Scientists using the precise control of ultrafast lasers accelerated electrons a distance of 20 centimeters at speeds usually reserved for particle accelerators the size of 10 football fields.
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), has achieved this feat using two laser pulses sent through a jet of hydrogen gas. The first pulse ripped through the hydrogen, punching a hole in it and creating a plasma channel. This channel guided a second, higher power pulse that picked up electrons from the plasma and dragged them into its wake, accelerating them to nearly the speed of light in the process.
With this technique, the team accelerated electrons to nearly 40% of the energy achieved in massive facilities like the mile-long Linac Coherent Light Source (LCLS), the accelerator at SLAC National Accelerator Laboratory . The article has been accepted in the journal Physical examination X on August 1, 2022.
“This is the first multi-GeV electron accelerator powered entirely by lasers,” says Milchberg, who is also affiliated with UMD’s Institute for Research in Electronics and Applied Physics. “And with lasers becoming cheaper and more efficient, we hope our technique will become the way forward for researchers in this field.”
The new work is driven by accelerators like the LCLS, a kilometer-long track that accelerates electrons to 13.6 billion electron-volts (GeV), the energy of an electron traveling at 99 .99999993% of the speed of light. The predecessor to LCLS is responsible for three Nobel Prize-winning discoveries about fundamental particles. Today, a third of the original accelerator has been converted to LCLS, using its superfast electrons to generate the most powerful X-ray laser beams in the world. Scientists use these X-rays to peer inside working atoms and molecules, creating videos of chemical reactions. These videos are essential tools for drug discovery, energy storage optimization, electronics innovation and much more.
Accelerating electrons to energies of several tens of GeV is not easy. SLAC’s linear accelerator gives electrons the boost they need by using powerful electric fields propagating through a very long series of segmented metal tubes. If the electric fields were stronger, they would trigger a thunderstorm inside the tubes and seriously damage them. Unable to push the electrons harder, the researchers chose to simply push them longer, providing more track for the particles to accelerate. Hence the mile-long slice through Northern California. To bring this technology to a more manageable scale, teams at UMD and CSU worked to boost electrons to nearly the speed of light using, appropriately enough, light itself.
“The ultimate goal is to reduce GeV-scale electron accelerators to a modest room size,” says Jaron Shrock, a graduate student in physics at UMD and co-first author of the work. “You take kilometer-scale devices, and you have another accelerating field that’s a factor of 1,000 stronger. So you take kilometer-scale to meter-scale, that’s the purpose of this technology.”
Creating these stronger accelerating fields in a laboratory uses a process called laser wakefield acceleration, in which a pulse of intense, tightly focused laser light is sent through a plasma, creating a disturbance and driving electrons into 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-first author of the work. “As the laser pulse travels through the plasma, because it’s so intense, it pushes electrons out of its way, like water being pushed out the bow of a boat. These electrons loop around the boat and gather right behind it, traveling in the wake of the pulse.”
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 allowed the UMD and CSU team to take advantage of wakefield acceleration more effectively than ever before was a technique the UMD team developed to tame the high-energy beam and prevent him from dispersing his energy too much. Their technique punches a hole in the plasma, creating a waveguide that keeps the beam’s energy focused.
“A waveguide allows a pulse to propagate over a much longer distance,” Shrock explains. “We have to use plasma because these pulses are so high in energy, they’re so bright, that they would destroy a traditional fiber optic cable. Plasma can’t be destroyed because, in a sense, it is. already.”
Their technique creates something akin to fiber optic cables – the stuff that carries fiber optic Internet service and other telecommunications signals – out of thin air. Or, more precisely, from carefully sculpted jets of hydrogen.
A conventional fiber optic waveguide consists of two components: a central “core” guiding the light and a surrounding “sheath” preventing the light from escaping. To make their plasma waveguide, the team uses an additional laser beam and a jet of hydrogen gas. As this additional “guiding” laser travels through the jet, it strips electrons from the hydrogen atoms and creates a plasma channel. The plasma is hot and quickly begins to expand, creating a “core” of lower density plasma and higher density gas on its fringe, like a cylindrical shell. Then, the main laser beam (the one that will gather the electrons in its wake) is sent through this channel. The very leading edge of this pulse also turns the higher density shell into plasma, creating the “coating”.
“It’s kind of like the very first pulse clears an area,” Shrock explains, “and then the high-intensity pulse goes down like a train with someone standing in front throwing the rails as it goes. measure.”
Using the UMD’s optically generated plasma waveguide technique, combined with the high-powered laser and expertise of the CSU team, the researchers were able to accelerate some of their electrons to staggering speeds of 5 GeV. That’s 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 say their technique is more versatile: it can potentially produce bursts of electrons thousands of times per second (as opposed to to about once per second), making it a promising technique for many applications, from high-energy physics to generating X-rays that can take videos of molecules and atoms in action like at LCLS . Now that the team has demonstrated the method’s success, they plan to refine the setup to improve performance and increase acceleration to higher energies.
“Right now, electrons are generated along the full length of the waveguide, 20 centimeters long, which makes their energy distribution less than ideal,” Miao says. “We can improve the design so we can control where they are injected precisely, and then we can better control the quality of the accelerated electron beam.”
While the dream of LCLS on a table isn’t yet a reality, the authors say this work shows a way forward. “There’s a lot of engineering and science to do between now and then,” says Shrock. “Traditional accelerators produce highly reproducible beams with all 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 reach energies on the scale of tens of GeV, we will need to stage several wakefield accelerators, passing the accelerated electrons from one stage to another while preserving the quality of the beam. So there is a long way between now and having an LCLS type installation based on the acceleration of the laser wake field.
Meter-Scale Plasma Waveguides Push Particle Accelerator Limits
B. Miao et al, Multi-GeV Electron Bunches from an All-Optical Laser Wakefield Accelerator, Physical examination X (2022). DOI: 10.1103/PhysRevX.12.031038
Provided by the University of Maryland
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