The LHC machine
Cooler than the outer space, it will produce 600 million particle collisions per second.
- What are sectors and octants in the machine?
- What are the important parameters for an accelerator?
- What are the main ingredients of an accelerator?
- What is so special about the LHC dipoles?
- What is so special about the cryogenic system?
- Why superfluid helium?
- Why do we talk about bunches?
- How many collisions per second take place at the LHC?
- How long do the beams last in the accelerator?
The LHC is not a perfect circle. It is made of eight arcs and eight ‘insertions’ (IP). The arcs contain the dipole ‘bending’ magnets, with 154 in each arc. An insertion consists of a long straight section plus two (one at each end) transition regions - the so-called ‘dispersion suppressors’. The exact layout of the straight section depends on the specific use of the insertion: physics (beam collisions within an experiment), injection, beam dumping, beam cleaning.
A sector is defined as the part of the machine between two insertion points. The eight sectors are the working units of the LHC: the magnet installation happens sector by sector, the hardware is commissioned sector by sector and all the dipoles of a sector are connected in series and are in the same continuous cryostat. Powering of each sector is essentially independent.
An octant starts from the middle of an arc and ends in the middle of the following arc and thus spans a full insertion. Therefore, this description is more practical when we look at the use of the magnets to guide the beams into collisions or through the injection, dumping, and cleaning sections.
We build accelerators to study processes whose probability varies with collision energy, and which are often rare. This means that for physicists the most important parameters are the beam energy and the number of interesting collisions. More specifically, in a collider such as the LHC the probability for a particular process varies with what is known as the luminosity - a quantity that depends on the number of particles in each bunch, the frequency of complete turns around the ring, the number of bunches and the beam cross-section. In brief, we need to squeeze the maximum number of particles into the smallest amount of space around the interaction region.
In an accelerator, particles circulate in a vacuum tube and are manipulated using electromagnetic devices: dipole magnets keep the particles in their nearly circular orbits, quadrupole magnets focus the beam, and accelerating cavities are electromagnetic resonators that accelerate particles and then keep them at a constant energy by compensating for energy losses.
Vacuum in the LHC: The internal pressure at the LHC will be 10–13?atm (ultrahigh vacuum), because we want to avoid collisions with gas molecules. There is ~6500 m3 of pumped volume in the LHC, like pumping down a cathedral!
Magnets: There is a large variety of magnets in the LHC, including dipoles, quadrupoles, sextupoles, octupoles, decapoles, etc. giving a total of about 9300 magnets. Each type of magnet contributes to optimizing a particle’s trajectory. Most of the correction magnets are embedded in the cold mass of the main dipoles and quadrupoles. The LHC magnets have either a twin aperture (for example, the main dipoles), or a single aperture (for example, some of the insertion quadrupoles). Insertion quadrupoles are special magnets used to focus the beam down to the smallest possible size at the collision points, thereby maximizing the chance of two protons smashing head-on into each other. The biggest magnets are the 1232 dipoles.
Cavities: The main role of the LHC cavities is to keep the 2808 proton bunches tightly bunched to ensure high luminosity at the collision points and hence, maximize the number of collisions. They also deliver radiofrequency (RF) power to the beam during acceleration to the top energy. Superconducting cavities with small energy losses and large stored energy are the best solution. The LHC will use eight cavities per beam, each delivering 2 MV (an accelerating field of 5MV/m) at 400 MHz. The cavities will operate at 4.5K (-268.7ºC)(the LHC magnets will use superfluid helium at 1.9 K or -271.3ºC). For the LHC they will be grouped in fours in cryomodules, with two cryomodules per beam, and installed in a long straight section of the machine where the transverse interbeam distance will be increased from the normal 195 mm to 420 mm.
The following table lists the important quantities for the LHC.
The dipoles of the LHC represented the most important technological challenge for the LHC design. In a proton accelerator like the LHC, the maximum energy that can be achieved is directly proportional to the strength of the dipole field, given a specific acceleration circumference. At the LHC the dipole magnets are superconducting and able to provide the very high field of 8.3 T over their length. No practical solution could have been designed using ‘warm’ magnets instead of superconducting ones. Indeed, if the magnets were made to work at a temperature of 4.5 K, they would produce a magnetic field of only 6.8 T.
The LHC dipoles use niobium-titanium (NbTi) cables, which become superconducting below a temperature of 10 K (–263.2°C), that is, they conduct electricity without resistance. In fact, the LHC will operate at the still lower temperature of 1.9 K (–271.3°C), which is even lower than the temperature of outer space (2.7 K or –270.5°C). A current of 11 700 A flows in the dipoles, to create the high magnetic field of 8.3 T, required to bend the 7 TeV beams around the 27-km ring of the LHC. For comparison, the total maximum current for an average family house is about 100 A.
The temperature of 1.9 K (–271.3°C) is reached by pumping superfluid helium into the magnet systems. Each dipole is 15 m long and weighs around 35 t.
