Compact electron accelerator reaches new speeds with nothing but light


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.

This feat was accomplished utilising two laser pulses transmitted through a stream of hydrogen gas by a team led by Professor of Physics and Electrical and Computer Engineering Howard Milchberg at the University of Maryland (UMD) and Jorge J. Rocca's team at Colorado State University (CSU). The first pulse shattered the hydrogen, piercing it with a hole and forming a plasma channel. The second, more powerful pulse was led by that channel and it scooped up electrons from the plasma and dragged them along in its wake, accelerating them to almost the speed of light.

With this method, the team was able to accelerate electrons to an energy that was roughly 40% of what was possible at large facilities like the kilometer-long Linac Coherent Light Source (LCLS), an accelerator at SLAC National Accelerator Laboratory. On August 1st, 2022, the paper was approved by the journal Physical Review X.

According to Milchberg, who is also connected to the University of Maryland's Institute of Research Electronics and Applied Physics, this is the first multi-GeV electron accelerator that is totally powered by lasers. "And we anticipate that our method will become the standard for researchers in this field as lasers grow more affordable and effective," the authors write.

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% the speed of light, are what inspired the latest 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 movies are essential resources for developing new drugs, improving energy storage, developing electronics, and many other things.

It takes a lot of effort 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—appropriately enough—light itself in order to scale down this technology to a more manageable level.

The ultimate objective, according to Jaron Shrock, a PhD student in physics at the University of Maryland and a co-first author on the paper, is to reduce GeV-scale electron accelerators to the size of a modest room. "You are using kilometer-scale devices and an accelerating field that is 1,000 times stronger. The aim of this technology is to translate kilometer-scale to meter-scale."

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

Bo Miao, a postdoctoral scholar in physics at the University of Maryland and a co-first author on the paper, believes that the laser pulse can be compared to a boat. "Due to its high intensity, the laser pulse pushes the electrons out of its path as it moves through the plasma, much like a boat's prow does with water. Traveling in the wake of the pulse, those electrons circle around the boat and assemble directly behind it."

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

Shrock explains that a waveguide "allows a pulse to travel across a significantly greater distance." "Due to the enormous intensity and brightness of these pulses, conventional fibre optic cables would be destroyed without the usage of plasma. Since plasma is already destroyed in a certain sense, it cannot be destroyed."

Their method generates fibre optic cables, which are used to transmit telecommunications signals and fibre optic internet service, out of thin air. Or, to be more specific, out of expertly crafted hydrogen gas jets.

A typical 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 very first pulse, according to Shrock, "clears out a region, and then the high-intensity pulse comes down like a train with someone standing at the front throwing down the tracks as it's going."

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 enormous accelerator (that honour belongs to a team at Lawrence Berkeley National Labs). 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 take movies 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). The team wants to tweak the setup to maximise performance and accelerate to greater energies now that they have proven the technology works.

The electrons are now produced along the entire 20-centimeter length of the waveguide, which results in a less than perfect energy distribution, claims Miao. We can enhance the design so that we can accurately regulate where they are injected, which will allow us to better manage the accelerated electron beam's quality.

Although the dream of LCLS on a tabletop is not yet a reality, the authors claim that this work illustrates a future course. Between now and then, a lot of engineering and science work needs to be done, according to Shrock. "Traditional accelerators produce extremely reproducible electron beams with uniform electron energy and directions of motion. The best ways to enhance these beam characteristics in multi-GeV laser wakefield accelerators are currently being discovered. Additionally, it's likely that we will need to stage many wakefield accelerators in order to obtain energies on the order of tens of GeV, transferring the accelerated electrons from one stage to the next while maintaining the beam quality. There is still a long way to go until we have a facility like LCLS that uses laser wakefield acceleration."

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