Laser Wakefield Acceleration Brings Particles to High Energies

Based at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, the Accelerator Test Facility (ATF) offers high-brightness laser and electron beams to users and provides plenty of opportunities for students such as Prabhjot Kaur to gain knowledge.

Prabhjot Kaur with a poster describing her work on laser wakefield acceleration, a technique that could bring particles to high energies in smaller-size particle accelerators. Image Credit: Photo courtesy of Prabhjot Kaur.

Kaur is an intern through DOE’s Science Undergraduate Laboratory Internships (SULI) program, administered at Brookhaven National Laboratory by the Office of Educational Programs. She has worked with Navid Vafaei-Najafabadi, an ATF user and assistant professor of physics and astronomy at Stony Brook University, on an experiment related to laser wakefield acceleration.

Laser wakefield acceleration is generally known as the “tabletop accelerator,” stated Kaur. “The goal is to use lasers to accelerate particles to greater energies over shorter distances than can be done with traditional accelerators—and therefore in a much smaller space.”

This technique may reduce the cost and remove other barriers encountered in an array of accelerator applications, such as X-ray radiation therapy for cancer, physics research, and processes like food sterilization.

How it Works

Focusing a strong laser beam into a diffuse substance such as hydrogen gas will ionize—or knock off electrons from—the hydrogen atoms. Such a process forces the electrons outward, leaving behind a plasma of charged ions or naked protons.

Researchers such as Vafaei-Najafabadi are looking for ways to administer electrons into the plasma wake, as this would allow them to ride the “waves” produced by the laser to obtain kinetic energy. Electrons fast-tracked in this manner can possibly be used for sterilizing food, producing X-rays for cancer therapy, or scanning cargo or packages.

With X-ray therapy, basically you accelerate electrons and make them strike a target material. That generates x-rays you can direct towards a tumor.

Prabhjot Kaur, Intern, Brookhaven National Laboratory

An analogous method can produce gamma rays for gamma-ray radiography, which “can be used to look inside materials,” added Kaur. “If we get shipments or packages, without opening them, we can look inside [for dangerous substances or items]—like when you go to the airport and transportation security officers look inside your luggage.”

According to Vafaei-Najafabadi, plasma wakefield acceleration may result in small and room-sized accelerators for modest energy particles. “These small accelerators would make this kind of science more widely available across the globe,” he observed.

Plasma wakefield acceleration may also bring the particles to ultra-high energies for applications in particle colliders, where there is a need for very high-quality particle beams.

Even though these particles can be accelerated to high speeds in a distance much shorter than a normal accelerator. What takes place within the plasma is not fully understood.

Navid Vafaei-Najafabadi, Assistant Professor of Physics and Astronomy, Stony Brook University

At ATF, studies are helping researchers to analyze such questions, added Vafaei-Najafabadi.

The presence of a high-quality electron beam in the same place as the high-power laser pulse is what makes ATF a unique facility among all the high-power laser labs in the U.S.,” he further explained.

This offers tools to investigate the workings of the plasma in ways that cannot be accessed elsewhere.

Probing the Plasma

A major aspect of Kaur’s study has been to produce an exclusive, high-resolution probe—that is, an imaging system where researchers can study the substructure of the plasma. This layout has given Kaur a glimpse of what it was like to work on the investigational side of physics, including all the trial and error techniques that accompany it.

The first challenge that I faced was learning about the theory of plasma physics. And in this physics, there’s also an experimental side. And sometimes things don’t go the way that you want them to go. On the spot, you have to think of solutions, figure out what the problems are. So, you also have to be an engineer.

Prabhjot Kaur, Intern, Brookhaven National Laboratory

According to her, one of the most difficult parts was developing the probe to analyze the plasma and finding a lens that has a sufficient resolution to observe its components.

I had to learn about how the optics work and what kind of equipment we needed,” she added. “I tested a bunch of different lenses—mechanical lenses, single lenses, and then we ended up using this piece called an objective.”

The aim was to identify a lens that has a resolution of 5 µm and is smaller than the thickness of a single strand of hair.

No matter what we did, what combination we used, the simple lenses would not work,” Vafaei-Najafabadi added. “We had to go to an objective, a compound lens, to get to the resolution that we want.”

Vafaei-Najafabadi explained that a compound lens is analogous to the eyepiece of the microscope. It is composed of several lenses right behind one another. “Each is supposed to compensate for the weakness of the others,” he added.

The Experimental Process

It was a challenge, trying to experiment with different equipment and trying to get that resolution,” added Kaur.

However, the trials and tribulations are all part of the process.

That’s part of the scientific work, and we train the students for that,” added Vafaei-Najafabadi. “You get to a problem, you have no idea why it’s not working, and then you come up with an hypothesis. And when you learn that that also doesn’t work, then you have to come up with a more refined hypothesis.”

Kaur also gained some experience by working on various experiments at the ATF.

This experience has allowed me to learn how experiments work in general. I like doing the engineering side, and I like learning about the theory. And even though there are many challenges, the SULI program solidified my desire to continue to do research,” she concluded.

In 2019, Kaur graduated from Stony Brook University, where she double-majored in applied mathematics/statistics and physics.

The ATF is a DOE Office of Science User Facility.

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