More on accelerators

Linear and circular accelerators

There are two main types of particle accelerators, linear and circular. The linear ones, or linacs, accelerate an injected bunch of particles only once. At the end of the accelerator the particles generally strike a target and a particle detector registers their interactions.

A typical linear accelerator is a made up of a series of so-called drift-tubes each of which has an alternating voltage applied to it . Inside the drift-tubes particles feel no force, so they just drift, but as soon as they enter the space between the drift-tubes, they are accelerated. The voltage applied to the drift-tubes is varied to ensure that the particles are always accelerated in the right direction.

Try accelerating a particle yourself with this game. Think of each battery as a drift tube, your job is to flip the polarities so that the particle is always accelerated in the right direction.

The drift tubes inside a real linear accelerator look like this.

Linear accelerators are often used to study collisions between exotic short-lived particles. To do this, they fire a beam of ordinary particles onto a target. These particles interact with the target to produce new particles of many kinds, including the short-lived ones of interest for a particular experiment. Using magnets, these exotic ones can be extracted and steered towards another target or even another beam of particles to study exotic collisions.

The world's most powerful linac is in California, at the Stanford Linear Accelerator Center, SLAC. It is over three kilometres long and has been turned into a particle collider. Two bunches of particles are accelerated and then curved around to collide head on with each other. CERN's biggest accelerator, the 27-kilometre Large Electron-Positron collider, LEP, does the same thing, but using a circular accelerator instead of a linac.

Circular accelerators have the advantage that the particles can be kept for a long time and that they can be successively accelerated lap after lap using the same equipment. Like linacs, circular accelerators can be used in two ways; to accelerate beams, extract them and fire them at targets, or as colliders.

Colliders are a more efficient way of exploiting the energy that has been pumped into the beams. In a circular collider the bunches of particles travelling in opposite directions are brought into collision at one or more points around the ring. At the collision points, two individual particles can collide head-on making use of all the energy of the initial particles. When a single beam strikes a target, a lot of its energy is dissipated in the target and is wasted as far as physics research is concerned.

An advantage of circular accelerators is that since collisions between individual particles in opposing bunches are relatively rare, bunches can be kept circulating for hours. The drawback, however, is that the particles they accelerate lose energy by emitting electromagnetic radiation as they are forced around the ring. This means that energy constantly has to be pumped in.

The LEP accelerator

LEP is CERN's flagship research machine. Commissioned in 1989, it has furnished four big experiments, called ALEPH, DELPHI, L3 and OPAL, with colliding beams for over a decade. LEP typically operates with four bunches of electrons and four bunches of positrons rotating in opposite directions. In principle, these bunches could collide with each other at eight points around the ring, but in practise only four points corresponding to the four experiments are used.

Each bunch contains some 250 billion particles. A bunch is just over a centimetre long and fractions of a millimetre high and wide. That gives you an idea of how incredibly tiny electrons are.

The bunches rush around LEP at a shade under the speed of light. Their speed is so close to that of light that if a bunch of electrons in LEP could be made to compete with a beam of light in a race to the moon, 384 500 kilometres away, the light would win by a margin of just 5 millimetres! At this speed, the particles in LEP make 11 200 laps per second, meaning that the bunches pass through each other 44 800 times per second in the four detectors. The fact that collisions between individual electrons and positrons only happen a few times per second is another indication of how small electrons are.

A schematic diagram of CERN's accelerators...

...buried underground, the white rings in this pictures show their locations.

The particles used for collisions in LEP begin their lives in the Linear accelerator for LEP, LIL. They are then accumulated in a device called the Electron-Positron-Accumulator, EPA, until there are enough of them to form a bunch. Bunches are then successively accelerated through smaller accelerators called the PS and SPS, both research machines in their own right, before being injected into LEP. There they receive their final boost of energy before being brought into collision inside the four experiments.

Charged particles follow curved paths in a magnetic field.

Particle bunches are maintained in their orbits using powerful dipole magnets...

...and they are accelerated by eletric fields in so-called accelerating cavities. The cavities at LEP can provide a field gradient of over 6 million Volts per metre (MV/m). Adding up the total accelerating power of LEP gives the colossal value of almost three billion Volts per lap.

When discussing particle energies, particle physicists use a unit derived from the electric field through which the particle has been accelerated. For instance, an eletron which is accelerated through a field of 1 Volt will have an energy of 1 electron Volt. In Joules this corresponds to about 10-19 J, a rather awkward unit to use in particle physics.

If an electron and a positron are accelerated in a field of 470 million Volts, each particle will have an energy of 470 million electron Volts or 470 MeV (Mega electron Volt) for short. If the two particles collide head-on the total energy of the collision will be 2 x 470 MeV = 940 MeV. This energy is sufficient to produce a neutron. That is, according to Einstein's famous E=mc2 we can also mesure the mass of the neutron in terms of electron Volts :

m = E/c2 = 940 MeV/c2

For simplicity we can avoid dividing by c, the speed of light, and remember that it's always there. That's why you'll often find physicists using electron Volts as units both of energy and of mass.

To generate the events on this CD-ROM, the electrons and positrons in LEP were accelerated to about 45 billion electron Volts, 45.625 GeV (Giga electron Volts) to be precise, resulting in collisions with an energy of 91.25 GeV. This energy corresponds exactly to what is needed to produce a Z particle.

It sounds like a lot of energy but in fact it is only about a tenth the kinetic energy of a flying mosquito. The difference is that it is concentrated into a tiny space, about a ten thousand million million times smaller than a mosquito, and that's what gives rise to the phenomena physicists are interested in.

Learn more about Accelerators in the Universe
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Introduction to accelerators

Particle Physics Education CD-ROM 1999 CERN