source:science
For 2 decades, physicists have strived to miniaturize particle accelerators—the huge machines that serve as atom smashers and x-ray sources. That effort just took a big step, as physicists in China used a small “plasma wakefield accelerator” to power a type of laser called a free-electron laser (FEL). The 12-meter-long FEL isn’t nearly as good as its kilometers-long predecessors. Still, other researchers say the experiment marks a major advance in mini accelerators.
“A lot of [scientists] will be looking at this like, ‘Yeah, that’s very impressive!’” says Jeroen van Tilborg, a laser-plasma physicist at the Lawrence Berkeley National Laboratory who was not involved in the work. Ke Feng, a physicist at the Shanghai Institute of Optics and Fine Mechanics (SIOM) who worked on the new FEL, isn’t claiming it’s ready for applications. “Making such devices useful and miniature is always our goal,” Feng says, “but there is still a lot of work to do.”
Particle accelerators are workhorses in myriad fields of science, blasting out fundamental particles and generating intense beams of x-rays for studies of biomolecules and materials. Such accelerators stretch kilometers in length and cost $1 billion or more. That’s because within a conventional accelerator, charged particles such as electrons can gain energy only so quickly. Grouped in compact bunches, the particles zip through a vacuum pipe and pass through cavities that resonate with microwaves. Much as an ocean wave propels a surfer, these microwaves push the electrons and increase their energy. However, if the oscillating electric field in the microwaves grows too strong, it will set off damaging sparks. So, the particles can gain a maximum of about 100 megaelectron volts (MeV) of energy per meter of cavity.
To accelerate particles in shorter distances, physicists need stronger electric fields. Firing a pulse of laser light into a gas such as helium is one way to generate them. The light rips electrons from the atoms, creating a tsunami of ionization that moves through the gas, followed by a wake of rippling electrons that produces an extremely strong electric field. That wakefield can scoop up electrons and accelerate them to 1000 MeV in just a few centimeters.
Physicists hoping to harness wakefields have shown they can generate very short, intense bursts of electrons. But within a burst, the energies of those electrons typically vary by a few percent, too much for most practical applications. Now, SIOM physicist Wentao Wang, Feng, and colleagues have improved the output of their plasma wakefield accelerator enough to do something potentially useful with it: power an FEL.
In an FEL, physicists fire electrons down a vacuum pipe and through a line devices called undulators. Within an undulator, small magnets above and below the beam pipe lined up like teeth, with the north poles of neighboring magnets alternating up and down. As electrons pass through the undulators, the rippled magnetic field shakes them back and forth, causing them to emit light. As the light builds up and travels along with the bunch of electrons, it pushes back on the electrons and separates them into sub-bunches that then radiate in concert to amplify the light into a laser beam.
The world’s first x-ray laser, the Linac Coherent Light Source (LCLS) unveiled in 2009 at SLAC National Accelerator Laboratory, is an FEL powered by the lab’s famous 3-kilometer long linear accelerator. Researchers in Europe and Japan have also built large x-ray FELs. But by shooting the electron beam from their plasma wakefield accelerator through a chain of three 1.5-meter-long undulators, the SIOM team has made an FEL small enough to fit into a long room.
To make that possible, SIOM physicists had to shrink the spread in the electrons’ energy to 0.5%. They succeeded by optimizing the laser and the gas target to better control the electrons’ acceleration send them more smoothly down the vacuum pipe, Wang says. Teams in the United States and Europe have explored more complex schemes for filtering out electrons of a specific energy, but the SIOM team took a simpler approach, van Tilborg says. “Everything is just a little better optimized,” he says.
Others had used plasma wakefield accelerators to coax light out of undulators before. But Wang and colleagues demonstrated amplification, showing the light’s intensity increases 100-fold in the third undulator, they report this week in Nature. “This a huge step forward,” says Agostino Marinelli, an accelerator physicist at SLAC.
The tiny FEL is a far cry from its bigger brethren, which generate beams billions of times brighter than other x-ray sources, with an energy spread as low as 0.1%. The new FEL produces much fainter pulses of longer wavelength ultraviolet light with an energy spread of 2%. SLAC researchers are also upgrading the LCLS to produce millions of pulses per second; the novel FEL can produce 5 per second.
Reaching x-ray wavelengths with the device will be difficult, Marinelli predicts. “These are very impressive results, but I would be very careful of extrapolating this to x-ray energies,” Still, the SIOM team says that’s their goal. “It is hard to say how long it will take to reach the hard x-ray wavelengths, maybe a decade or longer,” says Ruxin Li, an SIOM physicist and team member. “We look forward to that day.”