Stanford University researchers have built a prototype particle accelerator small enough to fit on a human hair.

The high-power particle accelerators used for some of the most monumental scientific investigations – such as the discovery of the Higgs Boson, or search for dark matter – are among the largest and most expensive machines in the world.

However, researchers at Stanford University – which operates the 3.2km long SLAC particle accelerator, which accelerates electrons to near light speed through a linear tunnel – have developed a prototype particle accelerator which can be built onto a silicon chip. The work is described in the latest issue of Science.

Much like SLAC, the accelerator boosts electrons in a straight line. The tiny accelerator uses infrared laser light to deliver energy to the articles, boosting them much more quickly than the microwave radiation used in SLAC. This allows for the accelerator to be miniaturised so extremely.

“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,” said Professor Jelena Vuckovic, the electrical engineer who led the project. “We want to miniaturise accelerator technology in a way that makes it a more accessible research tool.”

This miniaturisation is comparable to the transition in computing from mainframes to smaller PCs, which are useful for conducting research – such as by running simulations – as well as for everyday tasks.

The researchers carved a nanoscale tunnel out of silicone, sealed it within a vacuum, then boosted electrons inside it using a laser. It was important to fire the laser pulses at precisely the right moments and at the right angle in order to boost the electrons. To achieve this, they used an inverse design approach, beginning with algorithms which suggest the nanoscale structures which could allow for the right amounts of energy to be delivered in the right directions.

According to the researchers, inverse design can produce solutions that human engineers may not have ever considered. In this case, the algorithm suggested an “almost otherworldly” chip layout which allowed for 100,000 laser pulses to boost a bunch of electrons every second within a structure less than a hair’s width in size.

The prototype provides just one stage of acceleration: approximately 1,000 of these stages would be required to accelerate electrons to 94 per cent the speed of light (useful for research and medical purposes). However, the engineers suggest that this could be doable, as the prototype is a fully integrated circuit, meaning that all of the critical functions needed to accelerate particles are built into the chip already. They hope to pack the 1,000 necessary boosts into approximately an inch of chip by the end of 2020.

While this accelerator would be magnitudes less powerful and versatile than accelerators like SLAC, it could still be valuable in research and applications, including in medicine. Vuckovic also believes that the techniques used to create it could be scaled up to create other devices capable of generating particle beams useful for experiments probing mysteries in chemistry, materials science and biology.

Accelerator-on-a-chip technology could also lead to new cancer radiation therapies, suggested co-author Professor Robert Byer (head of the Accelerator on a Chip International Program). Medical x-ray machines tend to be room-sized and deliver beams of radiation which are difficult to focus on affected areas, requiring patients to wear lead shields and be held completely still during the therapy. However, this research shows that it could be possible to deliver electron beam radiation to a tumour with great precision.