44 | New Scientist | 27 July 2019atoms, 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 highestMA
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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, UKThe AWAKE
collaboration
at CERN aims
to build a
small plasma
accelerator“One plasma beam
boasts 10,000 times
the LHC’s rate of
acceleration”