Researchers Build Chip-Size Particle Accelerator

For the first time, scientists at Stanford University and the SLAC National Accelerator Laboratory have created a silicon chip that can accelerate electrons using an infrared laser to deliver, at less than a hair’s width, the sort of energy boost that takes microwaves several feet.

Until now, the university scientists have relied on a two-mile-long accelerator to deliver electrons through a vacuum pipe. Bursts of microwave radiation launch the electrons at a velocity just shy of the speed of light. The product of this electron projection is a beam powerful enough to help scientists around the world probe the atomic and molecular structures of both organic and inorganic materials.

Writing in the Jan. 3 issue of Science, a team led by electrical engineer Jelena Vuckovic, recent winner of the IET A F Harvey Engineering Research Prize, explained how it carved a nanoscale channel out of silicon, sealed it in a vacuum, and sent electrons through this cavity while pulses of infrared light were transmitted by the channel walls to speed the electrons along.

A section of a prototype accelerator-on-a-chip. Courtesy of Neil Sapra.

The accelerator-on-a-chip is just a prototype and operates at a fraction of the velocity of the larger accelerator, but Vuckovic said its design and fabrication techniques can be scaled up to deliver particle beams fast enough to perform cutting-edge experiments in chemistry, materials science, and biological discovery that don’t require the power of a massive accelerator.

“The largest accelerators are like powerful telescopes,” Vuckovic said. “There are only a few in the world and scientists must come to places like SLAC to use them. We want to miniaturize accelerator technology in a way that makes it a more accessible research tool.”

The team said its approach was similar to the evolution of computing, from the mainframe to the smaller but still useful PC. Robert Byer, a co-author of the paper, believes accelerator-on-a-chip technology could also lead to new cancer radiation therapies. Current medical x-ray machines are large enough to fill a room while delivering beams of radiation that are tough to focus on tumors and require patients to wear lead shields to minimize collateral damage.

“In this paper we begin to show how it might be possible to deliver electron beam radiation directly to a tumor, leaving healthy tissue unaffected,” said Byer, who leads the Accelerator on a Chip International Program, or ACHIP, a broader effort of which this current research is a part.

In their paper, Vuckovic and her team explain how they designed the chip to fire pulses of infrared light through silicon to hit electrons at a precise moment and angle for them to be accelerated forward faster than before.

To accomplish this, they turned the design process upside down. In a traditional accelerator, such as the one at SLAC, engineers draft a basic design and then run simulations to physically arrange the microwave bursts to deliver the highest possible acceleration. While microwaves measure four inches from peak to trough, infrared light has a wavelength one-tenth the width of a human hair. That difference is the reason infrared light can accelerate electrons in shorter distances. But this also means that the chip’s physical features must be 100,000 times smaller than the copper structures in a traditional accelerator.

Vuckovic’s team solved this problem using inverse design algorithms developed at her lab. These algorithms allowed the team to work backward by specifying how much light energy they wanted the chip to deliver and tasking the software with suggesting how to build the right nanoscale structures required to bring the photons into proper contact with the flow of electrons.

“Sometime inverse designs can produce solutions that a human engineer might not have thought of,” said R. Joel England, another co-author of the paper.

The design algorithm came up with a chip layout where the electrons flowing through the channel run a gauntlet of silicon wires, poking through the canyon wall at strategic locations. The laser pulses 100,000 times a second, each pulse sending a cluster of photons into another cluster of electrons, and accelerating them forward.

The team wants to accelerate electrons to 94% of the speed of light, or 1 million electron volts, to create a particle flow powerful enough for research or medical purposes.

The prototype chip provides only a single stage of acceleration, requiring the electron flow to pass through approximately 1000 stages to achieve the desired speed. However, because Vuckovic’s prototype is a fully integrated circuit, all of the critical functions needed to create acceleration are built right into the chip.

The team plans to add a thousand stages of acceleration by the end of 2020 to reach target velocity.

In anticipation of developing the 1 million electrons volts accelerator, team member Olav Solgaard has begun work on a possible cancer-fighting application. Solgaard is working on a way to channel high-energy electrons from a chip-size accelerator through a catheter-like vacuum tube that could be inserted right alongside a tumor, below the skin to avoid potential burns, using the particle beam to administer radiation therapy surgically.

“We can derive medical benefits from the miniaturization of accelerator technology in addition to the research applications,” Solgaard said.