Accelerating Particles with the Brightest Lasers on Earth

What happens when you focus one of the world’s most powerful lasers on a spot so tiny it can be hidden by a human hair?

Using the J-KAREN-P laser at the Kansai Photon Science Institute (KPSI) in Japan, a team led by researchers from the National Institutes for Quantum and Radiological Science and Technology (QST) in Japan investigated this extreme situation. As they report in the American Physical Society’s journal Physical Review Letters, the experimental results reveal a fundamental limit that’s key to optimizing the next generation of ultra-high-intensity lasers.

The J-KAREN-P (Japan-Kansai Advanced Relativistic ENgineering Petawatt) laser at KPSI. Credit: QST.

“[S]tate-of-the-art high-power laser facilities can produce extreme conditions like no other on earth,” explains Nicholas Dover, a postdoctoral researcher at QST and lead author of the new research paper. “To be precise, there is no other method we know of to concentrate as much energy into such a small space and time.”

This extra-concentrated energy can potentially be used for a wide assortment of technological, medical, and research applications and has spurred a lot of cutting-edge research. Dover works in the High-Intensity Laser Science Group at QST, led by Masaki Kando, which is particularly interested in using high-powered lasers to accelerate particles.

When a really intense laser is focused on a small, thin metal target (a foil), electrons in the target get a huge boost of energy—they can be accelerated to nearly the speed of light. These rapidly-traveling electrons generate an electric field on the surface of the target that accelerates ions, like protons, which then leave the foil at high speeds. “This is a completely new way of accelerating particles compared to conventional accelerators,” Dover says.

This image of laser-driven acceleration was generated by a supercomputer based on a 3D calculation. Credit: Nicholas Dover.

Laser-generated particle accelerators could potentially revolutionize accelerator technology, pushing the limits of exploration and treatment capabilities. But first, we need a comprehensive understanding of the physics at work. This includes a solid understanding of the connection between laser intensity, focus area, electron properties, and ion beam properties that’s verified by experimental data. That was the goal of this project.

“This research was really the culmination of a really demanding laser upgrade which took 10-20 people a few years,” Dover explains. “To give you an example of how sensitive [the laser system] is—if the temperature changes by even a degree or two in the lab, thermal expansion of the optical components and their holders will cause the laser to become unusable until we realign it. This isn’t helpful in the scorching Japanese summers!”

Once upgraded, the J-KAREN-P laser had a power of 300 TW (Terawatts). For comparison—the power plant with the greatest capacity in the world, Three Gorges Dam in China, can output a maximum of 0.0225 TW. Worldwide power consumption estimates come in at about 20 TW. “It is about the same amount of light as all the sunlight falling on Japan, except focused down to a couple of square micrometers instead of being spread over the whole country,” Dover says. “But only for a very, very short time!” The laser pulse lasts just quadrillionths of a second.

To get at the physics behind laser-generated particle acceleration, the researchers focused J-KAREN-P onto a stainless-steel foil target. Then, for laser intensities up to 5×1021 W/cm2, the team measured the speed and position of the electrons and protons that streamed off the foil. The researchers adjusted the laser intensity by either changing the energy of the laser or changing the size of the focus spot. For maximum intensity, they reduced the spot down to a radius of just 0.00015 cm.

Illustration of the experiment. The laser (bottom, center) is focused onto a steel foil. Accelerated electrons (e–) and protons (p+) leave the target in different directions. The particles travel through beam profilers and spectrometers that collect data on their position and speed. Credit: Dover, et al. / APS.

Initially, the results were disappointing. They didn’t match predictions made by models that worked well at lower laser intensities. The models predicted a strong correlation between laser intensity and proton speed. But as the laser beam’s intensity increased due to tighter focusing (smaller spot size), the experimental results showed a less-than-expected increase in speed.

Dover explains what happened next this way, “We thought that maybe there was some unknown quantity we had neglected that meant the laser intensity wasn’t what we thought it was. I started going to sit in a park near the lab to think about what was wrong. Then one day I was sitting there thumbing through an old paper and it dawned on me that the experiment was fine, but there was an assumption in the existing theories which we had broken.”

It turned out that the team’s results reflected a fundamental limit of particle acceleration under these conditions. As you reduce the size of the focus spot to increase the intensity, you also reduce the time the electrons have to accelerate. If the beam is too tight, you reduce this time so much that the electrons never reach their predicted speeds—consequently, the ions don’t either. By packing so much power into a such a tiny spot, the researchers had run into a limit they didn’t even know existed.

Dover (front) and other members of the team as captured during a promotional video filmed during the experiment. Credit: APS TV.

“We then quickly turned to calculations on a supercomputer which confirmed that what we had measured was actually a fundamental limitation on the particle acceleration due to our achievement of making such a small laser focal spot,” Dover explains. “This is an important finding, because it allows us to better optimize the design of future laser driven accelerators.”

Research in the field of high-powered lasers is exciting and progressing quickly. With their now-tested laser system, the team is set on improving the parameters of the particle beams they produce. They are also working on accelerating carbon ions using this technique, in an attempt to create a lower-cost, smaller-size accelerator system than currently exists for radiotherapy cancer treatment. “There are numerous technical milestones we need to demonstrate before this can be realized, so we are working hard to test the limits of our techniques,” Dover says.

–Kendra Redmond