The magnet coils for the LHC are wound from a cable consisting of up to 36 twisted 15-mm strands, each strand being made up in turn of up to 6400 individual filaments, each filament having a diameter as small as 7 micrometres (for comparison, a human hair is about 50 micrometres thick). The 27-km circumference of the LHC calls for some 7600 km of cable, corresponding to about
270 000 km of strand — enough to circle the Earth six times at the Equator. If all the component filaments were unravelled, they would stretch to the Sun and back five times with enough left over for a few trips to the Moon .
The LHC is the largest cryogenic system in the world and one of the coldest places on Earth. Such a cold temperature is required to operate the magnets that keep the protons on course (see question: “what is so special about the LHC dipoles?”). To maintain its 27-km ring (4700 tonnes of material in each of the eight sectors) at superfluid helium temperature (1.9 K, –271.3°C), the LHC’s cryogenic system will have to supply an unprecedented total refrigeration capacity - some 150 kW for refrigerators at 4.5 K and 20 kW for those at 1.9 K. The layout for the refrigeration system is based on five “cryogenic islands”. Each “island” must distribute the coolant and carry kilowatts of refrigeration over a long distance. The whole cooling process will take a few weeks.
The refrigeration process happens in three phases:
1) cool down to 4.5 K (-268.7ºC),
2) filling with liquid helium of the magnet cold masses
3) final cool down to 1.9 K (-271.3ºC).
The first phase happens in two steps: first helium is cooled in the refrigerators’ heat exchangers to 80 K by using about 10 000 t of liquid nitrogen. Then refrigerator turbines bring the helium temperature down to 4.5 K (-268.7ºC), ready for injection into the magnets’ cold masses. Once the magnets are filled, the 1.8 K refrigeration units bring the temperature down to 1.9 K (-271.3ºC). In total, about 120 t of helium will be needed, of which about 90 t will be used in the magnets and the rest in the pipes and refrigerator units. Liquid nitrogen is never directly injected into the LHC to avoid any possible source of asphyxiation in the underground tunnel.
The choice of the operating temperature for the LHC has as much to do with the ‘super’ properties of helium as with those of the superconducting niobium-titanium alloy in the magnet coils. At atmospheric pressure helium gas liquefies at around 4.2 K (–269.0 °C), but when it is cooled further it undergoes a second phase change at about 2.17 K (–271.0 °C) to its ‘superfluid’ state. Among many remarkable properties, superfluid helium has a very high thermal conductivity, which makes it the coolant of choice for the refrigeration and stabilization of large superconducting systems (see also question above).
In all, LHC cryogenics will need some 40 000 leak-tight pipe junctions, and 96 t of helium will be required by the LHC machine to keep the magnets at their operating temperature of 1.9 K. 60% of the helium will be in the magnet cold masses while the remaining 40% will be shared between the distribution system and the refrigerators. During normal operation most of the helium will circulate in closed refrigeration loops. Nevertheless, each year, a certain percentage of the inventory could be lost due to facility stops, leakage to the atmosphere, conditioning of installations and operational problems.
The protons of the LHC circulate around the ring in well-defined bunches. The bunch structure of a modern accelerator is a direct consequence of the radio frequency (RF) acceleration scheme. Protons can only be accelerated when the RF field has the correct orientation when particles pass through an accelerating cavity, which happens at well specified moments during an RF cycle. In the LHC, under nominal operating conditions, each proton beam has 2808 bunches, with each bunch containing about 1011protons.
The bunch size is not constant around the ring. Each bunch, as it circulates around the LHC, gets squeezed and expanded—for instance it gets squeezed as much as possible around the interaction points to increase the probability of a collision. Bunches of particles measure a few centimetres long and a millimetre wide when they are far from a collision point. However, as they approach the collision points, they are squeezed to about 16 micrometres (a human hair is about 50mm thick) to allow for a greater chance of proton-proton collisions. Increasing the number of bunches is one of the ways to increase luminosity in a machine. The LHC has opted for a bunch spacing of 25ns (or about 7m), which introduces many technical challenges. (The LHC’s predecessor, LEP, operated with as few as 4 bunches).
The bunch spacing of 25 ns corresponds to a frequency of 40MHz, which implies that bunches should pass each of the collision points in the LHC 40 million times a second. However, for practical reasons there are several bigger gaps in the pattern of bunches, which allow time for example for the ‘kicker’ magnets to come on in order to inject or dump beam. The average crossing rate is equal to the total number of bunches multiplied by the number of turns round the LHC per second: 2808 × 11245 = 31.6 MHz.
Each beam will consist of nearly 3000bunches of particles and each bunch will contain as many as 100billion particles. The particles are so tiny that the chance of any two colliding is very small. When the bunches cross, there will be only about 20collisions between 200billion particles. Bunches will cross on average about 30million times per second (see previous question), so the LHC will generate up to 600million particle collisions per second.
A beam might circulate for 10hours, travelling more that 10billion kilometres, enough to get to the planet Neptune and back again. At near light-speed, a proton in the LHC will make 11245 circuits every second.