Wing Sheung Chan

The Large Hadron Collider and the ATLAS detector 29 Figure 2.1.: A schematic representation of the CERN accelerator complex during the second operational run of the LHC. Circular accelerators are represented by ellipses labelled with their names and the years that they were built in. The LHC can be seen as the largest ellipse at the top. The four circles on the LHC represent the collision points where the four main detectors are located. The colour-coded arrows indicate the types and directions of particles being transmitted or accelerated in each part of the complex. This picture is adapted from Reference [65] . slightly higher (lower) than the ideal energy will travel faster (slower) through the cavities and will therefore be decelerated (accelerated) by the electromagnetic field due to the offset timing. As a result, charged particles are grouped in discrete packets known as “bunches”, which increases the chance of collision. On the LHC, the RF cavities are driven by high-power klystrons that modulate at a frequency of 400MHz . Each cavity can build an electric potential up to 2MV . Thousands of superconducting electromagnets are deployed along the LHC to steer, stabilise and focus the particle beams. Dipole magnets are used to provide strong magnetic fields that bends the particle beams, keeping them in a (nearly) circular orbit. The main dipole magnets on the LHC could generate magnetic fields of 8 . 3 T . Quadrupole magnets are used to tighten the beams by squeezing them in the vertical and horizontal directions alternately. Sextupole, octupole and decapole magnets are also installed to counteract higher-order effects such as the inhomogeneity of the main dipole magnets or electromagnetic interactions between bunches. Additionally, eight sets of so-called inner