Accelerator Experiments at RHIC Inform EIC Design

Newswise — When building a one-of-a-kind particle collider filled with groundbreaking technologies, it’s extremely helpful to have a place to test things out. Thankfully that’s the case for the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, home to the future Electron-Ion Collider (EIC).

The EIC at Brookhaven will be a unique facility for unlocking the secrets of the atomic nucleus and the strongest force in nature. It’s being designed and built in partnership with DOE’s Thomas Jefferson National Accelerator Facility. The EIC will reuse one ion accelerator ring of Brookhaven’s Relativistic Heavy Ion Collider (RHIC) while adding an electron accelerator ring, an electron storage ring, and other transformational equipment to create a brand-new state-of-the-art facility. But even before the switch to the EIC begins, RHIC provides a perfect testbed for tackling some of the EIC’s exciting accelerator challenges.

“We run accelerator physics experiments, or APEX studies, at RHIC every other week for about 16 hours,” said Haixin Huang, the accelerator physicist who’s been leading the APEX effort.

For many years, the highest-priority APEX studies were designed to test accelerator approaches that could improve RHIC’s performance. Over RHIC’s 24-year operational lifetime, a range of strategies — including those explored in APEX studies — have increased the collider’s collision rates to nearly 50 times its original design. APEX experiments have also explored technologies that could benefit future facilities or be of interest to the accelerator physics community in general.

“In recent years, we’ve geared these studies more and more into EIC-related topics,” Huang said — particularly since 2020, when DOE selected Brookhaven Lab as the site for the new collider.

Now, the APEX team has a series of successes and milestones to report. Several of these experiments have already had impacts on the EIC design.

Flattening particle beams

One of the central goals of any collider is to achieve high luminosity — a measure that’s related to how frequently particles from one beam collide with particles in the other. Those collisions are what produce the data needed to make discoveries about the building blocks of matter. More collisions yield more data and more discoveries.

At RHIC, two beams of protons or other ions (nuclei of atoms) circulate in separate ion rings in opposite directions. Collisions happen where the two rings cross. RHIC physicists have developed many ways to keep the ions in each beam packed as tightly as possible. This maximizes the chances that ions will smash into one another when the two beams meet.

But even with all this squeezing, the particle beams at RHIC can still be described as something like a rope, with horizontal and vertical dimensions of about the same size. For the EIC, where protons or ions traveling through one RHIC ring will collide with electrons circulating in a brand-new electron storage ring, scientists hope to flatten the proton/ion beam to something more like a ribbon. This flattening will increase the chances that a proton or ion will interact with an electron in the EIC’s oncoming beam.

“The overlapping region for the electron and proton in the EIC collisions will be much smaller than in RHIC collisions,” said Yun Luo, an accelerator physicist coordinating APEX studies related to the EIC and leading this beam-flattening effort. Electron beams are more naturally flat, he said. By flattening the proton/ion beam to match the electron beam’s flatness, the physicists hope to achieve luminosities 100 times higher than in the world’s only previous electron-proton collider, the Hadron-Electron Ring Accelerator (HERA) particle collider, which operated in Hamburg, Germany, from 1992 to 2007.

“This flattening of ion beams has never been demonstrated anywhere,” Luo said … until now. “During last year’s APEX studies, we proved in RHIC that we can generate and maintain these flat beams.”

Keeping beams cool

One of the best ways to squeeze particle beams to increase luminosity — and indeed to flatten a beam — is to keep the beam cool. This makes sense if you think of heat as related to particles’ motion. More motion pushes particles apart and raises the temperature.

Since the EIC’s ions and protons are all positively charged, they actively push away from one another. These repulsive electromagnetic interactions make the beam spread out and heat up. Adding something cool to extract heat can dampen the motion.

“We need cooling to make the beams smaller — the bunches of particles smaller and more tightly packed together at the interaction point — to create higher luminosity,” Luo said.

One approach, led by Brookhaven Lab accelerator physicist Alexei Fedotov and already demonstrated at RHIC, injects a beam of relatively cool electrons to travel together with the ion beam. Since these electrons are physically cooler than the ions, they extract heat just like the coolant in a refrigerator. In addition, their negative charges help to counteract the positively charged protons’ tendency to push one another apart. 

