Round and round again history 4
 Figure 1. The path of electrons in a magnetic field.
 If we fire electrons into a magnetic field they are deflected (figure 1). The force is always at right angles to their motion so they move in the arc of a circle. If the magnetic field covers a large enough area, the electrons will move in a complete circle. Now, we can put in some electrodes and accelerate the electrons as they go round. This is the principle of the synchrotron. Synchronised spinning Synchrotrons can be used for all sorts of particles. For positive particles, like protons, we need to swap the magneitc field and make sure that the particles are always leaving a positive electrode.
 Figure 2. This is a simplified picture of a synchrotron with only a few electrodes and coils. The voltages are swapped to make sure that the positive protons are always leaving a positive electrode. The coils provide a magnetic field that keeps the particles in orbit.
 Each time the protons go around the circle, the electrodes give them a push and accelerate them (just like the linear accelerator). The voltages on these electrodes have to be continually swapped over so that the protons are always leaving a positive electrode and being drawn towards a negative one (figure2). However, the rate at which the voltages swap over cannot be constant. As the protons speed up, they will spend less time between the electrodes so the voltages have to be swapped more rapidly. Also, the magnetic field has to be increased because a bigger force is needed to keep the faster electrons in the same orbit.

H3
 Both the increasing frequency of the voltage and the increasing magnetic field have to be synchronised with the speeding protons. Hence this accelerator is called a synchrotron. The largest synchrotron is at the European Laboratory for Particle Physics (known as CERN). It runs through a tunnel under the border between France and Switzerland. It has a 27 km circumference and can give an effective accelerating voltage of 100 GV (a hundred thousand million volts).

H3
The story so far
 • a synchrotron uses a tunnel that goes around in a circle • particles go round and round the tunnel, picking up energy as they pass through a sequence of electrodes • the largest synchrotron is the 27 km tunnel at CERN in Switzerland and France
 Question H4

a) The magnetic field strength in a synchrotron has to be increased as time goes on. Why is this?

b) Why do the voltages have to be swapped as the particles go round the loop?

c) In a linear accelerator, the frequency of the alternating voltage is constant. However, in a synchrotron, the frequency has to be increased as time goes on. Why is this?

 The story so far Particle accelerators are used to determine the structure of sub-atomic particles Linear accelerators use long tunnels with a sequence of electrodes to send particles craching into each other at huge energies Linear accelerators are limited by how long they can be made