The Particle Accelerators of the Future May Be Based on Surfing Antimatter

High-energy physics isn't planning on taking a post-Large Hadron Collider breather. In fact, the international research community had been scheming up a successor well before the LHC even started smashing protons. The most advanced project to this end is the International Linear Collider, which will become the longest particle collider in the world with at least 19 miles of tunneling.

Length is important because of a parameter known as the maximum energy gradient. If we imagine charge as gravity and particles as bowling balls, the maximum energy gradient would be like the steepness of a hill. A steep hill can make the bowling ball roll very fast in a short amount of space, but a less steep hill can accomplish that by adding length. So, once the maximum energy gradient is reached, we're forced to instead add more length to get he desired energies. Hence, long-ass tunnels.

What if there were another way? Physicists at the SLAC National Accelerator Laboratory in the United States have indeed come up with a more efficient alternative, according to a paper published last week in Nature. It's called plasma wakefield acceleration, which has been around since 2007 but, until now, with the pretty huge limitation of only being able to accelerate electrons. The desired collisions for next-generation experiments are electrons and positrons (the anti-matter partners of electrons) for the reason that they result in "cleaner" showers of collision products. The catch is that smashing positrons and electrons together takes a whole lot of energy.

The principle behind wakefield acceleration is pretty neat. It starts with a wad of plasma, which is where a bunch of atoms are basically shredded into free-floating electrons and naked atomic nuclei, e.g. positively charged ions. This wad is overall electrically neutral, but that's not so much atom by atom as it is an overall average. The positive and negative charges are all accounted for, but they're just not stuck together into neutral atoms. Any given plasma has a plasma "density," which relates to the proportion of free electrons floating around (vs. those bound to a nucleus).

A beam of electrons is then fired at the plasma, which has the effect of repelling all of the free electrons, which push away in dense waves. Immediately behind the first burst of electrons is a second, which is able to slide in behind these waves and effectively "surf" the resulting cavity. This is the electron version of wakefield acceleration, anyhow: two bolts. The new, positron version adds a twist.

First off, using positron bolts has a fairly obvious barrier. An electron bolt repels the like-charged electrons in the plasma while the positrons attract them, so instead of a cavity, the positrons face a wall of electrons. As a result, they slow down instead of speeding up. There is, however, a tipping point where a component of the plasma electric field switches sign and the positive charges repel. The positive charge pulls in a bunch of negative charges and, eventually, there are enough collected there that any positrons should be accelerated away.

The problem is that there aren't really any positrons left at this point to be sling-shotted back out. The solution reached by the SLAC group is to stretch the bolt of positrons out, so it's sort of a cylinder. The front part of the bolt "loads" the plasma for the tail, which takes advantage of the energy added to the plasma by initial positrons. Note that we're not talking free energy here.

"The overall energy of the bunch is obviously not going to be increased, because energy must be conserved," Sebastien Corde, a SLAC researcher and paper co-author, told Physics World. "We are just transferring energy from the front to the tail. What's important for particle colliders is that each particle has a very large energy."

In a separate Nature commentary, Philippe Piot, a physicist at Northern Illinois University not affiliated with the current study, explains a bit deeper: "Corde et al. observe that the large positron population (about 1 billion positrons) experiencing the accelerating field effectively 'loads' the wakefield and affects its shape, leading to an approximately uniform energy gain for the accelerated positrons. The experiment therefore demonstrates that, by using appropriate operating parameters, only one positron bunch is needed for acceleration: part of its trailing population is 'trapped' and accelerated quasi-uniformly to higher energies, and so splits from the initial bunch."

The improvement offered is substantial, perhaps as much as doubling the energies offered by a conventional accelerator, though there are still some barriers, including the fact that only a fairly small portion of positrons are actually accelerated with this method. "We have a good idea that it could work," Corde continued, "but it's also a technically challenging experiment."