Importantly, these cooling electrons are not the same electrons that participate in the EIC’s collisions. They’ll move in the same direction as the ions for just a short stretch of the accelerator and exit to dump their heat as a steady stream of new cooling electrons flows in to replace them.

“At the EIC, we are going to use this method to pre-cool the ion beams when they are injected into the ring, before they are accelerated to their collision energy,” Luo said.

While this new technology has been essential for RHIC physics, there will be differences at the EIC. The EIC cooling section where the two beams flow together will be significantly longer. The physicists will also need to minimize the angle at which the electron and ion beams overlap. And they’ll use higher electron bunch intensities and different electron patterns than were used at RHIC — for example, matching three electron bunches with every bunch of protons circulating in the collider.

“This year, we did some experiments to test these strategies. We verified that the baseline design parameters for this pre-cooling approach will work for the EIC,” Luo said.

Synchronizing electron and ion beams

Another challenge inherent in colliding electrons with protons at a wide range of energies — an important capability of the EIC — is that the speed of the protons will change at different energies while the speed of electrons stays almost constant. To get these particles to collide at the interaction point within the EIC’s detector at the same time, the physicists will steer the proton beam through different trajectories within the 70-millimeter-diameter beampipe depending on the beam energy.

“At RHIC, the beam orbit goes through the center of beampipe,” Luo said. “However, for the EIC, we have to put the proton beam through the inner or outer part of the beampipe to match the electrons’ revolution time.”

Brookhaven Lab accelerator physicist Guillaume Robert-Demolaize and his team developed a strategy to do this by changing the electric current flowing through the magnets that steer the particles around the circular accelerator racetrack. Changing the current changes the magnets’ field strength and therefore how strongly these magnets bend the paths of the electrically charged particles.

During the APEX studies, they tested this precision steering approach.

“The maximum change we demonstrated is 20 millimeters from the beampipe center in either direction to create a smaller or larger circumference for the proton beam,” Luo said. “Our experiments proved that we can change the orbit of the proton beam at the various EIC energies to synchronize the travel times for the protons and electrons.”

Keeping beams stable

As ion beams make about 80,000 turns per second around the EIC racetrack, there will be many chances for the particles that make up the beam to interact with one another and the environment inside the accelerator — including the beampipe and the radiofrequency cavities that produce their acceleration. These interactions can destabilize the beam.

“This will be a particular challenge at the EIC because its proton beams will have many more bunches and three times more total particles compared with those at RHIC. Plus, these higher-intensity bunches will be packed closer together,” Luo said.

During the APEX studies, the physicists couldn’t pack bunches closer together or increase the numbers of bunches to the levels expected for the EIC. But in an effort led by Brookhaven Lab accelerator physicist Alexei Blednykh, they were able to double or triple the bunch intensity to check the stability of the beam and begin to quantify its interactions with the surrounding beampipe and other accelerator components.

The team is still analyzing those results. But they’re already thinking about ways to counteract the instabilities — for example, by installing dampers that would provide corrective “kicks” to particles when they start to go astray.

Accelerator physicists are also thinking about how to deal with unwanted clouds of electrons that can be generated by protons as they circulate in the ring. These electrons come from interactions between the accelerated protons and residual gas in the beampipe or trapped within the accelerator’s metal components.

“The accelerator is not a perfect vacuum,” Luo explained. “When protons going around hit these gas atoms, they knock electrons off, and when those electrons hit the walls of the beampipe, they generate more electrons to form an electron cloud.”

Those electron clouds can generate heat in the beampipe, which, in a worst-case scenario, could cause the ion storage ring’s supercold superconducting magnets to “quench” — that is, lose their superconductivity.

“So, we’re trying to study these conditions in RHIC,” said Luo, who noted that the EIC’s higher-intensity ion bunches could produce even more of an issue with electron clouds.

“To reduce the electron cloud effect, we plan to insert a ‘beam screen’ into the beampipe,” Luo said.

