Early in August, scientists at the UK University of Manchester successfully developed a pocket-sized particle accelerator capable of projecting ultrashort electron beams with laser light at more than 99.99% of the speed of light. To achieve this result, the researchers had to slow light to match electron speeds using a specially designed metallic structure lined with quartz layers thinner than a human hair. This leap forward simultaneously offers the ability 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 it takes light to travel 1/100th of a millimeter), enabling researchers to create strobe photographs of atomic motion. It also paves the way for the development of high energy, high charge, high quality Terahertz (THz) driven accelerators, which promise to be cheaper and more compact—opening up the technology to a much wider range of applications.
Particle accelerator use is widespread with applications in basic particle physics research, materials characterization, radiotherapy in hospitals where they are used to treat cancer patients, radioisotope production for medical imaging and security screening of cargo. The basic technology (radio-frequency oscillators) underpinning these machines, however, was developed for radar during the Second World War.
In new research published August 12 in Nature Photonics, a collaborative team of academics show that their unique solution is to use lasers to generate terahertz frequency pulses of light. Terahertz is a region of the electromagnetic spectrum between infrared (used in TV remotes) and microwave (used in microwave ovens). Laser-generated THz radiation exists in the ideal millimeter-scale wavelength regime, not only making structure fabrication simpler but (most importantly) providing the half-cycle lengths that are well suited for acceleration of whole electron bunches with high levels of charge.
The University of Manchester’s Dr. Morgan Hibberd, lead author on the paper, 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 realized through refinements to increase our THz source energy, which are already underway.”
Professor Steven Jamison of Lancaster University, who jointly leads the program, explained, “The controlled acceleration of relativistic beams with terahertz frequency laser-like pulses is a milestone in the 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. With our demonstration of terahertz acceleration of particles traveling at 99.99% of the speed of light, we have confirmed a route to scaling terahertz acceleration to highly relativistic energies.”
While the researchers have an eye on the long term role of their concepts in replacing multi-kilometer scale research accelerators (such as Europe’s 3-km long X-ray-source in Hamburg) with devices mere meters in length, they expect the immediate impacts will be in the fields of radiotherapy and materials characterization.
Dr. Darren Graham, senior lecturer in physics at The University of Manchester said, “Achieving this milestone would not have been possible without the uniquely collaborative environment provided by the Cockcroft Institute, which has helped bring together scientists and engineers from the University of Lancaster, The University of Manchester and the staff from STFC at Daresbury Laboratory”.