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."

New Tabletop Instrument Tests Electron Mobility for Next Generation Electronics

From the Journal: 

Applied Physics Letters

By Catherine Meyers
https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=5454029727541311093

The table-top sized terahertz cyclotron resonance spectrometer
CREDIT: S. Hammersley, The University of Manchester

WASHINGTON, D.C., May 24, 2016 -- The National High Magnetic Field Laboratory, with facilities in Florida and New Mexico, offers scientists access to enormous machines that create record-setting magnetic fields. The strong magnetic fields help researchers probe the fundamental structure of materials to better understand and manipulate their properties. Yet large-scale facilities like the MagLab are scarce, and scientists must compete with others for valuable time on the machines.

Now researchers from the United Kingdom, in collaboration with industry partners from Germany, have built a tabletop instrument that can perform measurements that were only previously possible at large national magnet labs. The measurements will help in the development of next generation electronic devices employing 2-D materials, said Ben Spencer, a post-doctoral research associate working in Darren Graham’s group at the University of Manchester’s Photon Science Institute, who helped develop the new instrument.

The researchers describe their work in a paper in the journal Applied Physics Letters, from AIP Publishing.

Since the 1950s, experiments conducted with magnetic fields have played a pivotal role in the development of semiconductors devices -- like transistors and light-emitting diodes -- that have changed the world.

One magnetic field technique is called cyclotron resonance. In a magnetic field, the charged particles in a material start to move in circles around the magnetic field lines. The orbiting particles interact with light differently depending on properties like their mass, concentration, and on how easily they move through the material. By shining light on the material in the magnetic field and recording what frequency and how much light is absorbed, scientists can learn important information about how easily charged particles move, a critical property in electronic devices.

One of the main obstacles to wide-spread use of cyclotron resonance is that some materials require an extremely high magnetic field to get the charged particles to move fast enough to interact with the light.

Recently researchers created a small, high-powered magnet that can generate fields of around 30 Tesla, about 600,000 times stronger than the Earth’s magnetic field and 20 times stronger than the MRI scanners typically used in hospitals.

The new magnet is compact enough for a tabletop machine, yet the magnet can only generate a field in short pulses that each last for a fleeting one-hundredth of a second.

“The challenge in doing cyclotron resonance with these pulsed magnets is being able to record your data within the brief time period that the magnet is on,” Spencer said. “The breakthrough we have made is in the measurement technique.”

The magnetic coil at the heart of the system was developed by Prof. Hiroyuki Nojiri at the Institute of Materials Research, Tohoku University (Japan). It produces a pulsed magnetic field of up to 30 Tesla
CREDIT: S. Hammersley, The University of Manchester

Spencer and his colleagues used an approach called an asynchronous optical sampling technique to increase the number of measurements during one pulse to around 100. Previous experiments with a similar magnet system were limited to four measurements per pulse.

The team worked with researchers from Laser Quantum, a laser manufacturer, to incorporate ultrafast lasers into the new instrument. The “Taccor” lasers they used run at repetition rates of 1 billion cycles per second, more than 10 times higher than the typical repetition rates for ultrafast laser systems, Spencer said. The fast laser allowed data acquisition times on the order of one ten-thousandth of a second, which meant up to a hundred measurements could be taken during the transient magnet pulse.

“It is this leap forward that will now enable routine cyclotron resonance measurements on a tabletop in a laboratory environment,” Spencer said.

The team tested their system by measuring the properties of electrons at the interface of the two semiconductors AlGaN and GaN. Such interfaces could form an important part of new, energy-saving transistors.

“This work is feeding into a programme of work at Cambridge University on developing AlGaN/GaN-based high electron mobility transistors. These promise much lower power consumption than current devices, which will ultimately lead to energy savings in a wide range of consumer electronic devices. We are also starting to investigate a range of other two-dimensional materials using this instrument, including the new wonder 2-D material graphene,” Graham said.

Ultimately, the team hopes their new instrument could facilitate rapid progress in many areas of semiconductor device development. The system can be easily moved to different universities, and it makes it easy to think of a measurement, and simply perform it the next day, without having to apply for time at a national magnet facility, the researchers say.

“We’re sure that when people realise that we can do such measurements in the lab they will be lining up to use our instrument. We’ve already been contacted by several groups interested in having measurements made on their samples,” Graham said.

The high-speed ASOPS setup including the two turn-key 1 GHz Taccor lasers from Laser Quantum GmbH.
CREDIT: Laser Quantum GmbH

Funding: This work was supported by the United Kingdom’s Engineering and Physical Sciences Research Council.

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Article title: 

Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an ...

Authors: 

B. F. Spencer, W. F. Smith, M. T. Hibberd, P. Dawson, M. Beck, A. Bartels, I. Guiney, C. J. Humphreys and D. M. Graham

Author affiliations: 

University of Manchester, Laser Quantum GmbH, and the University of Cambridge