Compact electron accelerator reaches new speed with nothing but light

Compact electron accelerator reaches new speed with nothing but light

An image from a simulation in which a laser pulse (red) drives a plasma wave, accelerating electrons in its wake. The bright yellow spot is the region with the highest concentration of electrons. In one experiment, scientists used this technique to accelerate electrons to nearly the speed of light over a period of just 20 centimeters. credit: Bo Miao/IREAP

Using precise control of ultrafast lasers, scientists accelerate electrons over the 20-centimeter stretch typically reserved for particle accelerators the size of 10 football fields.

A team from the University of Maryland (UMD) led by Howard Milchberg, a professor of physics and electrical and computer engineering at Colorado State University (CSU) led by George J. In collaboration with Rocca’s team achieved this feat using two. laser pulses sent through a jet of hydrogen gas. The first pulse separated the hydrogen, punching a hole through it and creating a channel of plasma. that channel directed a second, higher-powered pulse that up electrons ejected from the plasma and pulled them with him, accelerating them to nearly the speed of light in the process.

With this technique, the team accelerated electrons to about 40% of the energy obtained at massive facilities such as the accelerator, the kilometer-long Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. the letter was accepted Physical Review X On 1st August 2022.

“This is the first multi-GeV electron accelerator powered entirely by a laser,” 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 technology to become the way to go for researchers in this field.”

Inspiring the new work are accelerators like the LCLS, a one-kilometre-long runway that accelerates electrons to 13.6 billion electron volts (GeV)—the energy of an electron that is moving at 99.999999993% of the speed of light. The predecessors of LCLS are behind three Nobel-prize winning discoveries about fundamental particles. Now, a third of the original accelerator has been converted to LCLS, using its super-fast electrons to generate the most powerful X-ray laser beam in the world. Scientists use these X-rays to look inside atoms and molecules to make videos of chemical reactions. These videos are vital tools for drug discovery, adapted energy storageInnovations in Electronics, and much more.

Accelerating electrons to energies of tens of GeV is no easy feat. SLAC’s linear accelerator gives the electrons the push they need using powerful electric fields spread across a very long chain of fractured metal tubes. If the electric fields were more powerful, they would create a lightning storm inside the tubes and seriously damage them. With electrons unable to push harder, the researchers opted to push them for longer, providing more runway for the particles to accelerate. Hence the kilometer long piece across Northern California. To bring this technology to a more manageable scale, the UMD and CSU teams worked to accelerate electrons to nearly the speed of light—quite well enough—light itself.

“The ultimate goal is to reduce the GeV-scale electron accelerator to a modestly sized room,” says Jaron Schrock, a graduate student in physics at UMD and co-first author on the work. “You’re taking kilometer-scale equipment, and you have another factor of 1,000 strong acceleration fields. So, you’re moving kilometer-scale to meter-scale, that’s the goal of this technology.”

Creating those strong accelerating fields in a laboratory employs a process called laser wakefield acceleration, in which a pulse of tightly focused and intense laser light is sent through a plasma, creating disturbances and pulling electrons with it. Is.

“You can imagine the laser pulse like a boat,” says Bo Miao, a postdoctoral fellow in physics at the University of Maryland and co-first author on the work. “As the laser pulse travels in the plasma, because it is so intense, it pushes electrons out of its path, like water pushed aside by the prowess of a boat. Those electrons loop around the boat. and gather right behind it, traveling to the awakening of the pulse.”

Laser wakefield acceleration was first proposed in 1979 and demonstrated in 1995. But the distance to which it could accelerate electrons remained limited to a few centimeters. To enable UMD and the CSU team to take advantage of wakefield acceleration more effectively than ever before, the UMD team took the lead in taming the high-energy beam and preventing 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,” Schock explains. “We need to use plasma because these pulses are so high energy, they are so bright, they would destroy conventional fiber optic cable, Plasma cannot be destroyed because in some sense it already is.”

Their technology creates something akin to fiber optic cables — the things that carry fiber optic Internet service and other telecommunications signals — out of thin air. Or, more accurately, from a carefully crafted jet 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 light from escaping. To create their plasma waveguide, the team uses an additional laser beam and a jet of hydrogen gas. As this additional “guiding” laser jet travels through, it rips electrons off the hydrogen atoms and creates a channel of plasma. The plasma heats up and expands quickly, forming a low-density plasma “core” and a high-density gas at its edge, like a cylindrical shell. Then, the main laser beam (the one that will collect electrons in its wake) is sent through this channel. The very front edge of this pulse turns the high-density shell into plasma, forming a “cladding”.

“It’s kind of like the first pulse clears an area,” Schorck says, “and then the high-intensity pulse comes down like a train, with someone standing in front and running down the tracks.” throws away.”

Using UMD’s optically generated plasma waveguide technology, combined with the CSU team’s high-powered laser and expertise, the researchers were able to accelerate some of their electrons to 5 GeV. This is still a factor of 3 less than SLAC’s giant accelerator, and not the maximum achieved with laser wakefield acceleration (this honor belongs to a team from Lawrence Berkeley National Labs). However, in the new work the laser energy used per GeV of acceleration is a record, and the team says their technique is more versatile: it can potentially produce electron bursts thousands of times per second (as shown in Fig. as opposed to about once per second), making it a promising technology for many applications, from high energy Physics for the generation of X-rays that can take video of molecules and atoms in action, like LCLS. Now that the team has demonstrated the method’s success, they plan to refine the setup to improve performance and increase acceleration at high energies.

“Right now, electrons are generated along the entire length of the 20-centimeter-long waveguide, which makes their energy distribution less than ideal,” Miao says. “We can improve the design so that we can control exactly where they are injected, and then we can better control the quality of the accelerated electron beam.”

While the dream of LCLS on the tabletop is not yet a reality, the authors say this work shows the way forward. “There’s a lot of engineering and science to be done between now and then,” Schorck says. “Conventional accelerators produce highly repeatable beams with the same energy and all electrons traveling in the same direction. We are still learning how to improve these beam characteristics in multi-GeV laser wakefield accelerators. This is also likely To achieve energies on the scale that are tens of GeV, we would need to phase out multiple wakefield accelerators, moving the accelerated electrons from one phase to another while maintaining the beam quality. Hence no more LCLS type facilities. There is a long way to go between relying on laser Wakefield acceleration.”

Meter-scale plasma waveguides push the particle accelerator envelope

more information:
B. Miao et al., Multi-GeV Electron Bunch from the All-Optical Laser Wakefield Accelerator, Physical Review X (2022). DOI: 10.1103/PhysRevX.12.031038

Citation: Compact electron accelerator reaches new speeds other than light (2022, 19 September) Retrieved 20 September 2022 from

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