Particle physics experiments are huge—they have to be, in order to accelerate particles with enough energy to properly study them. The Large Hadron Collider in Switzerland is nearly 17 miles around, while others are closer to the 2-mile range. But scientists working on a new experiment reported Wednesday that they’ve accelerated electrons to high energies in just 33 feet.
The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) at CERN in Geneva is a prototype for a new kind of machine that could accelerate electrons over a fraction of the distance needed by other accelerators. AWAKE still relies on protons created in CERN’s large rings, but “wakefield” accelerators like these could become a game-changing technology for the future of particle physics.
“What I think is exciting about wakefield acceleration is the possibility of developing for a future where accelerators will be much shorter with much reduced costs,” Matthew Wing, professor of physics at University College London, told Gizmodo.
Typically, particle physics experiments use oscillating electric fields, called radiofrequency cavities, and high-powered magnets to accelerate particles to high energies. But these experiments must grow quite large if scientists hope to produce the highest-energy particles.
A promising candidate to accelerate particles more efficiently is the wakefield accelerator. Physicists send a beam of either electrons, protons, or a laser through a plasma. Free electrons in the plasma move toward the beam, but overshoot it, then come crashing back, creating a bubble structure behind the beam and intense electric fields. If you inject particles, like more electrons, into the wake, it can accelerate the injected particles in a shorter amount of time with an electric field 10 or more times stronger.
Wing and his colleagues announced that for the first time, using a beam of protons from CERN’s 4.3-mile-round Super Proton Synchrotron (SPS), they were able to create a beam of electrons with 2 giga-electron volts (GeV) of energy in just 10 meters, or 33 feet, of plasma, according to the paper published in Nature. And they think that they could scale up the device to make a powerful accelerator a fraction the size of present-day electron accelerators.
“We could potentially get an electron beam to the tera-electron volt scale over a relatively short distance—which is almost unfeasible with [other] accelerators,” said Wing. That’s a thousand times more energy than the AWAKE electrons currently have.
For reference, an electron volt is how much energy an electron gains by moving across a single volt of electric potential. A cathode ray tube in an old television might accelerate electrons to 20,000 electron volts, or 20 keV. Two high-powered electron accelerators in the US can accelerate electrons to 6 billion and 50 billion electron volts (6 to 50 GeV), but are around a mile long or more. AWAKE was able to accelerate electrons to 2 GeV in just 33 feet of plasma. According to Wing’s simulations, AWAKE might soon be able to accelerate electrons to 50 GeV in 245 feet of plasma.
While the plasma field is only 33 feet long, it still requires a 4.3-mile-round SPS accelerator create the proton beam. But scientists don’t need to use protons to accelerate the electrons. They can also use laser pulses, and physicists have used the building-sized Berkeley Lab Laser Accelerator to create GeV-energy electrons with laser pulses.
CERN may one day be able to build a reasonably sized but incredibly powerful electron accelerator powered by proton beams from either the LHC or the SPS. Electrons, being smaller and simpler than protons, make a useful probe for learning more about complex structures like atomic nuclei.
Mark Hogan, a physicist working on accelerator science at the SLAC National Accelerator Laboratory in the US who was not involved in the study, expressed excitement at the new results. This was, after all, the first successful wakefield acceleration using a protons to create the wake. But he explained that there’s a lot more work for AWAKE on the horizon, including increasing the energy and number of accelerated electrons and focusing the electron beams. He also mentioned that this wouldn’t replace linear colliders like the planned International Linear Collider, but would be a complimentary technology.
Five to 10 years from now, Hogan said, we may begin to see the first physics experiments relying on wakefield technology, perhaps wobbling the accelerated electrons to produce high-powered laser beams that would be useful for biological or physics studies.
CERN will soon shut down for a few years for an upgrade, allowing AWAKE to upgrade as well. Wing has loftier goals. “I think we can soon achieve the highest-energy electron beam around.” [Nature]