With Just Light A Compact Electron Accelerator Hits New Speeds

By James Anderson •  Updated: 09/19/22 •  7 min read

Researchers have used ultrafast lasers under precise control to accelerate electrons over a 20-centimeter stretch to speeds typically only achieved in particle accelerators the size of ten football fields.

The feat was reached by using two laser pulses sent through a jet of hydrogen gas, by a team at the University of Maryland (UMD) headed by Professor of Physics and Electrical and Computer Engineering Howard Milchberg, collaborating with the team of Jorge J. Rocca at Colorado State University (CSU).

The first laser pulse broke through the hydrogen, piercing it and forming a plasma channel. The second, more powerful pulse was guided by that channel and scooped up electrons from the plasma, dragging them along in its wake, and accelerating them to almost the speed of light.

Everything Powered By Lasers

Experimental setup. LWFA drive laser pulse

Experimental setup. LWFA drive laser pulse. Credit: B. Miao, et al CC-BY

By using this technique, the team was able to accelerate electrons to an energy that was roughly 40% of what was possible at enormous facilities like the kilometer-long Linac Coherent Light Source (LCLS), an accelerator at SLAC National Accelerator Laboratory.

“This is the first multi-GeV electron accelerator powered entirely by lasers. And with lasers becoming cheaper and more efficient, we expect that our technique will become the way to go for researchers in this field,”

said Milchberg.

Accelerators like LCLS, which has a kilometer-long runway and can accelerate electrons to 13.6 billion electron volts (GeV), or the energy of an electron travelling at 99.99999993 percent the speed of light, are what inspired the current research. Three Nobel Prize-winning discoveries about basic particles were made thanks to LCLS’s predecessor.

The most potent X-ray laser beams in the world are produced utilising the super-fast electrons produced by the LCLS, which is now a third of the initial accelerator. These X-rays are used by scientists to view atoms and molecules in motion while generating movies of chemical reactions. These videos are essential resources for developing new drugs, improving energy storage, developing electronics, and many other things.

Lightning Storm Damage

It takes a lot to accelerate electrons to tens of GeV of energy. The linear accelerator at SLAC propels electrons by employing strong electric fields that go through a very long string of divided metal tubes. The tubes would suffer significant damage if the electric fields were any stronger because they would cause a lightning storm inside the tubes.

Since researchers are unable to push electrons harder, they have chosen to press them for longer, giving the particles more room to accelerate. This is why a kilometer-long swath of northern California was cut. The UMD and CSU teams tried to accelerate electrons to almost the speed of light using light itself in order to scale down this technology.

“The goal ultimately is to shrink GeV-scale electron accelerators to a modest size room. You’re taking kilometer-scale devices, and you have another factor of 1,000 stronger accelerating field. So, you’re taking kilometer-scale to meter scale, that’s the goal of this technology,”

said co-first author Jaron Shrock, graduate student in physics at UMD.

Laser Wakefield Acceleration

 a laser pulse drives a plasma wave, accelerating electrons in its wake

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

The process of laser wakefield acceleration, which involves sending a pulse of highly concentrated laser light across a plasma to produce a disruption and drag electrons in its wake, is used to produce those stronger accelerating fields in a lab.

First hypothesised in 1979, laser wakefield acceleration was demonstrated in 1995. However, the few centimetres that it could accelerate electrons across tenaciously remained its maximum range.

“You can imagine the laser pulse like a boat. As the laser pulse travels in the plasma, because it is so intense, it pushes the electrons out of its path, like water pushed aside by the prow of a boat. Those electrons loop around the boat and gather right behind it, traveling in the pulse’s wake,”

co-first author Bo Miao, a postdoctoral fellow in physics at the University of Maryland, said.

The UMD team developed a method to control the high-energy beam and prevent it from dispersing its energy too thin, which allowed the UMD and CSU team to utilise wakefield acceleration more efficiently than ever. Their method creates a waveguide by punching a hole through the plasma, which maintains the beam’s energy focus.

“A waveguide allows a pulse to propagate over a much longer distance. We need to use plasma because these pulses are so high energy, they’re so bright, they would destroy a traditional fiber optic cable. Plasma cannot be destroyed because in some sense it already is,”

Shrock explained.

Method Creates An Effect Similar To Fiber Optic Cables

A normal fibre optic waveguide has two parts: a centre “core” that directs the light and an outer “cladding” that shields it from evaporation. The team employs a jet of hydrogen gas and a second laser beam to create its plasma waveguide.

This additional “guiding” laser tears the electrons from the hydrogen atoms as it passes through the jet, forming a plasma channel. The hot plasma immediately begins to expand, forming a cylinder-shaped “core” of lower density plasma with a ring of greater density gas surrounding it.

The primary laser beam, which will collect electrons in its wake, is then directed via this channel. The “cladding” is produced when the very front edge of this pulse converts the higher density shell to plasma as well.

The researchers were able to accelerate some of their electrons to an astounding 5 GeV using the optically produced plasma waveguide technology developed at UMD and the high-powered laser and expertise provided by the CSU team. This is not quite the maximum attainable with laser wakefield acceleration and is still three times slower than SLAC’s massive accelerator.

However, the new work sets a record for the amount of laser energy utilised per GeV of acceleration, and the team claims their method is more adaptable: It is a promising method for various applications, from high energy physics to the creation of X-rays that can capture video of molecules and atoms in movement like at LCLS. It has the ability to produce electron bursts thousands of times per second (instead of around once per second).

Next Steps Forward

The team wants to tweak the setup to maximise performance and accelerate to greater energies now that they have proven the technology works. Future work will extend this technique to longer plasmas, higher laser intensities, and multiple staged modules to achieve electron acceleration to 100 GeV.

Although the dream of LCLS on a tabletop is not yet a reality, the authors claim that this work illustrates a future course.

“There’s a lot of engineering and science to be done between now and then. Traditional accelerators produce highly repeatable beams with all the electrons having similar energies and traveling in the same direction. We are still learning how to improve these beam attributes in multi-GeV laser wakefield accelerators. It’s also likely that to achieve energies on the scale of tens of GeV, we will need to stage multiple wakefield accelerators, passing the accelerated electrons from one stage to the next while preserving the beam quality. So there’s a long way between now and having an LCLS type facility relying on laser wakefield acceleration,”

concluded Shrock.

Study: B. Miao et al, Multi-GeV Electron Bunches from an All-Optical Laser Wakefield Accelerator, Physical Review X (2022).

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