Particle accelerators touch nearly every part of our daily lives. They are used in medical and scientific research – from nuclear physics to X-rays – and by industry to manufacture computer chips and to create shrink wrap, among many other applications. They even play an important role in our national security, inspecting cargo and more. Particle accelerators produce a beam of charged particles and can be several miles long, cost tens of millions of dollars and decades to build. One such accelerator is SLAC in the San Francisco Bay Area. It is the longest linear accelerator in the world at two miles in length and has been operational since 1966.
An international group of scientists are attempting to shrink the particle accelerator by placing it on a chip, reducing its size to a shoebox. How? By converging technologies of nanofabrication and solid-state lasers and combining them to make a miniaturized particle accelerator. Why shrink it? There are enormous cost and size benefits. A smaller, mobile accelerator could enable broader use among scientists and inventors, opening new doors in research and development across numerous fields and industries, such as health care, technology and more.
Putting an accelerator on a chip was first conceptualized by Dr. Robert Byer of Stanford University and Dr. Peter Hommelhoff of Friedrich-Alexander University (FAU) Erlangen-Nuremberg. Through funding from the foundation, FAU, Stanford and eight other international partner institutions formed the “Accelerator-on-a-Chip International Program” to build a scalable prototype. Achieving this is quite challenging. It requires many critical elements be developed and then come together to create a functioning prototype.
Making electrons surf
A key element to making an accelerator on a chip, is to use laser beams to accelerate electrons. Scientists must be able to control the oscillation of light very precisely with the movement of electrons so that they meet each other at just the right moment. It is similarly important to know when and where the maximum crest of a light wave hits a packet of electrons so that they can influence the outcome to a highly-specific degree. For this, light and electrons must unite within attoseconds (a billionth of a billionth of a second).
For the first time, a team of researchers, led by Dr. Peter Hommelhoff, achieved this critical milestone in the quest to shrink the particle accelerator. They developed a new technique in which they intersect two laser beams that oscillate at different frequencies to produce an optical field whose properties can be very precisely influenced. The most important property of this optical field is that it retains contact with the electrons – which is why it is called a travelling wave – so the electrons can continuously sense, or surf, the optical field. In this way, the optical field transmits its properties precisely to the particles. Their work was published in Nature Physics.
There are still several essential milestones to achieve before realizing a functioning accelerator on a chip. These include nanofabrication of the electron source, developing techniques to focus and bunch the electrons and stacking of the chips to achieve higher energies, led by research teams at Stanford and SLAC.
“As the Accelerator-on-a-Chip Program successfully achieves critical milestones, the idea of miniaturizing the particle accelerator and creating a small, scalable prototype becomes a greater reality,” says Gary Greenburg, Ph.D., program officer at the foundation. “Our hope is that with our initial funding the international team of researchers will demonstrate a working device that will lower the risk profile enough so that federal funding agencies and companies can step in to support its development on a large scale.”
Physicists Go Small: Let's Put A Particle Accelerator on a Chip (NPR, July 18, 2018)