Dual Lasers Lead to High-Energy LPA and View of Laser-Plasma Interaction

Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) used lasers and a supersonic sheet of gas to accelerate a high-quality beam of electrons to 10 billion electronvolts (eV) in just 30 cm. The energy and quality of the beam is a significant improvement compared to previous efforts.

The acceleration of a high-quality, 10-gigaelectronvolt (GeV) electron beam in less than one foot, using a petawatt (PW) laser, is a major step forward in laser-plasma acceleration. Laser-plasma accelerators could someday reduce the size and cost of the particle accelerators used in high-energy physics, medicine, and materials science.

Experts at the Berkeley Lab Laser Accelerator Center (BELLA) used dual laser beams focused onto a sheet of gas to achieve a milestone in laser-driven electron acceleration. Four members of the team are pictured with the device used to create the gas sheet. (From left) Alexander Picksley, Jeroen van Tilborg, Carlo Benedetti, and Anthony Gonsalves. Courtesy of Berkeley Lab/Marilyn Sargent.

The team used a dual laser system to create the beam. The first laser served as a drill, heating the plasma and forming a channel to guide the driver laser, which accelerated the electrons. The plasma channel directed the laser energy much like a fiber optic cable guides light, keeping the laser pulse focused over long distances.

The researchers used a series of gas jets to shape the plasma. The jets created a sheet of gas traveling at supersonic speed, and the lasers passed through the sheet to form the plasma channel. This setup allowed the researchers to fine-tune the plasma and modify its shape.

Because of its resilience, the gas sheet can be scaled to very high repetition rates — a potentially useful option for future applications, including particle colliders.

The researchers measured the high-intensity laser propagation throughout the channel-guided laser plasma accelerator by adjusting the length of the plasma channel on a shot-by-shot basis. They observed how a PW laser interacted with a long plasma channel frame by frame.

“Before, the plasma was essentially a black box. You knew what you put in and what came out at the end,” researcher Carlo Benedetti said. “This is the first time we can capture what’s happening inside the accelerator at each point, showing how the laser and plasma wave evolve, at high power, frame by frame.”

The researchers created an efficient beam from an accelerating structure that was dark current-free — that is, no background electrons in the plasma diverted power from the laser.

“If you have dark currents, they’re sucking up the laser energy instead of accelerating your electron beam,” researcher Jeroen van Tilborg said. “We’ve gotten to a point where we can control our accelerator and suppress unwanted effects, so we are making a high-quality beam without wasting energy. That’s essential as we think about the ideal laser accelerator of the future.”

The team observed transverse energy transport of higher-order modes in approximately the first 12 cm of the plasma channel, followed by quasi-matched propagation, and the gradual, dark current-free depletion of laser energy to the wakefield. The researchers quantified the laser-to-wake transfer efficiency limitations of the currently available PW-class laser systems, and demonstrated via simulation how control over the laser mode could improve accelerated beam parameters.

The high-quality electron beam, produced at a very high energy level using dual laser beams focused on a sheet of gas, opens the way for future high-efficiency machines. “We’ve jumped from 8 GeV to 10 GeV, but we’ve also significantly improved the quality and energy efficiency by changing the technology we use,” researcher Alexander Picksley said. “This is a milestone step on the path to a future plasma-based collider.”

The technology could be used to produce particle beams for cancer treatments, or serve as a power source for free-electron lasers that could be used to create advanced materials or provide insight into chemical and biological processes.

A plasma channel forms in a sheet of supersonic gas. Courtesy of Berkeley Lab/Alexander Picksley and Anthony Gonsalves.

“We’ve taken a big step towards enabling applications of these compact accelerators,” researcher Anthony Gonsalves said. “For me, the beauty of this result is we’ve taken away restrictions on the plasma shape that limited efficiency and beam quality. We have built a platform from which we can make big improvements, and are poised to realize the amazing potential of laser-plasma accelerators.”

Laser-plasma accelerators could be used to produce beams of muons to image difficult-to-explore areas like pyramids, volcanoes, mineral deposits, or the interior of nuclear reactors. In the future, the technology could power high-energy particle colliders to search for new particles and gain insight into the forces of the universe. Scientists at the Berkeley Lab Laser Accelerator Center (BELLA), where the experiment took place, are working to develop these very high-energy machines by connecting the building blocks together in a staged accelerator system.

“Coupling stages together gives us a realistic path to generate electrons between 10 and 100 GeV, and to build toward future particle colliders that can reach 10 TeV [teraelectronvolts],” Eric Esarey, director of the BELLA Center, said. “Once the laser energy from one stage is depleted, we send in a new laser pulse, boosting the electron energy from stage to stage in series.”

To create staged systems, sound diagnostics are essential. Reliable analytics help scientists understand the behavior of the plasma, laser, and electron beams, and give scientists precise control over the timing and synchronization of steps that can occur in a fraction of a second. The setup and precise controls provided by the dual laser system allowed the researchers to study the progression of the laser-plasma acceleration and create an efficient beam.

“With this study, we’ve advanced the particle energy of high-quality beams in very short distances, and the efficiency with which we can make them, by using precision diagnostics that give us great laser-plasma control,” researcher Cameron Geddes, director of Berkeley Lab’s Accelerator Technology & Applied Physics Division Division, said.

“Advancing laser-plasma accelerator technology has been identified as an important goal by both the U.S. Particle Physics Project Prioritization Panel and the Department of Energy’s Advanced Accelerator Development Strategy,” he said. “This result is a milestone on our way to staged accelerators that are going to change the way we do our science.”

The research is scheduled to be published in Physical Review Letters. A preprint is available at the online repository arXiv (www.arxiv.org/pdf/2408.00740).