Caren B. Les, email@example.com
LONDON – 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,”
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
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.