The inner wall of the beam screen will be coated with amorphous carbon to reduce the generation of unwanted electrons. Currently, Brookhaven Lab accelerator physicist Silvia Verdu Andres is leading a series of APEX studies to test this beam screen and make modifications to its design, if necessary.  

Measuring magnetic interference

One of this year’s APEX studies has already yielded a solution for what could have been a significant problem for the EIC — interference among the facility’s three rings of magnets.

These rings include one of RHIC’s existing ion rings, the new electron storage ring for carrying the colliding electrons, plus an electron accelerator ring to bring the electrons from their injection energy up to collision energy.

“We were concerned that the ion accelerator might generate a magnetic field in the tunnel that would be strong enough to interfere with the operation of the electron accelerator,” Luo said.

So, accelerator physicist Peng Xu led an experiment to measure the strength of the magnetic field generated by RHIC’s ion rings. They found that the “stray” magnetic field would be powerful enough to interfere with the magnets of the EIC electron accelerator if they were operating at the relatively low energy planned for electron injection.

As a result, EIC physicists modified the EIC design to inject the electrons into the ring at a higher, pre-accelerated energy. Raising the starting energy of the electrons before they enter the ring allows the electron accelerator to operate at a higher level that won’t be affected by the ion accelerator’s ambient magnetic field.

Gaining insight into neutrons

One goal of the EIC is to collide electrons with protons and nuclei to learn more about the proton’s inner building blocks, but what about the neutron, the other component of atomic nuclei? EIC scientists would like to learn about those, too. But neutrons cannot be accelerated because they have no electric charge. So, to gain access to neutrons, the EIC will collide electrons with simple nuclei.

One nucleus on the EIC menu is helium-3, which is made of two protons and one neutron. Scientists hope to use these simple nuclei to learn about neutron spin, a form of intrinsic angular momentum.

The first step in these experiments is to measure the spin direction, or polarization, of the particles that make up the helium-3 beam.

“If you want to study spin, you need the spins of all these particles to be in the same direction as much as possible,” said Huang. “That’s a unique feature of the EIC, that both beams will be polarized, the electron and the proton/ion.”

Measuring polarization can be tricky. The process scientists use can sometimes break the nuclei apart. If the number of breakups is too high, it can distort the polarization measurements.

So during this year’s APEX studies, Huang and colleagues put a helium-3 beam into RHIC and tested a device that could identify the breakup events.

“Our experiment shows that the breakup fraction is only a few percent, and it should not be a problem for the helium-3 polarization measurements in the EIC,” Huang said. “We can tag those events and, using this technology, we can veto them to make accurate measurements.”

Leveraging AI to optimize performance

A final set of experiments applied a form of artificial intelligence (AI) known as machine learning to optimize the performance of RHIC during this year’s run. While these experiments were not designed specifically for the EIC, the same approaches can be applied at that future facility, the scientists said.

For the first experiment, a team led by Xiaofeng Gu used machine learning to identify the best ways to adjust beam parameters to maximize RHIC’s luminosity.

“They used a computer-based AI program to analyze operating data from RHIC, including all the parameters that have been adjusted and the effects of those modifications on luminosity,” Luo said. “Then the machine predicted the best way to tune these parameters to get the highest luminosity.”

Another group, led by Chuyu Liu, explored how to use machine learning to disentangle the horizontal and vertical motion of the particles in RHIC’s proton beam.

“Particles in a beam have 3D motion — horizontal, vertical, and longitudinal,” Luo said. “Typically, when you adjust a particle’s horizontal position, its vertical position will change too.”

Anyone can see just how challenging these adjustments can be by exploring this beam-focusing simulation developed by Brookhaven Lab accelerator physicist Stephen Brooks.

The physicists’ goal is to decouple the particles’ horizontal and vertical motion — to find the correct settings and strengths for the magnets to make these corrections.

“These AI experiments are ongoing, but they demonstrate that machine learning is a very promising way to improve accelerator operation,” Luo said. “These techniques will benefit future EIC operations, too.”

The APEX studies for EIC and RHIC operations are funded by the DOE Office of Science.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, visit science.energy.gov.

Follow @BrookhavenLab on social media. Find us on Instagram, LinkedIn, X, and Facebook.