A new awakening?

FOR more than 80 years particle physicists have had to think big, even though the things they are paid to think about are the smallest objects that exist. Creating exotic particles means crashing quotidian ones (electrons and protons) into each other. The more exotic the output desired, the faster these collisions must be. That extra speed requires extra energy, and therefore larger machines. The first cyclotron, built in 1931 in Berkeley, California, by Ernest Lawrence, had a circumference of 30cm. Its latest successor, the Large Hadron Collider (LHC) at CERN’s laboratory near Geneva—which reopens for business in March after a two-year upgrade—has a circumference of 27km.

The bill for this big thinking, though, is enormous. The LHC, which started work in 2008, cost $5 billion. An even more ambitious American machine, the Superconducting Super Collider, would have had a circumference of 87km but was cancelled in 1993 after $2 billion had been spent building less than a third of the tunnel it would have occupied. Most particle physicists thus understand that the LHC may be the end of the road for their subject unless they can radically scale down the size and cost of their toys.

And that is what they are now trying to do. A group of them, working at CERN on what is known as the AWAKE collaboration, are experimenting with a way of shrinking their machines using a phenomenon called the wakefield effect. At the moment their devices are closer in size and power to the first cyclotrons than to the LHC. But even when scaled up, wakefield accelerators will not need to approach the LHC in size, for they should pack as much punch as conventional machines 30 times as big.

Rise and shine!

Existing accelerators send beams of charged particles along evacuated pipes interspersed with cavities filled with electric fields. Each field imparts a small kick to the passing particles. Protons are accelerated in circular machines such as the LHC, meaning they pass through every cavity many times. Electron accelerators are linear. The reason for the difference is that when a charged particle has its course bent (which is done using powerful magnets) it radiates energy away. This radiation is inversely related to a particle’s mass and, since protons weigh 1,836 times as much as electrons, radiative losses are far greater for electrons than protons. Electron accelerators, which physicists still like because they can use them to answer questions which protons have difficulty addressing, thus need particularly long (and therefore expensive) pipes and tunnels. The next planned electron accelerator, the International Linear Collider (ILC), would, were it to be built, extend for 31km but would have less than 3% of the LHC’s oomph. The AWAKE collaboration has therefore turned its attention to building a better electron accelerator.

The crucial difference between a conventional accelerator and a wakefield one is that the pipe through which the particles travel is not evacuated. Instead, it is filled with a thin gas heated to such a temperature that its atoms lose some of their electrons, becoming ions and forming a plasma. This plasma can then be used to create an appropriate electrical field.

The AWAKE collaborators plan to employ a plasma of vaporised rubidium (an element that is easily ionised) held in a 10 metre-long tube. Into it, using one of CERN’s existing circular accelerators, they will inject bunches of fast-moving protons. As these positively charged particles zip through the tube, they will attract the plasma’s free electrons (which are negatively charged), creating a region of concentrated negative charge in their wake. This will, in turn, be surrounded by a region of concentrated positive charge caused by the electron-deficient (and so positively charged) rubidium ions. These adjacent charged regions following in the wake of the passing protons will create a strong electric field (hence the name “wakefield”). Such fields can be used to accelerate bunches of electrons injected into the plasma alternately with the protons.

According to AWAKE’s spokesman, Allen Caldwell of the Max Planck Institute for Physics in Munich, the electric fields in a machine like this will be 100 times stronger than those of the ILC. A wakefield machine of equivalent strength would be about a kilometre long. It would, admittedly, also need a separate, circular accelerator to provide the protons. But that could be an existing machine no longer required for its original purpose.

None of this will happen quickly. Though the wakefield effect was discovered in 1979, previous attempts to exploit it have used lasers or electrons rather than protons to punch through the plasma. That works on a benchtop but has proved hard to scale up. Dr Caldwell’s computer models suggest that a proton machine, by contrast, will be so scalable. But they also suggest it will be complicated to build. Timing the injection of first the protons and then the electrons with sufficient precision is tricky, and the computing power needed to model the process has only recently become available. The team thus think they will need six or seven years to build a prototype reliable enough to act as a jumping-off point for a real accelerator.

Whether that would be soon enough for a wakefield-based design to replace the ILC in the minds of particle physics’s planners is moot. They might, understandably, be unwilling to bet the future of their subject on a relatively untested technology. But if their political paymasters balk at the $8 billion the ILC is expected to cost, and the AWAKE collaboration delivers the goods, then they may have no choice.