New particle accelerator can rest on your fingertip

Portable X-rays could result from a new particle accelerator chip that uses cheaper laser technology and less material to work. 

Imagine an X-ray machine small enough to fit on a table top.  

Stanford researchers have brought us one step closer to this technology by creating a particle accelerator smaller than a grain of rice. The device paves the way for cheaper and smaller accelerators that could mean big things for science and medicine.

"Our sponsor of this work is DARPA [the Defense Advanced Research Projects Agency], and DARPA wants us to develop an accelerator and an X-ray source that can be portable so that you could carry the X-ray machine into the field and use it to provide medical care for injured soldiers," said Robert Byer, principal investigator of the study.

Particle accelerators are typically clunky and costly. X-rays use accelerators to produce the images seen on film – electrons accelerated through a tube collide with atoms to create X-rays. Most accelerators use microwaves to accelerate the electrons to nearly the speed of light through a linear or circular track.

This track can be massive; the largest particle accelerator at CERN in Geneva runs about 17 miles around. But the Stanford team used an infrared laser to move their electrons through a channel finer than a human hair.

Byer compares the science of accelerators to a surfer.

"If you imagine a surfer riding a wave in the ocean, there's sort of two things a surfer has to do to ride the wave," he said. "One is be in the right place at the right time...and then you have to pre-accelerate,  you have to paddle to catch up to go about the about the same speed as the wave."

After getting electrons up to nearly the speed of light in a  conventional accelerator, researchers funneled the particles through a tiny channel less than 1/200th the width of a human hair. Instead of exposing the particles to microwaves, the team used lasers.  

The key part of their design was using nanoridges in the channel so that the particles could gain energy as they travel. The light waves from the laser have two electric forces: a positive one that accelerates an electron, and a negative one that slows it down.

The ridges allow the electron to be less affected by the negative charge that saps it of energy. Without them, Byer explained, the electrons would not gain any energy but just shimmy back and forth. 

"We have the gradient there so the electrons don't stand there and quiver, but they actually...surf the electric wave," he said.

The electrons accelerated at a speed about ten times faster than typical linear accelerators, and all within a much smaller channel.

"Lasers offer much higher peak power than microwaves, and because of its very high peak power – it's equivalent to having a very high wave in the ocean--because of that we can accelerate the electrons at a much higher rate," Byer said.

Byer said the primary motivation behind the research was to develop a machine for the study of particle physics that was affordable. But there are many more applications for accelerators.

Accelerators are also used to treat cancer in the form of particle therapy, which blasts tumors with radiation. Typical radiotherapy using X-rays can be harmful to surrounding tissue, but particle accelerators that deliver photons of radiation can be less harmful, say researchers. 

Byer said Varian Medical Center in Palo Alto sponsors accelerator work that goes into medicine.

"They would be very happy to have a much smaller, more compact accelerator structure so that they could deliver radiation or therapeutic service to the patient and do so in a small, more accurate way," he said. 

Byer said the new device is cheaper than typical accelerators for two reasons: it uses high-power lasers that are cheaper than microwave sources, and it's much smaller.

The laser-based device offers many opportunities for the future, Byer said.

"Today, the laser is integrated in your life in ways you'll never know. It makes your automobiles. It makes your iPhones. It delivers your medicine," he said. "After 50 years of the invention of the laser, it is an integral part of everything we do in this world." 

Ziran Wu, a staff scientist at SLAC and another team member of the project, says the device is a potentially great cost and space-saver, although the project is not entirely finished.

"It's a multi-year program," he said. "We are at year two and I think we are making very good milestone progress, but there' s still a long way to go."