Physicists in the UK and Germany have created a powerful yet highly compact X-ray source, which they claim could come to replace some of the world’s major research facilities.
X-ray beams have become a valuable tool for scientists because they can “see” deep inside matter, illuminating its internal structure at the atomic scale. Indeed, the technique is now applied across a wide range of science, from revealing the structure of viruses to tracking chemical reactions as they happen.
As scientists have begun to realize the full potential of X-rays, they have come to require much more advanced X-ray sources than those available at their academic institutions. This has led to the formation of a number of specialist facilities that provide the international research community with a centralized source of high-quality X-rays.
Shake it like a synchrotron
These centres, like ESRF in France and Diamond Light Source in the UK, generate X-rays as a form of “synchrotron” radiation. In the standard process, high-energy beams of charged particles are accelerated using electric and magnetic fields around loops that can be hundreds of metres in size. As the particles veer around the circle, they constantly shed energy in the form of X-rays tangential to the beamline.
In most modern synchrotrons, the particles are also passed through a periodic magnetic structure, known as an undulator, which forces the electrons to “wiggle” and emit X-rays each they change direction.
While these synchrotron facilities have proved very popular and provide researchers with an opportunity to mix with their international peers, they are not without their critics. Some feel that the high set-up and running costs are unnecessary, and it is notoriously difficult to provide all research applicants with sufficient time on the beam line. Moreover, the large size of the accelerators and the air-miles accumulated by all the international visits means that these facilities have a number of sustainability issues.
Determined to overcome these problems, Stefan Karsch of the Max Planck Institute for Quantum Optics and his colleagues have realized an alternative method for generating X-rays. They have developed a technique that has gained recognition in the last few years known as “laser wakefield acceleration”.
Ride the wave
The physicists begin by firing a 37 fs laser pulse at a cell of hydrogen atoms, energizing the electrons and causing some of them to try to break free. However, the positive attraction of the nucleus acts to cling onto the electrons and they end up oscillating back-and-forth about the nucleus – resulting in a form of plasma wave in the cell. Some of the other electrons then “ride” down this wave at relativistic speeds, generating X-rays as they change direction.
Laser wakefield acceleration has been demonstrated previously but never before to generate soft X-rays – that is, electromagnetic radiation that can resolve the structure of matter on the atomic scale. One of the important developments in this latest research was to introduce a miniature version of the undulator present in synchrotrons. The combination of a 1.5 cm accelerator and a 30 cm magnetic undulator enabled the physicists to accelerate electrons up to energies of 210 MeV.
The energy of these electrons is comparable with those in synchrotron facilities, producing X-rays with the same brilliance, but the generator is 10,000 times more compact. “In the long run we aim to replace large, costly synchrotron and linear accelerator facilities with something small and affordable, namely a high-power laser-driven plasma accelerator of cm-size,” Karsch told physicsworld.com.
The researchers now intend to develop their technique to generate X-rays with an even higher brilliance than those in synchrotrons. “Through an ongoing laser upgrade, we first aim at increasing the electron energy to 1–1.5 GeV to reach keV photon energies, which could serve as a first ultrashort X-ray source for application experiments on solids,” he said.