Ti:sapphire laser drives accelerator on chip

An advance that uses commercial lasers and low-cost mass-production techniques could set the stage for a generation of “tabletop” particle accelerators that are less expensive for use in science and medicine.

Current state-of-the-art accelerators use microwaves to boost the energy of electrons. Researchers have sought more economical alternatives, and this new technique, which uses ultrafast lasers to drive the accelerator, is a leading candidate.

A team that included scientists from the SLAC National Accelerator Laboratory and Stanford University used a device powered by a Ti:sapphire laser – a cheaper power source than those used in radio-frequency accelerators – to accelerate electrons at a rate 10 times higher than conventional technology in a nanostructured glass chip smaller than a grain of rice.

Nanofabricated chips of fused silica just 3 mm long were used to accelerate electrons at a rate 10 times higher than conventional particle accelerator technology. Photo courtesy of Brad Plummer/SLAC.

“We still have a number of challenges before this technology becomes practical for real-world use, but, eventually, it would substantially reduce the size and cost of future high-energy particle colliders for exploring the world of fundamental particles and forces,” said Joel England, the SLAC physicist who led the experiments. “It could also help enable compact accelerators and x-ray devices for security scanning, medical therapy and imaging, and research in biology and materials science.”

At its full potential, the new “accelerator on a chip” could match the accelerating power of SLAC’s 2-mile-long linear accelerator in just 100 feet, and deliver 1 million more electron pulses per second. The initial demonstration achieved an acceleration gradient (amount of energy gained per length) of 300 million eV per meter. That’s roughly 10 times the acceleration provided by the current SLAC linear accelerator, the researchers say.

“Our ultimate goal for this structure is 1 billion electronvolts per meter, and we’re already one-third of the way in our first experiment,” said Stanford professor and principal investigator Robert Byer.

Particles are generally accelerated in two stages. First, they are boosted to nearly the speed of light. Then, any additional acceleration increases their energy, but not their speed; this is the challenging part.

In the accelerator-on-a-chip experiments, electrons were accelerated to near-light-speed in a conventional accelerator, then focused into a half-micron-high channel within a fused silica glass chip 0.5 mm long. The channel was patterned with precisely spaced nanoscale ridges; IR laser light shining on the pattern generated electrical fields that interacted with the electrons in the channel to boost their energy.

The work was reported in Nature (doi: 10.1038/nature12664).

Turning the accelerator on a chip into a full-fledged tabletop accelerator will require a more compact way to get the electrons up to speed before they enter the device. A collaborating research group in Germany, led by Peter Hommelhoff at Max Planck Institute of Quantum Optics, has been looking for such a solution. The group simultaneously reports in Physical Review Letters its success in using a laser to accelerate lower-energy electrons.Laser accelerators could drive compact x-ray free-electron lasers – comparable to SLAC’s Linac Coherent Light Source – that are all-purpose tools for a wide range of research, Byer said. Another possible application is small, portable x-ray sources to improve medical care for people injured in combat as well as to provide more affordable medical imaging for hospitals and laboratories.