A particle accelerator on your fingertip

October 03, 2013 12:50 am | Updated 02:27 pm IST

The key to the accelerator chips is tiny, precisely spaced ridges, which cause the iridescence seen above.

The key to the accelerator chips is tiny, precisely spaced ridges, which cause the iridescence seen above.

Particle accelerators are important tools used to leverage the features of high-energy physics for research and diagnostics. Almost 49 per cent of the 26,000 accelerators worldwide in 2010 were used for some medical research ( Physics Today , 2010), such as in radiotherapy to kill cancer cells in the body. These devices are typically large and costly to build and operate, although it is in public interest to enhance access to them.

Advances in accelerator physics have given scientists the freedom to think up better particle accelerators that are smaller and cheaper. New acceleration techniques “may lead to much smaller devices so that pretty much any hospital might have advanced diagnostic tools at hand,” Dr. Peter Hommelhoff, Max Planck Institute for Quantum Optics, Germany, said in an email.

These devices are inherently sophisticated. Typically, they have a series of stages through which particles, like electrons, are consecutively accelerated using carefully directed electromagnetic fields, before being energized some more in the final stage. Then, they can be collided against each other or at stationary targets depending on the purpose.

In a step toward achieving this goal in an affordable way, scientists from Stanford University have built a particle accelerator that could sit on your fingertip. They have achieved this by eschewing the use of electromagnetic fields using high electric input, instead using lasers to cause the necessary acceleration.

The action takes place in a tiny glass channel less wide than 1/200th the thickness of the human hair. Infrared laser light with a wavelength exactly twice the height of the channel is shined across it. The light waves have electric fields that oscillate back and forth, respectively applying accelerating and retarding forces on the electrons. A team led by Dr. Robert L. Byer, Stanford University professor and the principal investigator etched nanoscale ridges in the channel so that, as electrons passed over crests, they would accelerate much more than they retarded when passing over troughs.

“The electrons achieve an acceleration gradient of 300 million electron volts per meter or 300 MeV/m,” said Dr. Byer in an email. This gradient, the amount of energy gained across some distance, is almost 10 times more than is achieved in the Stanford Linear Accelerator Centre, the world’s largest linear accelerator.

After injecting electrons already accelerated to 60 MeV into the channel, the system adds to that energy at “0.15 V per 0.5 mm,” consuming only 10 mW. At the moment, it can also handle only electrons and anti-electrons. Dr. Byer added, “In the future, we will operate at 10 million times more average power for applications in particle physics.”

He also foresees his technique providing greater access to medical research in the future, but is wary of the hurdles before such ‘accelerators-on-chips’ become practical tools. To achieve an acceleration gradient of 1,000 MeV/m, which Dr. Byer is aiming for, he says they will have to focus on better “coupling of power to the accelerator and focusing.”

A competing method that also uses lasers, called plasma acceleration, is taking shape. While Dr. Byer's method is easier to scale up, plasma acceleration offers higher acceleration over shorter distances, according to Prof. Simon Hooker, a physicist at Oxford University.

“In plasma accelerators, the acceleration gradient is about 1,000 times smaller than possible with conventional machines, leading to an enormous saving in space and cost. This would allow accelerators and radiation sources driven by them to be brought from national-scale facilities into university and industrial labs, and into hospitals,” wrote Prof. Hooker, who was not involved in Dr. Byer’s study, in an email.

While laser-driven accelerators are some way off from serving high-energy physics, it seems that with lower energies and immediate cost benefits, they are still very useful for less demanding but equally necessary research. As Dr. Hommelhoff, who has been working on using lasers to accelerate low-energy electrons, speculated, “I’d say that the future is bright for laser-driven particle accelerators.”

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