How efficient is a particle accelerator

"Strong waves from proton bundles"

At the CERN research center in Geneva, physicists do more than just study the building blocks of matter. You are also researching alternative concepts for particle accelerators. After four years of development, the scientists of the AWAKE experiment are now reporting a breakthrough in the journal “Nature”: For the first time, they are setting a proton-powered keel field accelerator in motion. In the first test runs, electrons reached an energy of two gigaelectron volts over a distance of just ten meters. In an interview, team member Matthew Wing from University College London explains how this new type of particle accelerator works and what exciting possibilities it offers.

World of physics: You are working on a new type of accelerator concept. Can you briefly explain the principle behind it?

Matthew Wing: We shoot high-energy proton pulses through a plasma - a mixture of positively charged atomic nuclei and electrically negative electrons. Similar to ships, this creates a kind of "keel wave" in which enormous electrical voltages prevail. These voltages are much higher than with conventional accelerators such as the LHC. If electrons are now fed into the keel, they can "surf" on the wave and are thus efficiently accelerated: over a distance of just ten meters, electrons can be brought to an energy of two gigaelectron volts.

Matthew Wing

There are already keel field accelerators that can be driven with laser pulses or electron beams. Why do you use protons for this?

Our results show for the first time that protons can be used effectively for the acceleration process. The decisive factor: Proton packets can store much higher energy than laser or electron pulses. Even high-energy laser pulses typically only have an energy of one joule. A packet of protons at our accelerator carries around four orders of magnitude more energy, around 19 kilojoules. The accelerator we are working on is the last pre-accelerator of the large LHC storage ring, where the Higgs particle was discovered, among other things.

What are the advantages?

Thanks to the higher energy, the proton bundle can travel comparatively long distances in the plasma and generate a particularly strong keel wave. Laser or electron pulses, on the other hand, can only cover distances of around one meter in the plasma before the wake effect subsides. In principle, protons are even suitable for distances of up to one kilometer and even more. This would make it possible to build significantly more compact and cheaper particle accelerators than is possible with conventional technology.

What applications do you anticipate for this new concept? There are many more laser sources than proton accelerators in the world.

In principle, keel accelerators are suitable for a whole range of applications - for example in medicine, in radiation therapy. You can also use the electron beam to drive a free-electron laser that provides high-quality X-rays for research into atomic processes. Our collaboration here at CERN is of course most focused on particle physics, even if some of our findings can also be used in other areas. For example, we are planning an experiment in which we want to use the generated high-energy electron beam to search for dark matter in a targeted manner. One could also let the electron beam collide with protons that are circling in the LHC. The most interesting thing for particle physicists would be a storage ring in which electrons and protons collide in short pulses in rapid succession. But that is still a long way off.

What are the plans for the near future?

We want to use the time until November to take as much data as possible and to characterize the electron beam - which emerges from the plasma at the back - more precisely. Because so far we only know its energy. We are currently working on measuring the parameters of the generated beam. To do this, we have to determine, among other things, the number of accelerated electrons. We have made good progress with the stability and reproducibility of the beam. At the end of the year, a two-year break in operations begins at CERN, during which various components are serviced and experiments rebuilt. From 2021 we want to try to generate high-quality electron beams. If all goes well, we will be able to carry out the first scientific experiments with our accelerator from the second half of the next decade.