A compact accelerator reaches new speeds with nothing but light


Scientists harnessing the precise control of ultrafast lasers they have accelerated electrons over a 20-centimeter stretch to speeds reserved for large particle accelerators.

A team from the University of Maryland (UMD) led by Professor of Physics and Electrical and Computer Engineering Howard Milchberg, in collaboration with the team of Jorge J. Rocca from Colorado State University (CSU), achieved this milestone using two laser pulses sent through a jet of hydrogen gas. The first pulse ripped apart the hydrogen, ripping a hole in it and creating a channel of plasma. That channel guided a second, higher-power pulse that pulled electrons out of the plasma and dragged them in its wake, accelerating them to nearly the speed of light in the process.

With this technique, the team accelerated electrons to almost 40% of the energy achieved in massive installations such as the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at the National Accelerator Laboratory SLAC. The article was accepted in the journal Physical Review X.

“This is the first fully laser-powered multi-GeV electron accelerator,” says it’s a statement Milchberg, who is also affiliated with UMD’s Institute for Electronic Research and Applied Physics. “And with lasers becoming cheaper and more efficient, we hope that our technique will become the way forward for researchers in this field“.

What motivates the new work are accelerators like LCLS, a kilometer-long track that accelerates electrons to 13.6 billion electron volts (GeV), the energy of an electron moving at 99.99999993% of the speed of light. The predecessor of LCLS is behind three Nobel prize-winning discoveries about fundamental particles.

Now one-third of the original accelerator has been converted to the LCLS, using its super-fast electrons to generate the world’s most powerful X-ray lasers. Scientists use these X-rays to look inside atoms and molecules in action, creating videos of chemical reactions. These videos are vital tools for drug discovery, optimized energy storage, innovation in electronics and much more.

Accelerating electrons to energies of tens of GeV is no easy task. SLAC’s linear accelerator gives electrons the boost they need using powerful electric fields that propagate in a very long series of segmented metal tubes. If the electric fields were stronger, they would trigger an electrical storm inside the tubes, severely damaging them. Unable to push the electrons harder, the researchers have opted to simply push them around longer, providing more track for the particles to accelerate. Hence the mile-long strip that runs through Northern California. To bring this technology to a more manageable scale, the UMD and CSU teams worked to propel electrons to nearly the speed of light by appropriately using light itself.

“The ultimate goal is to shrink GeV-scale electron accelerators down to a modest-sized room”says Jaron Shrock, a graduate student in physics at UMD and co-author of the paper. “You’re taking kilometer-scale devices, and you’ve got another factor of 1,000 of stronger acceleration field. So, you’re taking kilometer-scale to meter-scale, that’s the point of this technology.”

Creating those stronger acceleration fields in a laboratory employs a process called laser trail field acceleration, in which a pulse of intense, well-focused laser light is sent through a plasma, creating a disturbance and dragging electrons in its wake.

Laser trail field acceleration was first proposed in 1979 and demonstrated in 1995. But the distance over which it could accelerate electrons remained stubbornly limited to a couple of centimeters. What allowed the UMD and CSU team to harness wakefield acceleration more effectively than ever before was a technique the UMD team pioneered to tame the high-energy beam and keep it from spreading too far. His technique pierces a hole through the plasma, creating a waveguide that keeps the energy of the beam focused.

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A compact accelerator reaches new speeds with nothing but light