Scientists harnessing the precise control of an ultrafast laser accelerated electrons along 20 cm to speeds normally intended for particle accelerators the size of 10 football fields.
A University of Maryland (UMD) team led by Howard Milchberg Professor of Physics, Electrical and Computer Engineering, in collaboration with Jorge J. Roca’s team at Colorado State University (CSU), achieved this feat using two laser pulses It is sent through a jet of hydrogen gas. The first pulse ripped the hydrogen, punched a hole in it and created a plasma channel. That channel directed a second, higher energy pulse Electrons The plasma is pulled out and in its wake, accelerating it to nearly the speed of light in the process.
Using this technology, the team accelerated electrons to approximately 40% of the energy obtained at massive facilities such as the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at SLAC’s National Accelerator Laboratory. The paper was accepted into the journal X . physical review On August 1, 2022.
“This is the first fully laser-powered multi-GeV electron accelerator,” says Milchberg, who also belongs to the Research Institute of Electronics and Applied Physics at UMD. “As lasers become more expensive and more effective, we anticipate that our technology will become the way researchers in this field must take it.”
The new work galvanizes accelerators such as LCLS, a kilometer-long runway that accelerates electrons to 13.6 billion electronvolts (GeV) – the energy of an electron moving at 99.9999993% of the speed of light. The predecessor of the LCLS was behind three Nobel Prize-winning discoveries about fundamental particles. Now, a third of the original accelerator has been converted to LCLS, using its ultrafast electrons to generate the world’s most powerful X-ray lasers. Scientists use these X-rays to peer inside atoms and molecules in action, creating videos of chemical reactions. These videos are vital tools for improved drug discovery energy storageinnovation in electronics and much more.
Accelerating electrons to energies of tens of GeV is not easy. SLAC’s linear accelerator gives the electrons the boost they need using strong electric fields that propagate through a very long series of segmented metal tubes. If the electric fields were more powerful, it would trigger a thunderstorm inside the pipes and seriously damage them. Because they couldn’t push the electrons as hard, the researchers simply chose to push them longer, providing more runway to accelerate the particles. Hence the kilometer long slide through Northern California. To bring this technology to a more manageable range, the UMD and CSU team worked to boost electrons to nearly the speed of light using light itself — appropriately enough.
“The ultimate goal is to shrink GeV-scale electron accelerators to a modestly sized room,” says Jaron Schrock, a graduate student in physics at UMD and first co-author of the work. “You take kilometer scale devices, and you have another factor of 1,000 stronger acceleration fields. So you take kilometer scale to meter scale, that’s the goal of this technology.”
Creating those stronger accelerating fields in the lab uses a process called wakefield laser acceleration, in which a pulse of highly focused and intense laser light is sent through the plasma, creating a disturbance and drawing electrons in its wake.
“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 of its intense intensity, it pushes electrons out of its path, like water being pushed aside by the front of a boat. These electrons orbit around the boat and collect just behind it, traveling in a pulse wake.”
The wakefield field acceleration was first proposed in 1979 and demonstrated in 1995. But the distance that can increase the speed of electrons has been stubbornly limited to a few centimeters. What enabled the UMD team and CSU to take advantage of Wakefield acceleration more effectively than ever before was a technology the UMD team devised to tame the high-powered beam and prevent it from spreading its energy too thinly. Their technology punches a hole in the plasma, creating a waveguide that keeps the beam energy focused.
“The waveguide allows the pulse to propagate over a much longer distance,” Schrock explains. “We need to use plasma because these pulses are very high energy, they are very bright, and they can destroy conventional fibres. optical cable. Plasma cannot be destroyed because it is already there.”
Their technology creates something similar to fiber-optic cables – the things that carry fiber-optic internet service and other communication signals – out of thin air. Or, more precisely, from carefully sculpted hydrogen gas jets.
A traditional fiber-optic waveguide consists of two components: a central “core” that directs the light, and an enclosed “cap” that prevents the light from escaping outwards. To make the plasma waveguide, the team uses an additional laser beam and a jet of hydrogen gas. When this extra “guideline” laser travels across the plane, it rips electrons off the hydrogen atoms and creates a channel of plasma. The plasma is hot and begins to expand rapidly, resulting in the formation of a less dense “core” of plasma and higher-density gas at its edges, like a cylindrical shell. Then, the main laser beam (which will collect the electrons in its wake) is sent through this channel. The leading edge of this pulse converts the high-density envelope into plasma as well, creating the “cladding”.
“It’s kind of like the first pulse is cleaning an area, and then the high-intensity pulse goes down like a train with someone standing in the front throwing the bars as they go,” Schrock says.
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 an astonishing 5 GeV. That’s still three times less than the massive SLAC accelerator, not the maximum achieved with the Wakefield laser’s acceleration (that honor belongs to a team at Lawrence Berkeley National Laboratories). However, the laser power used for each GeV acceleration in the new work is a record, and the team says their method is more versatile: It can produce electron impulses thousands of times per second (as opposed to roughly once per second), making it a promising technology. For many applications, from high energy Physics for generating X-rays that can capture videos of molecules and atoms in action like LCLS. Now that the method has been proven successful, the team is planning to improve the setup to improve performance and increase acceleration to higher energies.
“Currently, electrons are generated along the entire waveguide length, 20 cm long, which makes their energy distribution less than ideal,” Miao says. “We can improve the design so that we can control exactly where to inject it, and then we can better control the quality of the accelerating electron beam.”
While the dream of tabletop LCLS is not yet a reality, the authors say this work shows a way forward. “There’s a lot of engineering and science to do now and then,” Schrock says. “Conventional accelerators produce high-frequency beams with all electrons having similar energies and traveling in the same direction. We are still learning how to improve these beam features in Wakefield field accelerators with multi-GeV lasers. We will also likely achieve energies on the scale of tens of GeV, we will need to regulate Many wakefield accelerators, to pass accelerated electrons from one stage to another while maintaining beam quality.So there is a long way between now and a LCLS-based facility. laser Wakefield acceleration. ”
B. Miao et al., Multiple GeV electron beams from the Wakefield all-laser field accelerator, X . physical review (2022). DOI: 10.1103/ PhysRevX.12.031038
University of Maryland
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