Last year, physicists working on the Advanced Wakefield collaboration at CERN added an electron source and beamline (pictured) to their plasma wakefield accelerator.
Maximilien Brice, Julien Ordan/CERN
The world’s biggest atom smasher is 27 kilometers in circumference and it cost $5 billion. But much smaller particle accelerators—perhaps just 1 kilometer long—may be on the horizon, delivering similar energies at a fraction of the cost, according to research published today.
The results mark “a major breakthrough” in the quest for cheaper high-energy colliders, says Ralph Assmann, an accelerator physicist at the German Electron Synchrotron (DESY) laboratory in Hamburg, who was not involved in the work.
As particles circulate in the Large Hadron Collider (LHC) at the CERN laboratory near Geneva, Switzerland, or some other conventional accelerator, physicists ramp up their energy by passing them through a series of metal cavities that resonate with radio waves, just as organ pipes ring with sound waves. Time things just right and the passing particles surf the radio waves’ electric fields to gain energy. But the higher the required energy, the more cavities are needed. That means longer—and more expensive—accelerators. The proposed International Linear Collider (ILC), for example, would smash electrons and positrons together at an energy of 250 billion electronvolts (eV) in a tunnel 20 to 40 kilometers long, and would cost at least $7 billion.
In contrast, the Advanced Wakefield (AWAKE) collaboration at CERN has developed an accelerator that consists mainly of a 10-meter-long tube of rubidium gas. Researchers inject into the gas bunches of protons from one of CERN’s lower power accelerators, as well as intense laser pulses. The latter ionize the gas, creating a plasma that consists of positively charged ions and negatively charged electrons.
As each proton bunch passes through the plasma at near–light speed, it draws surrounding electrons toward its axis of travel. While the proton bunch moves forward, the electrons converge on the axis, creating a pocket of negative charge in the bunch’s wake. The same process plays out slightly farther down the tube, creating a second clump of electrons ahead of the first, while the momentum of the original group causes those electrons to overshoot their mark and leave a region dominated by heavier, more static ions. The upshot is a “wakefield” of alternating positive and negative regions set up behind the proton bunch, which are then used to accelerate other particles.
Having observed wakefields last year, the AWAKE team has now carefully injected electron bunches into the plasma so that they get pushed by a negative region of charge behind them and pulled by a positive region ahead. This pushing and pulling ought to be so strong that electrons can be accelerated to very high energies over a much shorter distance than is possible with radiofrequency cavities.
With the particle bunches and laser pulses traveling at close to the speed of light, synchronizing the three sets of injections so that the electrons in effect surf the wakefield is far from straightforward. But in a study published in Nature, the collaboration reports having boosted electrons to 2 billion eV in its tube of plasma. That means that the electrons gain energy at 0.2 billion eV per meter, an “accelerating gradient” that is roughly double that of the best radiofrequency accelerator.
As impressive as that is, other teams have already done better. Creating wakefields using laser pulses or other electrons, rather than protons, physicists have previously achieved gradients of up to 100 billion eV per meter. But those gradients are limited to very short distances because laser pulses and electrons quickly run out of energy. Delivering the very high energies needed to study exotic new particles would, therefore, involve joining a long line of plasma accelerators together—a synchronization problem far more complex than that posed by a single accelerator, according to AWAKE deputy spokesperson Matthew Wing, a particle physicist at University College London.
In contrast, physicists think that by using protons they need only employ a single stretch of plasma. That’s because it’s possible to rev protons up to much higher energies to begin with. Indeed, Wing and colleagues have worked out that the 7-trillion-eV protons produced by the LHC would boost electrons to 1 trillion eV in a single 1-kilometer-long device—although in such a scheme you still need an accelerator as big and expensive as the LHC to begin with.
Wing says a cut-price version of the ILC is probably not on the cards at the moment. Such a machine would require exquisitely compact beams with a very precise energy, and plasma accelerators cannot yet produce such high-quality beams. But he reckons that AWAKE’s technology could be used within the next 20 years to transform the LHC into an electron-proton collider to investigate whether particles such as quarks and electrons have any internal structure. Such a facility, he estimates, would cost only about a 10th of its radiofrequency equivalent.
Before embarking on such an ambitious project, however, the collaboration needs to make tighter electron bunches and ramp up their acceleration. Over the next 5 years, the researchers hope to boost their accelerator’s gradient so that it reaches perhaps 10 billion eV over 10 to 20 meters. “At that point, we could start to think about using these beams for particle physics experiments,” Wing says.
Brian Foster, a particle physicist at the University of Oxford in the United Kingdom, is “highly impressed and pleasantly surprised” at how quickly AWAKE has turned a theoretical proposal (from 2009) into experimental reality. But he doubts that a plasma-based electron-proton collider could produce enough new physics results to justify what he foresees will be “the very considerable expense” of building and operating such a machine. Still, for physicists working on plasma accelerators the prospect of huge future savings makes the technology well worth pursuing. Surf’s up!
