Using precise control of ultrafast lasers, researchers have accelerated electrons over a 20-centimeter stretch to speeds normally reserved for particle accelerators the size of 10 football fields.
A team at 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 at Colorado State University (CSU), achieved this feat by using two laser pulses sent through a jet of hydrogen gas. The first pulse tore apart the hydrogen, punching a hole through it and creating a plasma channel. That channel directed a second, higher power pulse that picked up electrons out of the plasma and dragged them along in its wake, accelerating them to nearly the speed of light in the process.
With this technique, the team accelerated the electrons to nearly 40% of the energy achieved at massive facilities such as the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at SLAC National Accelerator Laboratory. The paper was accepted for the journal Physical examination X 1 August 2022.
“This is the first multi-GeV electron accelerator powered entirely by lasers,” says Milchberg, who is also affiliated with the Institute of Research Electronics and Applied Physics at UMD. “And with lasers becoming cheaper and more efficient, we expect our technique to be the way forward for researchers in this field.”
Motivating the new work are accelerators such as LCLS, a kilometer-long runway that accelerates electrons to 13.6 billion electron volts (GeV) – the energy of an electron moving at 99.99999993% of the speed of light. LCLS’s predecessor is behind three Nobel Prize-winning discoveries about fundamental particles. Now a third of the original accelerator has been converted to LCLS, using the superfast electrons to generate the most powerful X-ray laser beams in the world. Scientists use these X-rays to peer into atoms and molecules in action, making videos of chemical reactions. These videos are important tools for drug discovery, optimized energy storage, innovation in electronics and much more.
Accelerating electrons to energies of tens of GeV is no mean feat. SLAC’s linear accelerator gives the electrons the push they need by using powerful electric fields that propagate in a very long series of segmented metal tubes. If the electric fields were any stronger, they would set off a lightning storm inside the pipes and seriously damage them. Since they are unable to push electrons harder, scientists have chosen to simply push them further, giving more runway for the particles to accelerate. Hence the kilometer-long slice across northern California. To bring this technology to a more manageable scale, the UMD and CSU teams worked to boost electrons to nearly the speed of light by using — fittingly — light itself.
“Ultimately, the goal is to shrink GeV-scale electron accelerators into a modestly sized space,” says Jaron Shrock, a graduate student in physics at UMD and co-author of the work. “You take kilometer-scale units, and you have another factor of 1,000 stronger accelerating fields. So, you take kilometer-scale to meter-scale, that’s the goal of this technology.”
Creating the stronger accelerating fields in a laboratory uses a process called laser wakefield acceleration, in which a pulse of tightly focused and intense laser light is sent through a plasma, creating a disturbance and pulling electrons along in its wake.
“You can imagine the laser pulse as a boat,” says Bo Miao, a postdoctoral fellow in physics at the University of Maryland and co-author on the work. “As the laser pulse moves in the plasma, because it’s so intense, it pushes the electrons out of the path, like water pushed to the side by the bow of a boat. These electrons go around the boat and collect right behind it, and move into the pulse wakes up.”
Laser wakefield acceleration was first proposed in 1979 and demonstrated in 1995. But the distance over which it could accelerate electrons remained stubbornly limited to a few centimeters. What enabled the UMD and CSU team to harness wakefield acceleration more efficiently than ever before was a technique the UMD team pioneered to tame the high-energy beam and prevent it from spreading its energy too thin. Their technique punches a hole through the plasma, creating a waveguide that keeps the beam’s energy focused.
“A waveguide allows a pulse to propagate over a much longer distance,” explains Shrock. “We have to use plasma because these pulses are so high energy, they’re so bright, they would destroy a traditional fiber optic cable. Plasma can’t be destroyed because somehow it already is.”
Their technique creates something akin to fiber-optic cables — the stuff that carries fiber-optic Internet services and other telecommunications signals — out of thin air. Or, more precisely, out of carefully sculpted jets of hydrogen gas.
A conventional fiber optic waveguide consists of two components: a central “core” that guides the light, and a surrounding “cladding” that prevents the light from leaking out. To create their plasma waveguide, the team uses an additional laser beam and a beam of hydrogen gas. As this extra “guiding” laser moves through the beam, it rips the electrons off the hydrogen atoms and creates a plasma channel. The plasma is hot and rapidly begins to expand, creating a lower-density plasma “core” and a higher-density gas at the edge, like a cylindrical shell. Then the main laser beam (the one that collects electrons in the wake) is sent through this channel. The very leading edge of this pulse transforms the higher density shell into plasma as well, creating the “cladding”.
“It’s kind of like the very first pulse clears an area,” says Shrock, “and then the high-intensity pulse comes down like a train with someone standing in front throwing down the tracks as it goes.”
Using UMD’s optically generated plasma waveguide technique, combined with the CSU team’s powerful laser and expertise, the researchers were able to accelerate some of their electrons to a staggering 5 GeV. This is still a factor of 3 less than SLAC’s massive accelerator, and not quite the maximum achieved with laser wakefield acceleration (that honor belongs to a team at Lawrence Berkeley National Labs). However, the laser energy used per GeV of acceleration in the new work is a record, and the team says their technique is more versatile: It can potentially produce electron bursts thousands of times per second (as opposed to about once per second), making it a promising technique for many applications, from high-energy physics to the generation of X-rays that can take videos of molecules and atoms in action as at LCLS. Now that the team has demonstrated the success of the method, they plan to refine the setup to improve performance and increase acceleration to higher energies.
“Right now, the electrons are generated along the entire length of the waveguide, 20 centimeters long, which makes their energy distribution less than ideal,” says Miao. “We can improve the design so that we can control where exactly they are injected, and then we can better control the quality of the accelerated electron beam.”
While the dream of LCLS on a tabletop is not yet a reality, the authors say this work shows a way forward. “There’s a lot of engineering and science to be done between now and then,” says Shrock. “Traditional accelerators produce highly repeatable beams with all the electrons having similar energies and moving in the same direction. We are still learning how to improve these beam attributes in multi-GeV laser wakefield accelerators. It is also likely that to achieve energies on the scale of tens of GeV, we will need to stage several wakefield accelerators, and send the accelerated electrons from one stage to the next, while preserving beam quality. So there is a long way between now and having a laser-dependent LCLS type facility wakefield acceleration.”
Meter-scale plasma waveguides push the particle accelerator’s envelope
B. Miao et al, Multi-GeV electron beams from an alloptical laser Wakefield Accelerator, Physical examination X (2022). DOI: 10.1103/PhysRevX.12.031038
Provided by the University of Maryland
Citation: Compact electron accelerator reaches new speeds using nothing but light (2022, September 19) retrieved September 19, 2022 from https://phys.org/news/2022-09-compact-electron.html
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