Could plasma-surfing electrons star in future particle accelerators?

To find the tiny subatomic particle known as the Higgs boson, scientists had to build an accelerator with a 17-mile circumference -- but with a little unconventional technology, such giant machines could one day be a thing of the past.

A team led by researchers at Stanford’s SLAC National Accelerator Laboratory has used a plasma-based device that’s just a few inches longer than a paper-towel tube to punch particles up to energies 500 times higher than they could reach in the same distance in a typical accelerator.

“What might take you hundreds of meters to do in a regular machine, you can do in the space of just a meter, or a couple of feet,” said study coauthor and SLAC physicist Mark Hogan.

The device, called a plasma wakefield accelerator, superpowers electrons by surfing them on the wake of another burst of particles. Described in the journal Nature, the technology could one day allow researchers to build smaller or more powerful accelerators that could explore the mysteries of the universe beyond the limits of today’s technology.

Many of today’s accelerators shoot bursts of charged particles (either protons or electrons) through a giant tube, pushing them close to the speed of light. Smash two such beams together, and the collision produces a shower of new particles for physicists to study.

But there’s a limit to the energy that current accelerators can put into a particle beam — which means there’s a limit to what scientists can study. Who knows what particles beyond the Higgs boson remain undiscovered at higher energy levels?

“If we keep using the same technology to accelerate those particles, that means we’d have to keep building bigger and bigger accelerators,” said study lead author Michael Litos, a SLAC particle physicist.

The problem with building a bigger accelerator? That requires a lot of money, real estate and political support.

For this project, the pair prepped electrons by revving them up to about 20 billion electron volts using SLAC’s conventional main accelerator – but then switched to an experimental method using plasma. Plasma, Litos said, can support extremely high electric fields that could damage a typical accelerator.

They shot a burst of electrons – called a “drive bunch” – through a roughly 14-inch chamber filled with lithium plasma. Since a plasma’s protons and electrons float separately from each other, this drive bunch easily pushes all the plasma’s untethered electrons out of its way. (The plasma’s massive protons, however, remain right where they are.)

But shoving the plasma’s negatively charged electrons away from its positively charged protons is like stretching a rubber band: The two ends want to snap back together. There’s a lot of power in that snap, and the scientists wanted to take advantage of it.

So the researchers quickly shoot another burst of electrons -- called the “trailing bunch” -- into the plasma, straight into the drive bunch’s wake. As the wake closes right behind the trailing bunch, the plasma electrons whoosh back into place — and the force of that whoosh pushes the trailing bunch of electrons forward, giving them extra power.

The process is akin to a surfer riding a “boat wave”: Like a boat in the water, the “drive pulse” of electrons creates the wave, and the “trailing pulse” of electrons surfs that wave.

Precision is key to the process, the scientists added. They have to ensure that the billions of electrons in the bursts each gain the same amount of energy. In order to be scientifically useful, Litos said, all the electrons have to be surfing at the same speed, in a tiny, highly choreographed party wave.

Now, in this experiment, the researchers were able to put roughly 2 billion electron volts’ worth of energy into a burst of electrons. That’s not anywhere near the energy produced in accelerators such as the proton-smashing Large Hadron Collider, which discovered the Higgs boson at about 125 billion electron volts. (Keep in mind: The Large Hadron Collider is 17 miles long; this plasma container was about 14 inches.)

By daisy-chaining several yard-long tubes of plasma together, the scientists say they could make the trailing electrons surf to higher and higher energies by generating a fresh plasma wave in each chamber. The result: a small, extremely high-energy particle accelerator.

“The result might herald a new generation of compact ‘plasma afterburners’ that could boost the energy of conventional particle accelerators and potentially reduce the skyrocketing cost of high-energy physics machinery,” Mike Downer and Rafal Zgadzaj, physicists at the University of Texas at Austin who were not involved in the paper, wrote in a commentary.

The hope, the Stanford researchers said, is to use this technology in one of two ways. First, smaller, cheaper but equally powerful accelerators would mean that universities would no longer have to fly physicists across oceans to get to the machine they need. Hospitals, which use such devices for medical diagnoses and treatment, could just as easily stick an accelerator in their basements.

But the researchers say they could also make large plasma wakefield accelerators that could be hundreds, even thousands, of times more powerful than current accelerators – allowing them to explore new frontiers in particle physics.

“You want to keep probing at higher and higher energies to see what’s out there,” Litos said. “It gives us a more complete picture of the underlying physics of the universe.”

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