Particle accelerators, like the Large Hadron Collider near Geneva, Switzerland, allow scientists to study the most fundamental particles, but they operate on a massive scale. The tunnel that houses the collider is about 12 feet wide and stretches in a loop over 16 miles long.

The sheer size of these colliders means experiments are expensive to run and have to be coordinated among large collaborations of scientists. But imagine if we could shrink these accelerators, making them available in smaller labs worldwide.

One approach to making smaller, more powerful particle accelerators is plasma wakefield acceleration. Traditional accelerators use metallic structures to create an electric field which increases the speed and energy of the particle beam. Magnetic fields then guide the beam in the right direction. In plasma wakefield acceleration, a special laser-ionized plasma source (an energized gas, the fourth state of matter) accelerates particles which has the potential to make accelerators much more compact and thousands of times more powerful. 

In a recent study, a team of physicists at CU Boulder demonstrated the ability to align a laser-ionized plasma source with the electron beam in an ultra-precise and automated way, paving the way for future developments in making plasma wakefield accelerators a reality.

The plasma advantage

Valentina Lee during shift at FACET-II. (Image credit: SLAC/FACET-II)

The plasma source has a distinct advantage because it can handle electric fields much stronger than traditional accelerators, and it’s remarkably compact, says Valentina Lee, a physics graduate student and first-author on the team’s new paper. “Among the various types, the laser-ionized plasma source is the most appealing because it can be shaped in ways other sources cannot,” Lee adds.

“Using a laser to generate the plasma gives us total control over the three-dimensional shape of the plasma, which can be used to preserve the quality of the electron beam during acceleration,” says Lee. “This opens to the door to accelerating positrons (anti-electrons), in the future.”

The plasma accelerator designed by the group at CU measures only 40 centimeters long, about the length of a standard laptop screen. Yet, it provides as much energy to the electron beam as the 1-kilometer long conventional accelerator at SLAC National Accelerator Laboratory, where the Litos group's experiments take place. 

If plasma accelerators become practical, they could bring the power of large-scale accelerators to smaller research facilities, like university labs.

Precision alignment

Using one of the highest-powered lasers on campus in the Litos group’s lab, the team verified the optics for the plasma source before running the experiment with the electron beam at FACET-II, the Facility for Advanced Accelerator Experimental Tests at the SLAC National Accelerator Laboratory in California.

Experiment time at the national accelerator lab is limited as research groups bid for slots on the schedule. Running an experiment at a facility like SLAC involves a lot of upfront planning and coordination.

“There are only so many months of beam time per year, and only so many of those hours are available to different users for their experiments,” explains Michael Litos, associate professor of physics.  

For Lee, an experiment like this was something she had always hoped for. She had known she wanted to be a physicist since she was 8 years old, after children’s books sparked her interest. When choosing a research field, Lee wrote out what she liked. At the top of the list? Electromagnetics and lasers – giant lasers. So, when she found that plasma wakefield acceleration combined giant lasers with accelerating electrons, she was set. 

Lee traveled back and forth to SLAC during her research, often spending two weeks at a time “running shift” on site. “At SLAC, a shift runs about 16 hours, then you go home for 8 hours to rest. Even though 16 hours sounds like a long time, originally it was taking us 12 hours just to set up the experiment, then we had 4 hours to actually do the plasma wakefield research,” said Lee.

Realizing more time was needed for the experiment, Lee teamed up with former Litos group graduate student Robert Ariniello, who at the time was a project scientist at SLAC. Together, they developed a process to automate the alignment, saving about 10 hours of setup time.

Positioning the plasma source and electron beam requires ultra-precise work. “We’re aligning a large-scale electron beam from one kilometer away with a laser from 40 meters away, and they have to be aligned with better than 10 micron precision. That’s about a tenth of the width of a human hair,” says Lee.

Lee explains that their novel alignment technique relies on analyzing the plasma afterglow light at two longitudinal positions as they scan the plasma column across the electron beam. “Plasma always glows a little but when it’s really bright then you know you have the perfect alignment,” she says.

It worked. 

Seeing a working plasma wakefield accelerator for the first time was awe-inspiring for Lee. “I’ve been working in this field for seven years. We’ve talked about it and run simulations. To see it live for the first time was incredible,” she adds.

“Over the last year and a half, we’ve started seeing the high impact results that we’ve been working toward for most of a decade,” explains Litos. “This was an awesome experimental accomplishment which enabled the heart of Valentina’s thesis.”

With these results, Lee is finishing two more papers and will soon be writing her thesis. 

The group’s research will help open doors to accelerating positrons, one of the key challenges in plasma wakefield acceleration. The alignment will have to be even more precise, but with Lee’s alignment technique, the team thinks it’s possible.