Efforts to begin operation of the Large Hadron Collider (LHC), set to become the world’s most powerful atom smasher, suffered a setback on 19th September. The collider’s 27 km-long tunnel hosts several superconducting magnets. A faulty electrical connection between two of these magnets in one section of the tunnel has led to a major leak of helium, the LHC’s main coolant, into sector 34.
Before a full understanding of the incident can be established, however, the sector has to be brought to room temperature and the magnets involved opened up for inspection. This will take three to four weeks. Full details of this investigation will be made available once it is complete.
Coming immediately after the very successful start of the LHC operation on 10th September when scientists successfully sent a proton beam around the entire tunnel, the LHC tunnel incident is a major blow after years of painstaking preparation.
Before this setback, CERN scientists had intended to have the first collisions between two beams, each of which would have an energy of 5 trillion electronvolts, five times more energetic than those any other collider has managed. The plan was to host this first collision and then shut down the LHC in December for four months due to the high cost of powering the accelerator in the winter.
Next year, when the accelerator is set to run at full capacity, each proton beam will carry seven times more energy and have about 30 times the intensity of any beam at any other accelerator.
The most intense collisions will generate the heat, energy and densities that existed just a trillionth of a second after the Big Bang. Physicists hope that the LHC will lead them beyond the standard model of particle physics and to signs of extra dimensions, new types of elementary particles that could account for most of the mass in the universe and, perhaps, rapidly evaporating, microscopic black holes that the accelerator may forge.
The main structure of the LHC is a 27 km circumference accelerator ring of superconducting magnets. Its primary job is to boost the energy of particles which will be smashed at the various detectors located around the ring. Inside the tunnel are two separate vacuum tubes which channel particles at close to the speed of light. These "beam lines" travel in opposite directions.
The particles are guided around ring by a strong magnetic field. Thousands of magnets of different size and type are used to direct the beams around the accelerator. These include more than 1,200 "dipole" magnets, each of which is 15 m in length and are used to bend the beams around the loop. Nearly 400 "quadrupole" magnets, each 5 to 7 m long are used to focus the beams. A third type of magnet is used to squeeze the beams closer together to increase the chances of a collision.
Many of these magnets are built from coils of superconducting electric cable which conducts electricity with little resistance, and therefore little loss of energy. To achieve this effect the cables are cooled close to absolute zero (-273°C) using liquid helium. This is fed into a sealed network in the accelerator at eight "cryoplants" located around the ring.
All of this is monitored and managed from the control centre. Scientists will steer the particle beams to ensure that collisions occur at the detectors located around the circuit.
The LHC is not a perfect circle but a series of arcs. Magnet coils in the machine are wound from cable of up to 36 twisted 15 mm strands. Each strand is made up of 6,400 filaments, each filament seven micrometres in diameter (human hair is 50 micrometres thick). There are 7,600 km of cable, corresponding to 270,000 km of strand - enough to circle the Earth six times. If the component filaments were unravelled, they would stretch to the Sun and back more than five times.
CERN is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. India, Israel, Japan, Russia, the United States, Turkey, the European Commission and UNESCO have Observer status. View pictures here. Click ch/11. Visit www.cern.ch 40/08.
