A pocket-sized particle accelerator capable of driving ultra-short electron beams to velocities more than 99.99% of the speed of light has been demonstrated at STFC Daresbury Laboratory. In new research published in Nature Photonics, a collaborative team of academics based at the Cockcroft Institute, including researchers from the Femtosecond Lasers and Timing Group (ASTeC, STFC Daresbury Laboratory), have developed a unique solution using lasers to generate intense terahertz frequency pulses of light.
Terahertz (THz) is a region of the electromagnetic spectrum between infrared (used in TV remotes) and microwave (used in microwave ovens). Intense sources of terahertz radiation can be created by exciting a nonlinear crystal with a high power femtosecond laser. Laser-driven sources in excess of 1 GV/m are now possible, which is more than an order of magnitude larger than capable in a 'traditional' RF accelerating cavity. This new technology allows particles to be excited to higher energies over shorter distances, therefore reducing both the footprint and cost of a particle accelerator facility.
Laser-generated THz radiation exists in the ideal millimetre-scale wavelength regime, allowing for compact accelerating structures to be easily built. More importantly, the scale of these structures is well-suited to completing capturing and accelerating ultrashort bunches with high charge. This crucially allows terahertz radiation to both measure and manipulate particle bunches on time scales of less than 10 femtoseconds (0.000 000 000 000 01 seconds, or the time is takes light to travel 1/100th of a millimetre). In the future, terahertz driven particle accelerators could be used to track atomic motion in samples and construct so-called molecular movies for materials characterisation.
Lead author on the paper Dr Morgan Hibberd from The University of Manchester said: “The main challenge was matching the velocity of the accelerating THz field to the almost speed-of-light electron beam velocity, while also preventing the inherently lower velocity of the THz pulse envelope propagating through our accelerating structure from significantly degrading the length over which the driving field and electrons interact."
“We overcame this problem by developing a unique THz source which produced longer pulses containing only a narrow range of frequencies, significantly enhancing the interaction. Our next milestone is to demonstrate even higher energy gains while maintaining beam quality. We anticipate this will be realised through refinements to increase our THz source energy, which are already underway."
Professor Steven Jamison of Lancaster University who jointly leads the programme, explained: “The controlled acceleration of relativistic beams with terahertz frequency laser-like pulses is a milestone in development of a new approach to particle accelerators. In using electromagnetic frequencies over one hundred times higher than in conventional particle accelerators, a revolutionary advance in the control of the particle beams at femtosecond time scales becomes possible."
This research was part of an ongoing user access programme on the CLARA accelerator at STFC Daresbury Laboratory, and is a successful example of how the accelerator can generate impact for both UK and international collaborators. In the future, CLARA will be upgraded to 250 MeV and include a dedicated beam line for users, allowing new and exciting experiments to be performed that were previously not possible.