The LHC: a unique tool

The Large Hadron Collider (LHC) is a particle accelerator based at CERN, the European Organization for Nuclear Research, the world's largest particle physics laboratory. It is designed to be the most powerful instrument ever built to investigate the properties of fundamental particles.

The LHC is located in a 26.658 km long tunnel, about 100m underground. It will accelerate two separate beams of protons travelling in opposite directions at up to an energy of 7 TeV. At this energy the beams which are only few microns or so in diameter at the interaction point will pack a punch comparable to a high-speed train!

 

 

A view of the LHC

 

At four specific points along the LHC ring, the two beams are made to cross paths, to produce head-on collisions (hence the C of LHC). The proton collision energy will then be of 14 TeV. But the LHC will not be limited to the study of proton-proton collisions as it can also collide heavy ions, such as lead, with a collision energy of 1148 TeV. Since the constituents of ions are protons and neutrons, and belong to a family of particles called hadrons (hence the H of LHC)

 

CERN accelerator complex
The CERN accelerator complex with the path of the protons from the hydrogen source to the LHC

 

 

Before being injected into the LHC, proton beams will be prepared by CERN's existing "accelerator complex". This is a succession of machines with increasingly higher energies, injecting the beam each time into the next one, which takes over to bring the beam to an even higher energy.

To bend the 7 TeV protons around the ring, the LHC dipoles must be able to produce magnetic fields of 8.36 Tesla, a value which is made possible by the use of "superconductivity". This is the ability of certain materials, usually at very low temperatures, to conduct electric current without resistance and power losses, and therefore produce high magnetic fields. The LHC operates at about 300 degrees below room temperature (even colder than outer space!) and uses the most advanced superconducting magnet and accelerator technologies ever employed. 1,296 superconducting dipoles and more than 2,500 other magnets guide and collide the LHC beams. They range from small, normally conducting bending magnets to large, superconducting focusing quadrupoles. The LHC is the largest superconducting installation in the world.

Six experiments, with huge detectors, have been built to study what happens when the LHC's beams collide. They are designed to handle huge amounts of data, millions of gigabytes per second, and condense this into a more manageable, yet still daunting, stream of a few gigabytes per second. This data must then be stored and analysed by the thousands of particle physicists who have helped construct the detectors.

How an accelerator works

 

An accelerator usually consists of a vacuum chamber surrounded by a long sequence of vacuum pumps, magnets, radio-frequency cavities, high voltage instruments and electronic circuits. Each of these pieces has its specific function.

 

 

The vacuum chamber is a metal pipe where air is permanently pumped out (by the vacuum pumps) to avoid that the accelerated particles collide with normal matter (like air molecules) and annihilate or get deflected off course.

Inside the pipe, particles are accelerated by electric fields. These are provided by Radio-Frequency (RF) cavities. Each time charged particles traverse an RF cavity, the electric field inside the cavity gives them a "kick", i.e. some of the energy of the radio wave is transferred to them and they are accelerated. To make a more effective use of a limited number of RF cavities, accelerator designers can force the particle beam to go through them many times, by curving the beam trajectory into a closed loop. That is why most accelerators are roughly circular.

The curving of the beam's path, to make sure the particles stay within their circular track, is usually achieved by the magnetic field of dipole magnets (which have a North and a South pole, like the well-known horseshoe magnet). They are also called "bending magnets". This is because the magnetic force exerted on moving charged particles is always perpendicular to their velocity - perfect for curving the trajectory! The higher the energy of a particle, the stronger the field that is needed to bend the particle's path. This means that, as the maximum magnetic field is limited (to some 2 Tesla for conventional magnets, some 10 Tesla for superconducting ones), the more powerful a machine is, the larger it needs to be.

In addition to just curving the beam, it is also necessary to focus it. Just like a shot from a shotgun, a particle beam spreads out as it travels. Focussing the beam allows its width and height to be constrained so that it stays inside the vacuum chamber. This is achieved by quadrupole magnets (which have four poles), which act on the beam of charged particles exactly the same way a lens would act on a beam of light. They are also called "focussing magnets".

 


A view of the LHC Control room

 

 

These are some of the basic ingredients needed to make an accelerator. If you look around CERN's existing accelerators, like the PS or the SPS, you will see that there are many more objects such as:

  • other magnets (to perform "fine tuning" on the trajectory or of the focusing)
  •  injection/ejection elements( to put the beam into the accelerator or to take it out)
  •  measurement devices (to give the operators information on the behaviour of the beam
  •  safety elements (to ensure smooth operation of the accelerator).

All of these elements are controlled from a control centre, which is very much like the control centres used for space missions!

LHC operations

Having to accelerate two particles beams at the same time, the LHC is in fact "two machines in one". It will consist of two " superconducting magnetic channels" or "rings" housed in the same yoke and cryostat, a unique configuration that not only saves space but also gives a 25 % cost saving over separate rings!

 

 

The two rings will be filled with protons delivered from the SPS and its pre-accelerators at 0.45 TeV, and will accelerate them up to almost the speed of light, at an energy of 7 TeV

What we call a "proton beam" is in fact a succession of squeezed groups of protons called "bunches". The two LHC beams will consist each of 2808 bunches of 1.15×1011 particles each.

Once the 7 TeV energy is reached, the beams will counter-rotate for several hours, at a rate of 11,245 revolutions per second. Since the LHC crosses the Franco-Swiss border at two points along its circular path, that's 22,490 border crossings a second, luckily there are no border checks for protons!

At each turn, the beams will be forced to collide in determined places, where the experiments are located.

After about 10 hours the beams will become so degraded that the machine will have to be emptied and refilled.

 

First 3.5 TeV collisions on 30/03/2010
The image of the first collisions at 3.5 TeV on the 30 of March 2010