEBAF, Bodenstein has to determine if this design will work. After a year of preparations, he has begun simulating parts of the FFA arcs with Alex Coxe, a Ph.D. student at Old Dominion University.
Bodenstein and Coxe are tracking the movement of virtual electrons through simulations of different accelerator elements to test their performance. They also add in realistic errors, such as slight offsets of equipment that occur during installation.
Even with these intentional errors, initial simulations of the FFA arcs are promising.
“I’m mega excited to see the project taking off,” said Coxe, who recently presented the project’s first results at the 2022 Photonuclear Reactions Gordon Research Conference: Frontiers in Nuclear and Hadronic Physics. “We’re testing an upgrade that could allow new experiments for years to come.”
Bodenstein hopes to eventually combine simulations of the arcs with simulations of other parts of the machine to produce a start-to-end simulation of the upgraded CEBAF.
“I think that the LDRD is giving us an opportunity to really make this upgrade go quickly, because we're putting time and effort into seeing if this design is feasible,” he said.
At over 20 GeV, the final beam would be 100 times more powerful than existing FFA-based beams. It would also consume less electricity. CEBAF’s electromagnetic arcs use electromagnets, which require power to control them. The two FFA arcs would use permanent magnets, which don’t need power to maintain their magnetic fields.
Even if the LDRD project proves this upgrade feasible, a higher energy and greener CEBAF is still years away. For now, Bogacz and his team will continue to push this in-progress design while Bodenstein and Coxe simulate it with the help of Katheryne Price, Kirsten Deitrick, and Dennis Turner. And they are looking to bring on a new postdoctoral researcher to help with the analysis soon.
“This is our future, so we are working hard to make sure that it’s going to happen,” Bogacz said.
By Chris Patrick
Contact: Kandice Carter, Jefferson Lab Communications Office, firstname.lastname@example.org