Synchrotron x-rays from a tabletop source

Caren B. Les, caren.les@photonics.com

A tabletop instrument in development may have the potential to produce bright, spatially coherent synchrotron x-rays similar in energy and quality to those generated by some of the largest x-ray facilities in the world. Such x-rays could enable imaging of complex systems on nanometer and femtosecond scales, accommodating a variety of applications not feasible with the larger-scale facilities.

“The uses of synchrotrons are many – the main application until now has probably been in crystallography – allowing us to determine the structure of chemicals as complex as simple viruses or as technologically important as semiconductors and magnetic materials,” said Dr. Zulfikar Najmudin, a reader at Imperial College London and leader of the investigative team, which includes members from the University of Michigan, Ann Arbor, and Instituto Superior Técnico in Lisbon, Portugal.

An x-ray radiograph of a resolution test target demonstrates that the x-rays produced with a tabletop laser-plasma interaction have a source size of 1 μm and peak brightness comparable to that of third-generation synchrotrons. Courtesy of Dr. Zulfikar Najmudin, Imperial College London.

X-ray techniques also are used to examine mechanical systems, medical samples, and artwork and archaeological artifacts – all applications that require transporting a sample to a large-scale synchrotron facility such as the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, or the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory in California. “Our compact source, though not quite as bright, could be implemented much more widely,” Najmudin said.

He and his team, whose report was published online in Nature Physics on Oct. 24, 2010, demonstrated the use of new laser-driven plasma accelerators to produce the intrinsically ultrafast beams of hard x-rays.

It has taken a couple of major advances to bring plasma accelerators to the forefront, he said. “Rapidly advancing laser technology has made the prospect of driving a plasma accelerator with lasers a real possibility.”

Lasers now can focus light to more than 10¹⁹ W/cm² (compared with 10³ for the brightest sunlight) and in extremely short laser pulses, with durations less than 30 fs (only about 10 oscillations of the laser). These short, very intense pulses are ideal for driving a plasma accelerator in a scheme called a laser wakefield.

Najmudin’s and other groups have accelerated electrons to GeV levels. “At this kind of energy, the electrons begin to emit x-rays strongly if they are accelerated transversely in what is usually called a wiggler, or undulator,” he said. “Hence, it became obvious that, with our accelerator, we should be able to witness strong radiation emission, too.”

Conceptually, the experiment is simple: “The intense laser beam passes through a [helium] gas puff, instantly ionizing it,” Najmudin explained. “Due to its enormous ‘light pressure,’ electrons are expelled away from the light pulse, whilst ions remain immobile, thus creating a large electric field due to this charge separation.

“The electric field can indeed be so large that it can pull some of the background electrons and accelerate them so much that they can stay in the accelerating phase of the accelerating structure – in much the same way a surfer with enough speed can be pulled along by an ocean wave. The fields are so huge that they can accelerate electrons up to the GeV level in merely a centimeter.”

Apart from the laser, the whole device is very small. The acceleration takes only a centimeter, and the chamber to house the components – including a focusing optic and a magnet to separate the electron and x-ray beams – is about 1 m in length.

“But it does need a state-of-the-art laser,” Najmudin said, “such as the Gemini laser at the Rutherford-Appleton Laboratory in the UK, where we have demonstrated ~GeV acceleration of electrons, or the Hercules laser at the University of Michigan, where the experiment documented in our article was performed.”

Any accelerating charge radiates electromagnetic radiation, he said. In the first synchrotron accelerators, this radiation was found to come off whenever the electron beam was accelerated around a corner by a magnet (to keep it going in a circle). As a result, all x-rays given off by electron beams in an accelerator are usually called synchrotron x-rays.

“If made to ‘wiggle’ transverse to their motion, the radiation is then reinforced by each wiggle and so is produced in a beam in the direction of motion of the particle,” he said. “This is how a typical synchrotron facility works – such as the ESRF or the ALS – using magnets in a device called an undulator to cause the electrons to wiggle and so emit x-rays.

“In plasma, this is further simplified because the wakefield, as well as having accelerating fields, also has focusing fields. Hence, electrons that move off the laser axis are pulled back, overshoot the center and then start into an oscillation around this axis.”

This means that the plasma acts as accelerator and wiggler all in one. And because of the huge fields of the device, the acceleration is greater and the oscillation wavelength significantly shorter, “meaning that the radiation can be in the x-ray regime for smaller electron energies,” Najmudin said.

“The small spatial size also means that the device will have high spatial resolution and exhibit spatial coherence, which allows the application of some advanced imaging techniques, such as phase contrast imaging, [and] x-ray imaging of finer details than typically possible in a medical x-ray,” he added.