As particle accelerators get larger, and particle physicists seek ever-higher energies for their particle beams, an international team of researchers is experimenting with a radical new design for future accelerators, and it involves a laser!
The latest results published by the AWAKE collaboration at CERN are demonstrating that this ‘proof of concept’ experiment is on track.
AWAKE injects a "drive" bunch of protons from CERN’s SPS accelerator into a plasma column created by ionising a gas with a laser. As the particles travel through the plasma, they generate a type of wave called a wakefield that rapidly picks up energy before a second beam of electrons is accelerated by the wakefield.
Plasma wakefield research is focused on the capacity to reach high accelerator energies or ‘gradients’ over short distances. The accelerator gradient for conventional accelerators is limited to around 100 MeV (million electronvolts) per metre. Results from last year proved that electrons injected into AWAKE at relatively low energies of around 19 MeV, “rode” the plasma wave, and were accelerated by a factor of around 100, to an energy of 2 GeV (billion electronvolts) over a distance of only 10 metres.
The use of plasma to accelerate particles is a potential alternative to traditional accelerating methods that rely on radiofrequency electromagnetic cavities. It has long been known that plasmas are capable of supporting very strong electric fields. The challenge for researchers is to understand the best way to take advantage of this capability to create ‘same size’ particle accelerators with much higher energies, or much smaller (and cheaper) accelerators with the same energy than we have today. To put this into context, with existing technology a linear collider generating TeV energies would be 30-50km long. Using proton-driven plasma wakefield techniques, the length would be just a few kilometres.
“There are obvious economic advantages,” says Matthew Wing (UCL), Deputy Spokesperson for the AWAKE collaboration. “It is not just the civil engineering costs of building a linear collider experiment, but the reduced amount of instrumentation that would be required.”
The technology could also be used to create tabletop accelerators for medical applications, but this is very much in the future -the first stage is to demonstrate that it works.
AWAKE’s most recent results have been published in the journal Physical Review Letters, at the editor’s suggestion – reflecting the interest of the physics community in this experiment.
The first paper describes how the collaboration has innovative imaging techniques to watch how the initial injection of protons splits itself into bunches.
The second paper reports on what happens to the protons when they ‘feel’ the laser pulse.
While the CERN accelerator complex takes a two-year break for maintenance and upgrade, AWAKE researchers will continue to analyse the data that they’ve collected, and prepare for the return of proton beams in 2021.
Last updated: 22 March 2019