Milestone: Miniature Particle Accelerator Works

A FAU research team has succeeded for the first time in accelerating electrons in a nanometer-sized structure

Particle accelerators are indispensable tools in many areas of industry, research and medicine. The space requirements of these machines range from a few square meters to large research centers. Using lasers to accelerate electrons within a photonic nanostructure represents a microscopic alternative that has the potential to significantly reduce costs and make devices significantly less bulky. So far, no significant energy gain has been proven. In other words, it could not be proven that the speed of the electrons actually increased significantly. A team of laser physicists from FAU has now succeeded in demonstrating the first nanophotonic electron accelerator - at the same time as colleagues from Stanford University. The FAU researchers have now published their results in the journal “Nature” .

When you hear “particle accelerator,” most people probably think of the Large Hadron Collider (LHC) in Geneva, the approximately 27-kilometer-long ring-shaped tunnel that researchers from all over the world use to study unknown elementary particles. However, such huge particle accelerators are the exception. We tend to encounter them in other places in everyday life, for example in medical imaging procedures or in the radiation treatment of tumors. But even then, these devices are several meters tall and still quite bulky, and their performance needs improvement. In order to improve and make existing devices smaller, physicists around the world are working on dielectric laser accelerators, also known as nanophotonic accelerators. The structures used are only 0.5 millimeters long, and the channel through which the electrons are accelerated is only about 225 nanometers wide, making these accelerators as small as a computer chip.

The particles are accelerated by ultra-short laser pulses, which illuminate the nanostructures. “The dream application would be to place a particle accelerator on an endoscope in order to be able to carry out radiation therapy directly on the affected area in the body,” explains Dr. Tomáš Chlouba, one of the four first authors of the recently published work. This dream may be for the FAU team from the Chair of Laser Physics under the leadership of Prof. Dr. Peter Hommelhoff, consisting of Dr. Tomas Chlouba, Dr. Roy Shiloh, Stefanie Kraus, Leon Brückner and Julian Litzel, are still a long way off, but with the demonstration of the nanophotonic electron accelerator they have now taken a decisive step in the right direction. “For the first time we can really talk about a particle accelerator on a chip,” enthuses Dr. Roy Shiloh.

Guided electrons + acceleration = particle accelerator
A little over two years ago, the team achieved their first major breakthrough: They managed to use the alternating phase focusing (APF) method from the early days of acceleration theory to control the flow of electrons in a vacuum channel over long distances (see press release from September 23, 2021 ). This was the first major step towards building a particle accelerator. Now all that was left was to accelerate in order to generate large amounts of energy. “With this technology, we have now succeeded not only in guiding electrons in these nanofabricated structures over a length of half a millimeter, but also in accelerating them,” explains Stefanie Kraus. What may not sound like a major achievement to many people is a major success for accelerator physics. “We achieved an energy of 12 kiloelectron volts. That’s an energy gain of 43 percent,” explains Leon Brückner. In order to accelerate the particles over such large distances (as seen from the nanoscale), the FAU physicists combined the APF method with specially developed columnar geometric structures.

However, this demonstration is just the beginning. The aim is now to increase the energy and electron current gain to such an extent that the particle accelerator on a chip is sufficient for medical applications. To do this, the energy gain would have to be increased by a factor of around 100. “In order to achieve higher electron currents at higher energies at the exit of the structure, we have to expand the structures or place several channels next to each other,” says Tomáš Chlouba, explaining the next steps of the FAU laser physicists.

Head-to-head race among physicists
What the Erlangen laser physics team achieved was demonstrated almost simultaneously by colleagues from Stanford University in the USA: their results are currently being checked, but can be viewed in a repository. The two teams are working together on the realization of the “accelerator on a chip” in a project funded by the Gordon and Betty Moore Foundation .

“In 2015, the ACHIP team led by FAU and Stanford had a vision for a revolutionary approach to particle accelerator design,” said Dr. Gary Greenburg of the Gordon and Betty Moore Foundation, “and we are pleased that our support has helped make this vision a reality.”