Tabletop synchrotron: high-energy, coherent X-rays

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  • Published: Nov 1, 2010
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Tabletop synchrotron: high-energy, coherent X-rays

X-ray revolution on its way

Details of a tabletop synchrotron device has been revealed by an international team of scientists in the journal Nature Physics. The new device could revolutionise X-ray work and preclude the need for large-scale synchrotrons in many structural studies without compromising resolution or atomic detail.

Researchers from Imperial College London, the University of Michigan and Instituto Superior Téchnico Lisbon, Portugal have described a tabletop instrument that produces synchrotron X-rays, whose energy and quality rivals that produced by some of the largest facilities in the world. The new device could produce a tightly focused beam of high energy X-rays for examining everything from complex protein structures to the integrity of aircraft wings. 


High energy

The development of high-energy light sources to probe the details of a wide range of materials for research and commercial purposes has moved forward rapidly in recent years. The Diamond Light Source synchrotron facility in Didcot, England, for instance is providing scientists with high-resolution data. However, high-power, high-quality synchrotron X-ray sources are very large scientific installations and costly to build and maintain. Diamond, for instance, has a 500 metre circumference and cost GBP263 million to construct. Nevertheless, the appetite of scientific and medical research for yet higher precision instrumentation is a major driver towards developing synchrotrons still further, miniaturisation is just one aspect of that drive.

The international team has now demonstrated that they can replicate much of what large-scale synchrotrons do, but on a laboratory scale. Their system uses a tiny jet of helium gas and a high power laser to produce an ultrashort pencil-thin beam of high-energy and spatially coherent X-rays as demonstrated using Fresnel diffraction.


Crystallisation woes

"This is a very exciting development," explains Imperial College's Stefan Kneip, lead author on the study published in Nature Physics. "We have taken the first steps to making it much easier and cheaper to produce very high energy, high quality X-rays. Extraordinarily, the inherent properties of our relatively simple system generates, in a few millimetres, a high-quality X-ray beam that rivals beams produced from synchrotron sources that are hundreds of metres long. Although our technique will not now directly compete with the few large X-ray sources around the world, for some applications it will enable important measurements which have not been possible until now."

The benchtop X-ray system produces an extremely short pulse length with a point of origin just 1 micrometre across, and a very narrow beam is available to researchers. The ultra-short pulse time could allow researchers to measure atomic and molecular interactions occurring on the femtosecond timescale, for instance.

Team leader Zulfikar Najmudin explains: "We think a system like ours could have many uses. For example, it could eventually increase dramatically the resolution of medical imaging systems using high energy X-rays, as well as enable microscopic cracks in aircraft engines to be observed more easily. It could also be developed for specific scientific applications where the ultrashort pulse of these X-rays could be used by researchers to 'freeze' motion on unprecedentedly short timescales."

In order to develop the new system, the team carried out a relatively straightforward experiment at the Center for Ultrafast Optical Science at the University of Michigan using state-of-the-art laser facilities. They shone a very high power laser beam, HERCULES, into a jet of helium gas held in a vacuum chamber, which produces a tiny column of ionised helium plasma. In this plasma wake field, the laser pulse creates an inner bubble of positively charged helium ions surrounded by a sheath of negatively charged electrons.

This charge separation has the effect of creating a powerful electric field, a thousand times or more higher than conventional particle acceleration is capable of, which accelerates electrons within the plasma up to the gigaelectronvolt range forming an energetic beam that wiggles. The wiggling electrons produce a highly collimated co-propagating X-ray beam which the team measured in these experiments as proof of principle. The process is equivalent to synchrotron X-ray production but happens on the microscopic scale.

Previously, external 'wigglers' have been used to wiggle the electron beams produced by wake fields and so generate synchrotron radiation at visible or near visible wavelengths and modest brightness. The new X-ray source uses the wiggles, more formally betatron oscillations, of the electron beam to generate synchrotron X-rays that have energies of 1-100keV, which are a thousand times brighter than previous laser driven X-ray sources.

The team has now for the first time described the technical characteristics of the beam and presented test images that demonstrate its performance. "Our technique can now be used to produce detailed X-ray images," adds Najmudin. "We are currently developing our equipment and our understanding of the generation mechanisms so that we can increase the repetition rate of this X-ray source. High power lasers are currently quite difficult to use and expensive, which means we're not yet at a stage when we could make a cheap new X-ray system widely available. However, laser technology is advancing rapidly, so we are optimistic that in a few years there will be reliable and easy to use X-ray sources available that exploit our findings".

"The entire system currently fits into two or three rooms of a university lab," Kneip told SpectroscopyNOW. "The laser takes most of that space, the interaction itself is just a few millimetres in diameter. With the focusing optics of the laser, the interaction chamber is about 1 cubic metre." He added that, "Laser technology progresses rapidly and within a few years, the entire system should fit into a single room. I think this would be a space requirement that would make our system attractive for a lot of application environments. Progress in laser technology will not only help to shrink this down further and further, but also improve beam parameters such as repetition rate, average brightness and stability."

 



The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Credit: Kneip et al/Nature - Details of a tabletop synchrotron device has been revealed by an international team of scientists in the journal Nature Physics. The new device could revolutionise X-ray work and preclude the need for large-scale synchrotrons in many structural studies without compromising resolution or atomic detail.
The spectrally integrated X-ray beam profile shows an elliptically elongated beam profile in the direction of laser polarization.

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