Scientists create compact particle accelerators that drive electron beams nearer speed of light

 Ben Robinson, University of Manchester

https://phys.org/news/2020-08-scientists-compact-particle-electron-nearer.html

Scientists have successfully developed a pocket-sized particle accelerator capable of projecting ultra-short electron beams with laser light at more than 99.99% of the speed of light.

"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 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 to a 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 radio-therapy and in materials characterisation.

Dr. Darren Graham, Senior Lecturer in Physics at The University of Manchester said: "Achieving this milestone would not have been possible without the unique collaborative environment provided by the Cockcroft Institute, which has helped bring together scientists and engineers from University of Lancaster, The University of Manchester and the staff from STFC at Daresbury Laboratory."

To achieve this result, the researchers have had to slow down light to match the speed of the electrons using a specially designed metallic structure lined with quartz layers thinner than a human hair.

This huge 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 is takes light to travel 1/100th of a millimeter). This will enable them to create strobe photographs of atomic motion.

This successful demonstration paves the way to the development of high-energy, high-charge, high-quality Terahertz (THz) driven accelerators, which promise to be cheaper and more compact. Reducing the size and cost of accelerator technology, will open up these incredible machines to a much wider range of applications.

Particle accelerators are widespread with applications in basic research in particle physics, materials characterisation, 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 today 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, 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.

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."

Laser trick produces high-energy terahertz pulses

From the color difference of two slightly delayed laser flashes (left) a non-linear crystal generates an energetic terahertz pulse (right).CREDIT DESY, Lucid Berlin

https://www.eurekalert.org/pub_releases/2019-06/ded-ltp061319.php

A team of scientists from DESY and the University of Hamburg has achieved an important milestone in the quest for a new type of compact particle accelerator. Using ultra-powerful pulses of laser light, they were able to produce particularly high-energy flashes of radiation in the terahertz range having a sharply defined wavelength (colour). Terahertz radiation is to open the way for a new generation of compact particle accelerators that will find room on a lab bench. The team headed by Andreas Maier and Franz Kärtner from the Hamburg Center for Free-Electron Laser Science (CFEL) is presenting its findings in the journal Nature Communications. CFEL is jointly run by DESY, the University of Hamburg and the Max Planck Society.

The terahertz range of electromagnetic radiation lies between the infrared and microwave frequencies. Air travellers may be familiar with terahertz radiation from the full-body scanners used by airport security to search for objects hidden beneath a person's garments. However, radiation in this frequency range might also be used to build compact particle accelerators. "The wavelength of terahertz radiation is about a thousand times shorter than the radio waves that are currently used to accelerate particles," says Kärtner, who is a lead scientist at DESY. "This means that the components of the accelerator can also be built to be around a thousand times smaller." The generation of high-energy terahertz pulses is therefore also an important step for the AXSIS (frontiers in Attosecond X-ray Science: Imaging and Spectroscopy) project at CFEL, funded by the European Research Council (ERC), which aims to open up completely new applications with compact terahertz particle accelerators.

However, chivvying along an appreciable number of particles calls for powerful pulses of terahertz radiation having a sharply defined wavelength. This is precisely what the team has now managed to create. "In order to generate terahertz pulses, we fire two powerful pulses of laser light into a so-called non-linear crystal, with a minimal time delay between the two," explains Maier from the University of Hamburg. The two laser pulses have a kind of colour gradient, meaning that the colour at the front of the pulse is different from that at the back. The slight time shift between the two pulses therefore leads to a slight difference in colour. "This difference lies precisely in the terahertz range," says Maier. "The crystal converts the difference in colour into a terahertz pulse."

The method requires the two laser pulses to be precisely synchronised. The scientists achieve this by splitting a single pulse into two parts and sending one of them on a short detour so that it is slightly delayed before the two pulses are eventually superimposed again. However, the colour gradient along the pulses is not constant, in other words the colour does not change uniformly along the length of the pulse. Instead, the colour changes slowly at first, and then more and more quickly, producing a curved outline. As a result, the colour difference between the two staggered pulses is not constant. The difference is only appropriate for producing terahertz radiation over a narrow stretch of the pulse.

"That was a big obstacle towards creating high-energy terahertz pulses," as Maier reports. "Because straightening the colour gradient of the pulses, which would have been the obvious solution, is not easy to do in practice." It was co-author Nicholas Matlis who came up with the crucial idea: he suggested that the colour profile of just one of the two partial pulses should be stretched slightly along the time axis. While this still does not alter the degree with which the colour changes along the pulse, the colour difference with respect to the other partial pulse now remains constant at all times. "The changes that need to be made to one of the pulses are minimal and surprisingly easy to achieve: all that was necessary was to insert a short length of a special glass into the beam," reports Maier. "All of a sudden, the terahertz signal became stronger by a factor of 13." In addition, the scientists used a particularly large non-linear crystal to produce the terahertz radiation, specially made for them by the Japanese Institute for Molecular Science in Okazaki.

"By combining these two measures, we were able to produce terahertz pulses with an energy of 0.6 millijoules, which is a record for this technique and more than ten times higher than any terahertz pulse of sharply defined wavelength that has previously been generated by optical means," says Kärtner. "Our work demonstrates that it is possible to produce sufficiently powerful terahertz pulses with sharply defined wavelengths in order to operate compact particle accelerators."

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DESY is one of the world's leading particle accelerator centres. Researchers use the large?scale facilities at DESY to explore the microcosm in all its variety - ranging from the interaction of tiny elementary particles to the behaviour of innovative nanomaterials and the vital processes that take place between biomolecules to the great mysteries of the universe. The accelerators and detectors that DESY develops and builds at its locations in Hamburg and Zeuthen are unique research tools. DESY is a member of the Helmholtz Association, and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent).

Reference:

Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation; Spencer W. Jolly, Nicholas H. Matlis, Frederike Ahr, Vincent Leroux, Timo Eichner, Anne-Laure Calendron, Hideki Ishizuki, Takunori Taira, Franz X. Kärtner, and Andreas R. Maier; Nature Communications, 2019; DOI: 10.1038/s41467-019-10657-4