Particle accelerator on a chip promises more accessible research tool
Although the acceleration, demonstrated as an extra 0.915 keV gained along a 30µm-long channel, is only a fraction of what’s achievable with giant particle accelerators such as the 2 miles long instrument at Stanford’s SLAC National Accelerator Laboratory, it is designed at a scale several orders of magnitude smaller. Hence, the researchers anticipate that hundreds or even thousands of such silicon-based particle accelerators, only a few micrometers in size, could be operated in cascade to accelerate particles in useful high-energy beams.
The researchers carved a nanoscale channel out of silicon only 30µm-long, sealed it in a vacuum and sent electrons through this cavity while pulses of infrared light – to which silicon is transparent – were transmitted by the channel walls to speed the electrons along. The accelerator-on-a-chip demonstrated in Science is just a prototype, but the design and fabrication techniques used are easily scalable and in the future, small portable accelerators could deliver particle beams accelerated 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. There are only a few in the world and scientists must come to places like SLAC to use them,” explains Jelena Vuckovic, electrical engineer at Stanford and team leader on this research. “We want to miniaturize accelerator technology in a way that makes it a more accessible research tool.”
Team members liken their approach to the way that computing evolved from the mainframe to the smaller but still useful PC. Accelerator-on-a-chip technology could also lead to new cancer radiation therapies, said physicist Robert Byer, a co-author of the Science paper. Again, it’s a matter of size. Today, medical X-ray machines fill a room and deliver a beam of radiation that’s tough to focus on tumours, requiring 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 tumour, 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 graduate student Neil Sapra, the first author, explain how the team built a chip that fires pulses of infrared light through silicon to hit electrons at just the right moment, and just the right angle, to move them forward just a bit faster than before.
To accomplish this, they turned the design process upside down. In a traditional accelerator, like the one at SLAC, engineers generally draft a basic design, 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 in the hundreds of nanometers. That difference explains why infrared light can accelerate electrons in such short distances compared to microwaves. But this also means that the chip’s physical features must be 100,000 times smaller than the copper structures in a traditional accelerator. This demands a new approach to engineering based on silicon integrated photonics and lithography.
The gray structures are nanometer-sized features carved in to
silicon that focus bursts of infrared laser light, shown in yellow
and purple, on a flow of electrons through the centre channel.
As the electrons travel from left to right, the light focused in
the channel is carefully synchronized with passing particles
to move them forward at greater and greater velocities.
Credit: Neil Sapra
Vuckovic’s team solved the problem using inverse design algorithms that her lab has developed. These algorithms allowed the researchers 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.
“Sometimes, inverse designs can produce solutions that a human engineer might not have thought of,” said R. Joel England, a SLAC staff scientist and co-author on the Science paper.
The design algorithm came up with an original chip layout including nanoscale mesas separated by a channel, all etched out of silicon. Electrons flowing through the channel run a gantlet of silicon wires, poking through the canyon wall at strategic locations. For each of the laser pulses (running at 100kHz), a burst of photons hits a bunch of electrons, accelerating them forward.
The researchers want to accelerate electrons to 94 percent of the speed of light, or 1 million electron volts (1MeV), to create a particle flow powerful enough for research or medical purposes. 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, such a device would still pale in power alongside the capabilities of the SLAC research accelerator, which can generate energy levels 30,000 times greater than 1MeV. But 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, a co-author 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 tumour, 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.
Stanford University – www.stanford.edu
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