In order to probe the structure of matter we must collide particles at high energies to effectively smash the particles and 'see' what is inside. In order for particles to be accelerated, they must be charged and stable. Particle accelerators exist everywhere, you're looking at one now. The monitor consists of a cathode ray tube, which accelerates electrons to collide with a luminescent screen, which produces the picture. However, the Particle accelerators used in high energy physics to need particles to be travelling at much faster speeds, speeds approaching that of light. Modern accelerators fall into two categories, Linac and Synchrotron.

Linac

Linacs were developed during the late 1920's. They consist of a large linear tube, which is in a near vacuum state. This prevents the particles being accelerated colliding with air molecules and be scattered into a random direction. The particles are accelerated using radio frequency alternating potential difference applied to a cavity, known as an RF cavity. RF cavites work by attracting particles as they approach the cavity and repelling them as they leave the cavity by means of an electric field. Hence, the accelerated particles must be charged.

The Linac consists on a series of such cavities hence the particles are accelerated each time they pass though a cavity. The accelerated particles collide with either a fixed target or another beam of accelerated particles. The largest Linac in the world is the situated at Stanford university in the USA. It is 3.2 km long and can accelerate particles to energies of 50 GeV.

Synchrotron

A Synchrotron consists of a tube shaped large ring through which the particles travel. Inside the tube, a near vacuum state exists to ensure the particles being accelerated do not collide with molecules of air. The particles are accelerated using a series of RF cavities situated around the ring in order to keep the particles in orbit at a constant radius. This achieved by using focussing magnets. These magnets produce a magnetic field, which bend the particles around the ring. However, as the particles in the accelerator are accelerated they achieve faster and faster speeds. Hence, a grater force is required to bend the particles around the magnet to keep the particles in a constant orbit, thus stopping the particle flying though the side of the detector. This is achieved by steadily increasing the magnetic field strength as the speed of particles is increased. The speed of the particles and the magnetic field strength are synchronised hence the name Synchrotron.

The series of focussing magnets around the ring are ore super conducting electromagnets, which can produce field strengths in the region of 5T. As the charged particles travel around the Synchrotron they produce electromagnet radiation known as synchrotron radiation, synchrotron radiation is produce by accelerating charged particles travelling in a circular path. The amount of energy lost though this radiation is dependent on the energy of the particle - the greater the energy of the particle the greater the energy loss. In addition, the radius of the synchrotron ring, the lower the radius the, the greater the energy loss. At higher energies, this synchrotron energy becomes greater therefore, it becomes important to minimise this energy loss if we are to achieve the largest speed of particles possible. This is achieved by having a large a ring as possible. The Hadron Electron Ring Accelerator, HERA has a diameter of 2 km. The accelerator is capable of accelerating protons to an energies of 820 GeV and electrons to 30 GeV.