Laser research lights way to mini particle accelerators

New research could help cut the size of powerful particle accelerators from kilometres to metres in length, paving the way for their wider use in science, energy and medicine.

An international group of scientists is hoping that a powerful new type of accelerator, in which particles ‘surf’ in the wake of pulses of energy created by optical fibre lasers, could be small enough to fit within a typical lab – and they are drawing up plans of how to build one.

Such small accelerators could be used to conduct high-level scientific research with cheaper facilities than is currently possible. The Large Hadron Collider (LHC) used to find the particle that appears to be the Higgs boson, for example, sits in a 27km-long circular tunnel.

The proposed technology could also produce more practical equipment for generating proton beams for cancer treatment, or for running accelerator-driven nuclear reactors that would use existing nuclear waste or the abundant element thorium for fuel.

‘The problem with accelerators at the moment is that to accelerate electrons you need a big electric field,’ explained Dr Bill Brocklesby, a researcher at the Optoelectronics Research Centre (ORC) at Southampton University, which is part of the EU-funded International Coherent Amplification Network (ICAN) running the project.

‘RF accelerators based on radio frequency electric fields are limited to how big those fields can be before you get breakdown (sparks). That means if you can only have a certain amount of electric field then to get the electrons to higher energies you need to make then go further, and that’s why the LHC is 27km long.’

The accelerator proposed by ICAN would use a different principle known as laser wakefield acceleration whereby a laser pulse is fired into a small tube of gas or plasma to create a wake of electric potential that can accelerate particles to very high speeds over very short distances.

Fibre optics could be used to create lasers with higher peak and average powers.

To produce the short, intense pulses at sufficient frequencies and efficiencies, the ICAN team want to use thousands of optical fibres to split, amplify and then recombine the laser beam, giving it both the high average and high peak power.

The problem with existing laser accelerators, such as the experimental 40-joule BELLA at the University of California, Berkeley, is that they can only fire the short, intense pulses needed at a rate of one per second (1Hz) – not fast enough for practical scientific use. They are also very inefficient.

The ICAN team believe that optical fibre lasers could be the answer. As well as working at up to 90 per cent efficiency, these very long, thin lasers have a stable glass structure and high surface area that makes them much better at heat management, and so better at coping with the high average power needed.

But the fibres’ size also limits the peak power they can handle without the pulse distorting. This is where the idea comes in of splitting the laser beam into thousands of fibre channels, amplifying the signals and then adjusting their phases so they can be precisely recombined.

The ICAN project will require thousands of identical fibre lasers.

The ICAN group has nearly concluded a design and manufacture feasibility study for a demonstration laser, for which Southampton has provided the expertise in fibre manufacturing, and is hoping to secure funding to continue the research.

‘The next project will be to put together a laser system, something like a 10J pulse at 10kHz as a demonstration, with something like 1000 fibres,’ said Brocklesby. ‘That will be orders of magnitude better than you can do with conventional laser technology.’

This would put the team some way towards achieving the goal of a 30J laser operating at 13khz, set by the International Committee for Future Accelerators (ICFA) and the International Committee for Ultrahigh Intensity Lasers (ICUIL).

‘One of the big engineering challenges is making the fibre in such a way that we can produce 1000 lengths of maybe a metre long, which all have exactly the same properties in terms of the laser gain and things like this,’ said Brocklesby.

The other advantage of using optical fibre lasers is that the telecoms industry has already established much of the expertise and technology needed to build these devices, he added.

ICAN is made up of 14 laboratories around the world including the LHC operator CERN (the European Organisation for Nuclear Research) and Friedrich-Schiller University in Jena, Germany, and coordinated by École polytechnique in France.