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CERN ACCELERATING SCIENCE
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ACCELERATORS
CERN hosts a gigantic complex of particle accelerators.
But what are these machines and how do they work?
The linear accelerator Linac4 under construction (Image: CERN)
ACCELERATORS
CERN hosts a gigantic complex of particle accelerators.
But what are these machines and how do they work?
The linear accelerator Linac4 under construction (Image: CERN)
ACCELERATORS
CERN hosts a gigantic complex of particle accelerators.
But what are these machines and how do they work?
The linear accelerator Linac4 under construction (Image: CERN)
ACCELERATORS
CERN hosts a gigantic complex of particle accelerators.
But what are these machines and how do they work?
The linear accelerator Linac4 under construction (Image: CERN)
ACCELERATORS
CERN hosts a gigantic complex of particle accelerators.
But what are these machines and how do they work?
The linear accelerator Linac4 under construction (Image: CERN)
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WHAT IS AN ACCELERATOR?
An accelerator propels charged particles, such as protons or electrons, at high
speeds, close to the speed of light. They are then smashed either onto a target
or against other particles circulating in the opposite direction. By studying
these collisions, physicists are able to probe the world of the infinitely
small.
When the particles are sufficiently energetic, a phenomenon that defies the
imagination happens: the energy of the collision is transformed into matter in
the form of new particles, the most massive of which existed in the early
Universe. This phenomenon is described by Einstein’s famous equation E=mc2,
according to which matter is a concentrated form of energy, and the two are
interchangeable.
The Large Hadron Collider is the most powerful accelerator in the world. It
boosts particles, such as protons, which form all the matter we know.
Accelerated to a speed close to that of light, they collide with other protons.
These collisions produce massive particles, such as the Higgs boson or the top
quark. By measuring their properties, scientists increase our understanding of
matter and of the origins of the Universe. These massive particles only last in
the blink of an eye, and cannot be observed directly. Almost immediately they
transform (or decay) into lighter particles, which in turn also decay. The
particles emerging from the successive links in this decay chain are identified
in the layers of the detector.
Animation showing the path of the particles in the accelerator complex up to
their collisions in the LHC. (Video: Daniel Dominguez/CERN)
HOW DOES AN ACCELERATOR WORK?
Accelerators use electromagnetic fields to accelerate and steer particles.
Radiofrequency cavities boost the particle beams, while magnets focus the beams
and bend their trajectory.
In a circular accelerator, the particles repeat the same circuit for as long as
necessary, getting an energy boost at each turn. In theory, the energy could be
increased over and over again. However, the more energy the particles have, the
more powerful the magnetic fields have to be to keep them in their circular
orbit.
A linear accelerator, on the contrary, is exclusively formed of accelerating
structures since the particles do not need to be deflected, but they only
benefit from a single acceleration pass. In this case, increasing the energy
means increasing the length of the accelerator.
As physicists have been explored higher and higher energies, accelerators have
become larger and larger: the size of an accelerator is a compromise between
energy, the radius of curvature (if it’s circular), the feasibility and the
cost.
Colliders are accelerators that generate head-on collisions between particles.
Thanks to this technique, the collision energy is higher because the energy of
the two particles is added together.
The Large Hadron Collider is the largest and most powerful collider in the
world. It boosts the particles in a loop 27 kilometres in circumference at an
energy of 6.5 TeV (teraelectronvolts), generating collisions at an energy of 13
TeV.
HOW IT WORKS
THE ACCELERATING CAVITIES
THE MAGNETS
WHAT ARE THE CHARACTERISTICS OF AN ACCELERATOR?
The type of particles, the energy of the collisions and the luminosity are among
the important characteristics of an accelerator.
An accelerator can circulate a lot of different particles, provided that they
have an electric charge so that they can be accelerated with an electromagnetic
field. The CERN accelerator complex accelerates protons, but also nuclei of
ionized atoms (ions), such as the nuclei of lead, argon or xenon atoms. Some LHC
runs are thus dedicated to lead-ion collisions. The ISOLDE facility accelerates
beams of exotic nuclei for nuclear physics studies.
The energy of a particle is measured in electronvolts. One electronvolt is the
energy gained by an electron that accelerates through a one-volt electrical
field. As they race around the LHC, the protons acquire an energy of 6.5 million
million electronvolts, known as 6.5 tera-electronvolts or TeV. It is the highest
energy reached by an accelerator, but in everyday terms, this is a ridiculously
tiny energy; roughly the energy of a safety pin dropped from a height of just
two centimetres. But an accelerator concentrates that energy at the
infinitesimal scale to obtain very high concentrations of energy close to those
that existed just after the Big Bang.
Luminosity is a key indicator of an accelerator’s performance: it indicates the
number of potential collisions per surface unit over a given period of time. The
instantaneous luminosity is expressed in cm-2s-1 and the integrated luminosity,
corresponding to the number of collisions that can occur over a given period, is
measured in inverse femtobarn. One inverse femtobarn corresponds to 100 million
millions (potential) collisions.
WHAT IS LUMINOSITY?
WHY 13 TEV ?
WHAT TYPE OF ACCELERATORS ARE AT CERN?
CERN operates a complex of nine accelerators and two decelerators. These
accelerators supply experiments or are used as injectors, accelerating particles
for larger accelerators. Some, such as the Proton Synchrotron (PS) or Super
Proton Synchrotron (SPS) do both at once, preparing particles for experiments
that they supply directly and injecting into larger accelerators.
The Large Hadron Collider is supplied with protons by a chain of four
accelerators that boost the particles and divide them into bunches.
The accelerators are controlled by operators 24 hours a day from the CERN
Control Centre.
CERN'S ACCELERATORS
CURRENT ACCELERATORS
LHC
LINAC4
PS BOOSTER
LINAC3
LEIR
SUPER PROTON SYNCHROTRON
HIE-ISOLDE
CLEAR
PROTON SYNCHROTRON
ANTIPROTON DECELERATOR
ELENA
FUTURE ACCELERATORS
Imagining, developing and building an accelerator takes several decades. For
example, the former LEP electron-positron accelerator had not even begun
operation when CERN scientists were already imagining replacing it with a more
powerful accelerator. That was in 1984, twenty-four years before the LHC
started.
Since 2010, scientists have been working on the LHC’s successor, the
High-Luminosity LHC. Approved by the CERN Council in 2016, this second
generation LHC is expected to start after 2025. CERN scientists are also working
on accelerator studies for beyond 2040, such as the Future Circular Collider
(FCC), the Compact Linear Collider (CLIC) and the Muon Collider. Work is also
being done on alternative acceleration techniques for example with the AWAKE
experiment.
HIGH-LUMINOSITY LHC
FUTURE CIRCULAR COLLIDER
COMPACT LINEAR COLLIDER
MUON COLLIDER
AWAKE
PAST ACCELERATORS
Many accelerators developed several decades ago are still in operation. The
oldest of these is the Proton Synchrotron (PS), commissioned in 1959. Others
have been closed down, with some of their components being reused for new
machines, at CERN or elsewhere. Travel back into the past of CERN accelerators.
LARGE ELECTRON-POSITRON COLLIDER
SYNCHROCYCLOTRON
CERN NEUTRINOS TO GRAN SASSO
LOW-ENERGY ANTIPROTON RING
INTERSECTING STORAGE RINGS
LINAC1
LINAC2
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