TO GET to the very bottom of physics, there has always been one rule: size matters. The first particle smashers of the early 1960s were little wider than a dining room table. A decade later, the Tevatron, a circular collider in the US, had a circumference of 6 kilometres. Today’s largest machine, the Large Hadron Collider (LHC), has one four times as long. Now there are plans to build colliders 100 kilometres in circumference: about the size of New York City.
Physicists get a lot of flak for these enormous – and enormously expensive – aspirations. Nature is tenacious, however, and wresting its most closely held subatomic secrets from it has always meant accelerating particles over longer and longer distances before smashing them together. But a new shortcut is emerging in a weird, cloud-like state of matter known as a plasma. Inject particles into this febrile stuff, and they can accelerate a thousand times faster than before.
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This is more than wishful thinking. Plasma accelerators have been advancing steadily over the past few decades, and while they have yet to pose a serious threat to the dominance of conventional facilities, that might be changing. Several recent developments suggest that plasma accelerators could soon give big beasts like the LHC a run for their money. Ultimately, the hope is that these small machines will let us tackle some of the biggest questions in physics: why our universe is filled with matter and not antimatter, for instance, or what constitutes dark matter. It seems the ironclad rule of particle physics is about to be broken.
Certainly, technology everywhere else is shrinking. But conventional particle accelerators have always suffered from an intrinsic unshrinkability. Beneath their hugely complicated steering and control systems are lots of little metal pipes. These individually kick particles forward with short, strong electric fields. The stronger the fields between these pipes, the more powerful the kicks – but only up to a point. Beyond a certain field strength, the space between the pipes becomes conducting, discharging the fields like a lightning bolt and stopping the acceleration. To make the overall accelerator more powerful, therefore, you have to add not stronger kicks but more of them – in other words, more pipes.
“Plasma accelerators could soon give big beasts like the LHC a run for their money”
It was this limit that led to circular, instead of linear, particle smashers: in a circle, particles can keep going round until they reach the desired energy. Within reason. Even circular accelerators have to be very big, otherwise particles simply shed their energy as radiation – or else get flung out of the ring as they skid around tight corners. Hence the LHC. Only a circular collider 27 kilometres in circumference could smash opposing beams of protons with enough energy – up to 13,000 gigaelectronvolts (GeVs) – to produce the famous Higgs boson. Although impressive for confirming a decades-old prediction, the finding of the Higgs in 2012 has been the LHC’s only elementary particle discovery to date, and has left particle physicists wondering what, if anything, to do next. “Many of us thought 27 kilometres was the limit,” says Ralph Assman, a leading scientist at the German accelerator lab DESY.
Very large hadron colliders
Not everyone, though. One long-mooted successor to the LHC is the International Linear Collider (ILC), a $7.5 billion machine up to 30 kilometres long. Japan was set to host it until its government failed to commit in earnest this year. If the plan still goes ahead, the ILC wouldn’t technically be as powerful as the LHC, but it would allow much more precise studies of the Higgs and other particles by smashing together not protons, but electrons and positrons. As these are simpler particles, they would generate very clean collisions.
CERN, the particle physics laboratory near Geneva, Switzerland, that hosts the LHC, has a similar plan to the ILC, albeit up to 50 kilometres long, in the form of the Compact Linear Collider (CLIC). But even that is dwarfed by the lab’s proposed Future Circular Collider (FCC), a machine that would have a whopping 100-kilometre circumference to smash together either electrons and positrons, or protons and protons, at energies up to 100,000 GeV. This would cost up to $25 billion. Meanwhile, China is mulling over a Circular Electron Positron Collider (CEPC) with an equivalent 100-kilometre circumference, but a more economical $6 billion price tag.
These gargantuan machines seem essential to the future of particle physics. But for some researchers, a cheaper, smaller alternative is worth pursuing. As a senior scientist at CERN during the time of the celebrated Higgs discovery, Assman could easily have gone on to work on ever bigger colliders. But scarcely had the champagne corks landed at CERN than he had chosen a new path: plasmas. “I thought the main challenges with conventional accelerators had been overcome,” he says of his decision to move to DESY in 2012. “I thought it would be cooler to innovate new technology, and make it smaller, instead of bigger.”
After solids, liquids and gases, plasmas are sometimes considered the fourth state of matter: an ethereal mix of electrons and the positively charged atomic nuclei, or ions, from which they were stripped. As the electrons and ions move around, tiny electric fields are created and destroyed, making plasma the perfect medium for carrying charged particles. Unlike the hollow space separating a conventional accelerator’s metal tubes, for example, plasma is at no risk of suddenly becoming conductive and neutralising an electric current: it is conductive already.
