by Robert Lea, science journalist, Medium

At Stanford University in California there exists a powerful juxtaposition in the field of particle acceleration. On the hillside overlooking the university sits SLAC National Accelerator Laboratory, which houses a two-mile long linear particle accelerator. Meanwhile, in its shadow, scientists at SLAC and Stanford have developed a particle accelerator that does the same job but can also fit on a silicon chip.

Of course, such a disparity in size does create other major differences. While the giant accelerator fires streams of electrons through a vacuum pipe with bursts of microwave radiation pushing them to near light speed, the pocket-sized particle accelerator can accelerate electrons to only a fraction of that velocity. This is still an impressive feat, however, as the infrared laser used by the mini-accelerator can deliver enough energy in just the width of a hair to push the electron to this speed. The same boost in the larger equipment takes the microwaves many feet to impart.

The accelerator-on-a-chip is just a prototype, but its design and fabrication techniques can be scaled up to deliver particle beams accelerated enough to perform cutting-edge experiments that don’t require the power of a massive accelerator. By placing these chips in a series, the researchers hope to create an electron beam that can move at 94% the speed of light—an energized particle flow that can be used for research and in various medical applications.

Writing in the latest issue of Science, the Stanford/SLAC team led by electrical engineer Jelena Vuckovic explain how they were able to carve a nanoscale channel from silicon, seal it in a vacuum and send electrons through this cavity while pulses of infrared light—to which silicon is as transparent much in the same way glass is to visible light—were transmitted by the channel walls to speed the electrons along.

“The largest accelerators are like powerful telescopes,” Vuckovic says. “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 researchers say their approach is similar to developments in computing that saw the downscaling of mainframe computers to smaller personal computers. Robert Byer, a coauthor of the Science paper, adds accelerator-on-a-chip technology could also lead to new cancer radiation therapies, taking advantage of downsizing. Current medical X-ray machines fill a room and deliver a beam of radiation that is difficult to focus on tumors, thus requiring patients to wear lead shields to minimize harm to other tissues. “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,” says Byer, who leads the Accelerator on a Chip International Program, or ACHIP, a broader effort of which this current research is a part.

Turning Particle Acceleration on Its Head

To build a chip that fires pulses of infrared light through silicon in order to hit electrons at just the right moment and angle to accelerate them, the team has had to turn that traditional design of particle accelerators upside down.

In a traditional accelerator, such as that at SLAC, engineers generally draft a basic design, and then run simulations to physically arrange the microwave bursts to deliver the greatest possible acceleration. But microwaves measure 4 inches from peak to trough, while infrared light has a wavelength one-tenth the width of a human hair. This difference explains why infrared light can accelerate electrons in such short distances in comparison to microwaves. This also means that the chip’s physical features must be 100,000 times smaller than the copper structures used in a traditional accelerator. This demands a new approach to engineering based on silicon integrated photonics and lithography.

Vuckovic and her team approached this design challenge by using inverse design algorithms that her lab has developed. They worked backwards by specifying just how much light energy they wanted the chip to deliver and tasked the software with suggesting how to build the right nanoscale structures required to bring the photons into proper contact with the flow of electrons.

“Sometimes, inverse designs can produce solutions that a human engineer might not have thought of,” explains R. Joel England, a SLAC staff scientist and co-author on the Science paper.

The design algorithm led to a chip layout that seems almost otherworldly—the imaging of nanoscale mesas, separated by a channel, etched out of silicon. Electrons flowing through the channel run a gauntlet of silicon wires, poking through the canyon wall at strategic locations. Each time the laser pulses—which it does 100,000 times a second—a burst of photons hits electrons, accelerating them forward. All of this occurs in less than a hair’s width, on the surface of a vacuum-sealed silicon chip, made by team members at Stanford.

The researchers want to accelerate electrons to 94 per cent of the speed of light—or 1 million electron volts (1MeV)—to create a particle flow powerful enough for research or medical purposes. This prototype chip provides only a single stage of acceleration, and the electron flow would have to pass through around 1,000 of these stages to achieve 1MeV. That may not be as daunting at it initially seems, Vuckovic adds, because this prototype accelerator-on-a-chip is a fully integrated circuit. That means all of the critical functions needed to create acceleration are built right into the chip and increasing its capabilities should be reasonably straightforward.

The researchers plan to pack a thousand stages of acceleration into roughly an inch of chip space by the end of 2020 to reach their 1MeV target. Although that would be an important milestone, it still lacks the raw power of the SLAC research accelerator, which can generate energy levels 30,000 times greater than 1MeV. Despite this, Byer believes that, just as transistors eventually replaced vacuum tubes in electronics, light-based devices will one day challenge the capabilities of microwave-driven accelerators.
Meanwhile, in anticipation of developing a 1MeV accelerator on a chip, electrical engineer Olav Solgaard, another coauthor on the paper, has already begun work on a possible cancer-fighting application.

Today, highly energized electrons aren’t used for radiation therapy because they would burn the skin. Solgaard is working on a way to channel high energy electrons from a chip-sized accelerator through a catheter-like vacuum tube that could be inserted below the skin, right alongside a tumor, 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 concludes.