SLAC And UCLA Researchers Build Plasma Accelerator That Boosts Electron Energy And Brightness At The Same Time

Demonstrated at SLAC’s FACET-II national user facility, the innovative technology could pave the way for shrinking future accelerators and X-ray light sources and enhancing their capabilities.

Key takeaways:

• In a series of breakthroughs, researchers developed a new way to simultaneously create brighter and more energetic electron beams.
• Using a novel plasma wakefield accelerator and FACET-II’s high-power beams, teams from UCLA and SLAC National Accelerator Laboratory accelerate electrons to energies beyond expectations.
• This transformer technology opens the door to make compact, cost-efficient particle accelerators for advancing science and technology.

Researchers from the Department of Energy’s SLAC National Accelerator Laboratory and the University of California, Los Angeles (UCLA), have designed innovative technology that can generate both high-energy and high-brightness electron bunches in an accelerator that is a fraction of the size of current particle accelerators. This breakthrough has the potential to shrink the size of future particle colliders and X-ray free-electron lasers that researchers use to gain insight into nature’s fundamental building blocks and processes.

In the new study, the UCLA-led team developed a novel plasma wakefield accelerator (PWFA), in which electrons gain energy by “surfing” a plasma wave rather than drawing energy from the electromagnetic field inside metal structures of conventional accelerators. Powered by the unique electron beams from SLAC’s Facility for Advanced Accelerator Experimental Tests (FACET-II), the plasma accelerator more than doubled the energy of newly created electrons in a 4-meter-long plasma chamber that would have otherwise required a conventional linac over a kilometer long. In addition, it also boosted the beam brightness more than 10-fold over the FACET-II beam. The result is a new world record for plasma accelerators, achieving an unprecedented combination of high energy gain and high beam quality.

“This work could only be done here at SLAC because of the capabilities we have at FACET-II that are not found elsewhere,” said Mark Hogan, FACET-II director and a co-author of the study, published in Nature Communications. “Our high-energy, high-peak-current beams and our infrastructure for plasma sources and advanced diagnostics allow researchers in our advanced accelerator community to try new science and technologies for next-generation accelerators.”

Miniaturizing particle accelerators
For decades, researchers have been using powerful electron beams to probe the natural world. Those beams are also at the heart of state-of-the-art X-ray free-electron lasers, like SLAC’s world-leading Linac Coherent Light Source (LCLS), whose brilliant X-rays enable unprecedented studies of atoms and molecules in motion, paving the way for the discovery of quantum materials, new medicines and energy technologies.

However, these cutting-edge machines typically need massive particle accelerators, in which electrons gain energy from electric fields within metal cavities, to produce the required high-energy, high-brightness electron beams.

One promising technology to make shorter accelerators replaces metal cavities with cells of ionized gas, or plasma, that can sustain much larger acceleration gradients, potentially leading to the same energy boost over much shorter distances. In these plasma wakefield accelerators, a first electron bunch – the drive bunch – travels through the plasma, exciting a plasma wave. Then, a second electron bunch “surfs” that wave to higher and higher energies.

While this idea isn’t new and high energy gains have been demonstrated before, increasing the energy of the electrons is only half the battle. “One of the main challenges in our field has been to boost an electron beam's energy without sacrificing its brightness – a critical measure of its quality,” said Chaojie Zhang, lead author of the paper and a scientist at UCLA.

More than an accelerator: a plasma wakefield acceleration transformer
Now, the UCLA-SLAC team found a way to do both: boost beam energy and brightness at the same time.

Led by Chandrashekhar Joshi, professor of electrical and computer engineering, the UCLA researchers developed a novel plasma source for their experiments at FACET-II.

“Our plasma source is unique in the way it integrates different functions in three stages,” explained Zhang.

The first stage acts as a plasma lens, focusing a drive bunch from FACET-II to a small enough size to excite the plasma wake. The drive bunch, now a fraction of the diameter of a human hair, starts pushing the plasma electrons out of the way, forming a bubble that is similar to the wake behind a traveling boat.

“That bubble is where all the magic happens,” Hogan said. In a conventional PWFA, a second electron bunch from FACET-II would get injected into the bubble where it would gain energy.

“What sets this work apart from previous efforts is in the second plasma stage, where the plasma density drops sharply, stretching and enlarging the bubble very quickly,” Hogan said. “This traps the plasma electrons pushed away by the drive bunch and forms a new electron bunch at the back of the bubble.” In other words, with this new technology, there is no need to inject a second bunch into the plasma; the plasma transforms energy from the drive bunch into a new, self-created bunch with high energy and high brightness.

Finally, in the last two meters, the new bunch surfs the plasma wave, sustained by energy from the drive bunch, and rides it for a big energy boost.

I was extremely excited when we saw the energy gain. That was a big surprise to the whole team and far exceeded our initial expectations. -Chaojie Zhang.

Two birds, one stone
The researchers found that their setup created new bunches with energies above twice that of the drive bunch. “I was extremely excited when we saw the energy gain,” said Zhang. “That was a big surprise to the whole team and far exceeded our initial expectations.”

But there was more. The results also pointed to a simultaneous 10-fold increase in bunch brightness, overcoming the previous trade-off between boosting beam energy or preserving beam brightness. Another bonus of this technique: Increasing the brightness of a newly created bunch proved easier than doing so for an externally injected second bunch.

“The energy and quality of this bunch is unusual and unique to plasma-created beams and far exceeds previous plasma wakefield accelerator efforts,” said Hogan. “This is a milestone toward proving these accelerators can produce the quality needed to be useful for practical accelerator applications.”

‘Turning challenges into opportunities’
The team will next try to produce even higher-brightness bunches with other desired characteristics. “The plasma wakefield accelerator transformer could potentially be a beam source for the plasma-driven attosecond X-ray technique, which could lead to opportunities to expand the portfolio of science we do at LCLS,” said Hogan.

For X-ray free-electron lasers such as LCLS, “this experiment demonstrates a new paradigm for high-brightness beam generation, a potential game changer,” said Agostino Marinelli, professor of photon science and of particle physics and astrophysics at SLAC. “This could lead to dramatic improvements in the X-ray peak power, energy range and time resolution.”

“The success of this experiment was due to years of collaborative efforts and persistence,” added Zhang. “Along the way, we turned challenges into opportunities. When we realized our original design wouldn't work, we had to innovate and pivot our experimental approach.”

“This new accelerator concept is an important step forward in advancing compact and cost-efficient accelerators for future colliders and light sources,” said Joshi.

Additional contributors included researchers from the University of Oslo, the Polytechnic Institute of Paris and the University of Colorado, Boulder. Simulations were performed using resources of the National Energy Research Scientific Computing Center (NERSC).

This research was supported by DOE Office of Science. FACET-II, LCLS and NERSC are DOE Office of Science user facilities.

About SLAC
SLAC National Accelerator Laboratory explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by researchers around the globe. As world leaders in ultrafast science and bold explorers of the physics of the universe, we forge new ground in understanding our origins and building a healthier and more sustainable future. Our discovery and innovation help develop new materials and chemical processes and open unprecedented views of the cosmos and life’s most delicate machinery. Building on more than 60 years of visionary research, we help shape the future by advancing areas such as quantum technology, scientific computing and the development of next-generation accelerators.

SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science. 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.