New Scientist - July 27, 2019

44 | New Scientist | 27 July 2019

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.”

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

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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.” ❚

Jon Cartwright is a consultant 

for New Scientist based in

Bristol, UK

The AWAKE

collaboration

at CERN aims

to build a

small plasma

accelerator

“One plasma beam

boasts 10,000 times

the LHC’s rate of

acceleration”