This article was modified on 22/02/2011
3 February 2011—Forget for a moment about the quest to build bigger high-energy particle accelerators. Last week, at the MEMS 2011 conference, in Cancun, Mexico, researchers instead explained their efforts to create a smaller one.
Their chip-size cyclotron can guide argon ions with around 1.5 kiloelectronvolts of energy down a 5-millimeter accelerating track before whipping them around a 90-degree turn. The system boosts the ions’ energy by 30 electronvolts. That’s not very much energy, but unlike its larger cousins, this accelerator has no need for bulky magnets and instead uses an electric field set up between parallel electrode guide rails to accelerate and steer its particle beam. The device’s designers at Cornell University, in Ithaca, N.Y., say that with more research, similar electrostatic mini-accelerators might be used in shoebox-size scanning electron microscopes or portable particle-ray guns for cancer treatment.
The Large Hadron Collider, the world’s largest particle accelerator, which is buried underneath the border of France and Switzerland, can slam particles together in collisions that have nine or ten orders of magnitude more energy than the ions that traveled through the tiny Cornell device. "They need that to break open the nucleus and see what’s inside," says Yue Shi, an electrical and computer engineering graduate student who developed the accelerator on a chip.
Instead, Shi, funded by the U.S. Defense Advanced Research Projects Agency, is working to create a device that might accelerate ions to energies of hundreds of kiloelectronvolts on a chip not much bigger than a few square centimeters, and a suitcase-size device capable of accelerating ions to hundreds of megaelectronvolts, in the hopes of developing portable particle accelerators.
Shi constructed three versions of the accelerator—two on silicon-on-insulator (SOI) chips and one on a printed circuit board. Each had a straight, segmented acceleration track and either a 1-, 2-, or 4-mm turning radius. To test the design, she fired a stream of argon ions with around 1.5 keV of energy from a commercial ion source into each chip’s tracks. Electric fields between four segments in each chip’s acceleration track gave the ions a kick before they raced into the turn. Then another electric potential between two electrode curbs pulled ions around the bend. Only those ions with just the right amount of energy made it through. So, by detecting ions at the finish line, Shi confirmed that they truly got a boost.
If a small accelerator based on this design could bestow 1 MeV of energy to ion beams, it would have a broad range of applications, says Amit Lal, who worked with Shi and leads Cornell’s SonicMEMS Laboratory. Lal’s group works to create chip-scale power sources, such as a radioisotope-based generator for powering the electronics in cyborg insects. This particle accelerator is an offshoot of that research.
Lal also foresees more fantastic uses for the device. Doctors already use high-energy particle beams to kill cancer cells, he says, explaining that protons shot at living tissue give off heat as they slow down. Such radiation therapy requires devices that take up an entire room, he says, but tinier accelerators might make treatments more feasible for smaller clinics or allow more localized beams to irradiate fewer healthy cells. "Think of a scalpel with a proton beam coming out of it," he says.
Developing this proof-of-concept device into a commercial tool will first require overcoming some technical hurdles. Right now, the chip accelerates ions from a commercial argon ion source with a 75-micrometer-wide beam. Shi compares shooting that wide source into the accelerating channel to threading a needle, and much of the beam is lost. In the future, she hopes to use on-chip plasma sources to ionize atoms and energize them to around 100 eV before they even enter the electrostatic accelerator. She also notes that the accelerator she presented last week doesn’t focus the beam, which also leads to lost ions. Finally, she points out that the fastest ions that coursed through the accelerator during this initial research only had around 2 keV of energy—not much more than their starting energy of 1.5 keV from the commercial ion source—and that’s still three orders of magnitude lower than what she seeks.
Reaching the 1 MeV goal is certainly possible, she says. Having now shown that the ions can execute tight turns, Shi believes that future designs could navigate the ions repeatedly through accelerating strips for more energy.
The Cornell device is not the only mini-accelerator in development, or even the smallest. Instead of electrostatics, Gil Travish, who is developing a "micro-accelerator platform" at the University of California, Los Angeles, wants to use the electric fields in laser light to speed particles on their way. Travish’s group is starting to build a device that he describes as a 1-µm-thick "sandwich" with two mirrors above and below a gap only one wavelength of light high and several hundred wavelengths wide. As the light from a laser oscillates in that gap, an electron passing through the peak electric field will receive a tremendous boost—around a gigaelectronvolt per meter or a megaelectronvolt per millimeter. His team hopes to start beam tests in a prototype device in the next six months.
Travish says that it’s important to have multiple approaches, such as Cornell’s electrostatic work, for building these tiny particle accelerators. The high frequency of the laser light planned for use in the UCLA device means that only the zippiest of particles would make it through before the oscillating, light-wave-induced electric field reverses. That’s fine for electrons, but the argon ions that Cornell has accelerated, he notes, would need to be moving very quickly to make it through any laser accelerator. Otherwise, "the wave would just wash over the particle, and it would gain almost nothing," he says.
Funded in part by the U.S. Defense Threat Reduction Agency, which protects against the threat of weapons of mass destruction, the UCLA team imagines that their particle beam might also one day appear in medical devices or in unmanned aerial vehicles that could examine suspicious buildings using X-rays. "I think that in the next half decade you’ll start to see a real awakening," Travish says about the possibilities in particle accelerators’ new realm.
This article appeared in print as “A Chip-Scale Particle Accelerator."