Appropriately for a technology that has been likened to particles going surfing, plasma acceleration originated in California, a half hour’s drive from the beach. In 1979, John Dawson and Toshiki Tajima at the University of California, Los Angeles, (UCLA) published a theoretical paper that would form the basis for all subsequent work on plasma accelerators. The pair’s idea was to fire a laser into a gas of atoms, creating a plasma and dividing its electrons from its positively charged ions. Trailing in the wake of the laser, this division of negative and positive charges would create a hugely enhanced electric field. Any additional electron injected at just the right place would be propelled across this field, surfing the plasma wake and accelerating over a thousand times faster than it would in a conventional machine.
More than a decade after Dawson and Tajima’s proposal, a group led by Dawson’s colleague at UCLA, Chandrashekhar Joshi, managed to put this into practice, accelerating injected electrons by 7 megaelectronvolts over just a few millimetres. Lasers at that time were comparatively weak, however, and, in order to reach higher energies, he and others realised it could be better to use a pulse of electrons from a conventional accelerator to create the plasma, divide it and be accelerated by it. In this set-up, most of the electrons would lose their energy in creating the wake, while those at the back of the pulse would catch the surf. “Quickly, people realised you didn’t need lasers at all,” says Assman.
Gaining momentum
Led by accelerator scientist Robert Siemann, the Stanford Linear Accelerator Centre (SLAC) in California took up the challenge. By 2005, it had demonstrated that some of the electrons from its existing accelerator could be turbo-boosted by 3 GeVs over 10 centimetres. Two years later, it demonstrated 15 times the energy gain in under a metre – nearly 10,000 times the rate of acceleration at the LHC. “That’s still the record,” says Joshi, whose UCLA group worked with SLAC on the experiment. “But it’s not a fundamental limit.”
“One plasma beam boasts 10,000 times the LHC’s rate of acceleration”
Such swift progress obscures a few sticking points. While maximum energy is important, so is giving all particles about the same boost. Assman believes that the spread of energies generated by plasma accelerators is currently 10 times too broad, which would make the interpretation of particle collisions difficult. Meanwhile, Joshi is concerned by reliability. “Particle colliders are big and expensive for a reason,” he says. “Just like when you turn a switch and expect a light to come on, so particle accelerators have to work 24/7 continuously for weeks at a time.”
One barrier to reliability could be linking and aligning a series of laser or electron pulses, which would be needed to reach the highest energies. Unless, that is, you take the approach of AWAKE, an international collaboration at CERN, which is experimenting with protons – veritable cannonballs next to electrons. Send protons into a plasma, and they can generate an almighty wake that flings injected electrons forwards in one fell swoop. “You don’t need several stages, which complicates a beamline,” says Edda Gschwendtner, the project leader.
Even then, electron acceleration is only one half of the puzzle. To make sure all the energy from a collision goes into making new stuff, electrons need to be collided not with other electrons, but with their antimatter opposites, positrons. These, says Assman, are a different ball game entirely. “If you inject positrons in the same place as you would electrons, they are not accelerated, but decelerated.” Joshi’s group at UCLA did manage to accelerate positrons in 2004, using a positron beam to both create the plasma wake and serve as the source for the accelerating particles, but even he admits that progress on positron acceleration is “way behind” electron acceleration.
In the meantime, plasma accelerators may be of more immediate practical benefit. Without generating any collisions, compact accelerators could make advanced types of radiation therapy for cancer more widely available. They could also probe cutting-edge materials, or enable security staff to check for hidden explosives. In fact, plasma accelerator spin-offs like these could be just five or 10 years away, says Gschwendtner.
Still, a future in particle physics beckons. Earlier this year, building on a recent surge in laser technology, Berkeley Lab in California used a laser pulse with a power of 850 trillion watts to achieve electron energies of nearly 8 GeVs over 20 centimetres in a plasma accelerator.
Numbers like these should give pause for thought: they imply that energies on a par with those achieved at the LHC could be reached in less than a couple of hundred metres, instead of the marathon distances needed at present. If you weigh up the rate at which the new technology is progressing against the 30 years or so necessary to build a next-generation circular collider, says Assman, then plasma accelerators look like a decent bet for the future of particle physics. “On this timescale, we might have a good alternative.”
