Scientists Made Electrons 'Surf' on Waves of Plasma in Groundbreaking Physics Experiment

Scientists at the European Organization for Nuclear Research (CERN)—operators of the world’s largest particle physics lab—have conducted a groundbreaking experiment demonstrating a new technique for accelerating electrons to very high energies over short distances.

The latest achievement could enable engineers to drastically reduce the size of future particle accelerators, cutting down on the vast amounts of money normally required to build them. The high-energy particle collisions these facilities produce enable physicists to probe the fundamental laws of nature, providing the basis for advancements in a huge variety of different fields.

Researchers from CERN’s AWAKE project (short for "Advanced WAKEfield Experiment") successfully accelerated electrons using waves of plasma, generated by a beam of protons, for the first time, according to a study published in the journal Nature. Electrons and protons are both subatomic particles with negative and positive charges respectively, while plasma is the fourth fundamental state of matter, produced by removing the electrons from gas atoms in a process known is ionization.

Despite the fact that AWAKE's technology is still at an early stage in development, the acceleration that the scientists achieved over a given distance is already significantly higher that what is possible in conventional particle accelerators such as CERN’s Large Hadron Collider (LHC)—the largest and most powerful of these facilities in the world, consisting of a 17-mile "ring" of magnets.

“The huge size of the LHC and future collider designs based on conventional technologies show that we have to develop new and alternative ideas for accelerating particles to even higher energies, and that’s the context of AWAKE,” Edda Gschwendtner, Technical Coordinator and CERN Project Leader for AWAKE, told Newsweek.

Using waves of plasma, or “wakefields,” to accelerate electrons is not an entirely new idea. In fact, physicists first proposed the method in the 1970s. But the actual technology required to do this has only recently been developed.

Conventional particle accelerators make use of so-called cavities, explains Gschwendtner. “These are, in principle, evacuated metal boxes or cylinders where large oscillating electric fields are excited by electromagnetic energy into them,” she said. “These fields accelerate the particles.”

In plasma wakefield acceleration, on the other hand, the particles get accelerated by essentially “surfing” on top of a plasma wave. This process involves two different beams: the beam of particles that is the target for the acceleration (known as the “witness beam”) and the beam that generates the plasma waves, or wakefield, itself (known as the “drive beam”).

In the AWAKE experiments, the scientists heated rubidium, a type of metal, until it turned into a gas. They then targeted this gas with a laser beam in order to ionize it—removing electrons from the atoms—thus converting it into plasma.

Then, they shot the drive beam—which in this case is made of protons—into the plasma causing it to oscillate in a wavelike pattern, much like how a boat moving through water generates waves in its wake. The team also injected a beam of electrons—the witness beam—into the oscillating plasma. The electrons in this beam were then accelerated by the waves of plasma.

“Imagine a boat on a lake and some surfers waiting for a ride,” Gschwendtner said. “The boat passes by the surfers—it produces waves—the surfers jump on the wave, and ride with it. They get accelerated.”

“We do the same in plasma wakefield acceleration,” she explained. “We use plasma (the lake), send a drive beam (the boat) through the plasma to create the wakefields (the waves) and then inject particles to be accelerated (the surfers). By doing that one can potentially produce accelerating fields by a factor 1,000 [times] larger than in conventional cavities.”

A handful of research groups have achieved plasma wakefield acceleration already, however, the AWAKE demonstration is the first to generate the plasma waves using protons. This has advantages as protons are capable of carrying high energies over large distances.

“The plasma wakefields can be produced either by powerful lasers, electron beams or also by an intense proton beam, which is tried for the first time in AWAKE,” Gschwendtner said.

“Protons are especially interesting as they carry a high energy and can therefore penetrate deeper into the plasma than drive beams of electrons and lasers,” she said. “Therefore, wakefield accelerators relying on protons for their drive beams can accelerate their witness beams for a greater distance and consequently to higher energies.”

The proof-of-concept AWAKE experiments managed to accelerate electrons to a factor of 40 times higher over ten meters than acceleration in the LHC using conventional technologies.

Over the coming years, the AWAKE scientists hope to refine their method and accelerate the electrons to higher and higher energies while maintaining the quality of the electron beam. The ultimate goal is to build a high-energy particle accelerator based on this technology.

59_MA2_1629 The plasma cell used in the AWAKE experiments. CERN

“Accelerating particles to larger and larger energies gives us more understanding of the world of elementary particles and the fundamental forces between them,” Gschwendtner said. “With the recent discovery of the Higgs [boson] we have a fairly consistent idea about what matter consists of. But there are many open questions to be studied.”

The Higgs boson is a fundamental particle—one that is not composed of other particles—which gives all other particles mass. Scientists confirmed its discovery in 2013 but many questions remain regarding its properties. Nevertheless, fascinating new details about the particle emerged only this week: CERN researchers announced that they had finally witnessed it decaying into another fundamental particle known as a bottom quark—a process scientists have wanted to observe since it was first detected.

The AWAKE team are not the only ones trying to reduce the space required to conduct particle acceleration experiments. Scientists from Imperial College London have recently developed a technique that could be capable of accelerating antimatter in a space around a thousand times smaller than what current accelerator facilities require. (Antimatter is any material composed of the antiparticle to the corresponding particle of ordinary matter).

This approach and the research being carried out at AWAKE could help to advance the science of exotic particles and help researchers to unravel some of the biggest mysteries in physics, such as the properties of the Higgs boson and the nature of hypothetical dark matter and dark energy, which are thought to make up much of the universe but have never been directly observed.

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