The Large Hadron Collider(LHC) is a particle accelerator located near Geneva designed to accelerate protons and ions, with the aim of creating very high energy collisions. The LHC has a circumference of just under 27km, and produces approximately 1 billion collisions per second when run. The LHC is not a cheap operation, and cost 4.3 billion Swiss Francs (£3.7bn), with running costs over the period 2009-2012 of 1.1 billion Swiss Francs (£940mn). The LHC also consumes roughly 600Gwh of energy per year, or roughly 1% of Switzerland's entire energy consumption.
The LHC was built in the same tunnel formerly used by the LEP (Large Electron-Positron Collider). Plans to build the LHC date from the early 1980s, with construction being started in 1998 and lasting 10 years until completion and the first test in 2008. The LHC is run by CERN(the European Organisation for Nuclear Research).
The basic design of the structure consists of two beam pipes within one concrete tunnel. As a result, there are actually over 50 km of beam pipes in the LHC, twice as large as the circumference of the tunnel. The pipes intersect at four different points in the tunnel, where collisions occur. Particles are accelerated in opposite directions and collide at the intersection. To prevent the particles from colliding with air molecules, a vacuum must be created inside the pipe, with an air pressure of 10^-13 atmospheres, or roughly the same as interstellar space. In addition, separate vacuums are created around the magnets and the liquid helium distribution line. This is because they must be insulated to prevent heating (they are kept at ~1.9K), and the vacuum prevents energy transfer by either conduction or convection. Creating the vacuum is a long process where the pipes are cooled to the point that the gases inside condense on the walls and are pumped out over the course of two weeks. To maintain the vacuum in the room-temperature sections, a titanium-zirconium-vanadium alloy is used which is able to remove all gases bar the noble gases and methane.
The particles are accelerated by both linear and circular accelerators. In a linear accelerator, the pipe is divided into many sections of alternating electrical potential. The potential of the sections flips back and forth rapidly, such that the particles travelling through are always in the position such that they are accelerated. Circular accelerators are similar, but have the advantage of taking up far less space than a linear accelerator of similar power. As a downside, circular accelerators emit large quantities of synchrotron radiation (a form of bremsstrahlung where particles are accelerated perpendicular to their velocity by a magnetic field). Due to the large percentage of synchrotron radiation which is X-rays, and is difficult to insulate from, circular accelerators are more dangerous than their linear counterparts
The collisions are then monitored by the detectors. By tracing the path of particles produced by collisions, their momentum can be predicted, and therefore their mass and their identity. For example, particles with a very high momentum will not be significantly affected by the electromagnets, and their path will only be slightly curved, while particles with a very low momentum will have their paths more significantly changed. As the mass of known subatomic particles is known, then it can be deduced which particle has been observed, or if the mass does not match that of any known particle, then the collision is flagged and investigated further.
Due to the extremely high rate of collisions, a vast amount of computing power is needed to sort through the data. Due to the extreme difficulties with finding such computing power in one location, Tim Berners-Lee, then working at CERN, decided to create the World Wide Web in 1990, by joining HTTP(HyperText Transfer Protocol) with TCP(Transmission Control Protocol). This laid the foundations for the Worldwide LHC Computing Grid(WLCG), a network of thousands of computers designed to process all of the data gathered from collisions, and to share it almost instantaneously with physicists around the world, with the CERN facility in Switzerland as the central node.
9 days after the first run of the LHC, a quenching incident occurred, leading to changes being made to the accelerator. A quenching incident occurs when a superconducting magnet is heated past the transition temperature of the material, causing it to revert to a normal state. As the resistance of the magnet is now above zero, rapid heating of the magnet occurs due to energy loss, causing the magnets around it to heat. These magnets then revert to their normal state and begin heating in a chain reaction. In the case of this quenching incident, the liquid helium used as a coolant for the magnets evaporated, and approximately 6 tonnes of it was lost. In addition, 53 magnets were damaged sufficiently to need to be replaced. The original cause of the quenching incident was traced back to an electrical fault.
Even though the causes of quenching are now known, it is still taken as a given that quenching incidents will happen on occasion, and as a result, certain changes have been made to prevent the incidents from causing serious harm to the accelerator. Two detectors have been added which detect when the resistance of the system increases significantly, as a result of magnets reverting to their normal state. In such an event, the energy in the magnets is then dumped into blocks designed for this purpose, preventing damage to the magnets caused by a potentially explosive change of state of the liquid helium into a gas. As a result of this incident, the system was shut down and was not fully operational until November 2009, a full 14 months afterwards, mostly due to this incident, but also partially due to vacuum leaks which needed to be fixed.
On the 4th of July 2012, the Higgs boson was finally observed at CERN, supporting the Brout-Englert-Higgs mechanism, and shedding light on the mechanism by which mass is generated. Next week's blog post will be on the Higgs boson, make sure to read it.
Sources:
Comments