Team rivals synchotron with table-top device

Synchotron X-ray facilities are usually big and expensive. For example, the UK’s Diamond Light Source synchrotron facility is half a kilometer in circumference and cost £263 million to build.

But researchers from Imperial College London, the University of Michigan and Instituto Superior Téchnico Lisbon have developed a tabletop instrument producing synchrotron X-rays of an energy and quality that rival some of the largest X-ray facilities in the world.

It 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.

“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 meters long,” says Dr Stefan Kneip of Imperial.

“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 X-rays produced from the new system have an extremely short pulse length.  They also originate from a point about one micron across, creating a narrow X-ray beam that allows researchers to see fine details in their samples.  

“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,” says Imperial’s Dr Zulfikar Najmudin.

“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.”

The team shone the very high power laser beam, named Hercules, into a jet of helium gas to create a tiny column of ionised helium plasma.  In this plasma, the laser pulse creates an inner bubble of positively charged helium ions surrounded by a sheath of negatively charged electrons.

Due to this charge separation, the plasma bubble has powerful electric fields that both accelerate some of the electrons in the plasma to form an energetic beam and also make the beam ‘wiggle’. 

As the electron beam wiggles it produces a highly collimated co-propagating X-ray beam.

The process is similar to what happens in other synchrotron sources, but on a microscopic scale. The acceleration and X-ray production happens over less than a centimetre, with the whole tabletop X-ray source housed in a vacuum chamber about a meter on each side.

This miniaturisation leads to a potentially much cheaper source of high quality X-rays, and also results in their unique properties.

“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,” says Najmudin.

“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.”