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How do Synchrotrons Work?

It is a fundamental principle of physics, that when charged particles are accelerated they give off electromagnetic radiation. An everyday example of this effect is the radio-transmitter in which the particles being accelerated are the electrons in the transmitter mast; here the accelerations are such that the radiation produced is in the radio-frequency range. The most common synchrotrons also use electrons though their speed and acceleration is such that they produce electromagnetic radiation that is not only in the radio-frequency range but also in the infra-red, visible, ultra-violet and X-ray portion of the electromagnetic spectrum.

However we must first discuss the "building block" of synchrotrons, the so-called dipole magnet which produces a vertical magnetic field, H, in the gap between its poles (see below).

The dipole magnet has two vital roles in the synchrotron. First, we recall another basic principle of physics that if an electron, travelling in a direction v (horizontal in above diagram), intersects a magnetic field H in a direction perpendicular to v (H is vertical in above diagram) then it will experience a force F (called the Lorentz force) which is in a direction perpendicular to both v and H ("inwards" as shown in the case above). Since the electron is moving with velocity v, F produces a centripetal acceleration causing the electron to move in a circular orbit. The second vital feature is that because the electron is being accelerated within the dipole magnet it will emit electromagnetic radiation; however we will return to this second essential ingredient later. By ganging up a series of such dipole magnets around a circle of the appropriate radius, it is obviously possible to make an electron move around a closed loop (see below) consisting of curved (within the dipole magnets) and straight (between the dipole magnets) sections.

This would comprise a simple synchrotron, often referred to more properly as a ring or storage ring, though there are several other aspects and components to consider. First we need a source of energetic electrons to feed into the ring and this is done using a linear accelerator (linac) which produces electrons at energies which can range from hundreds of MeV (106 eV) to several GeV (109 eV). With some synchrotrons (e.g. the SRS) a small "booster synchrotron", sited in between the linac and the main synchrotron, is temporarily used during "start-up" (referred to as injection) just to bridge some of the energy gap between the output-MeV of the linac and the input-GeV required of the main synchrotron ring. A key aspect of injection, however, is that the electrons are injected in discrete pulses so that the electrons exist inside the storage ring as bunches, typically one or two hundred bunches distributed around the whole ring. This is essential for an effective action of another synchrotron component, a radio-frequency generator/cavity, of which there could be several around the ring. The purpose of this device is to synchronously (hence the name synchrotron) feed energy to the electron bunches circulating in the ring to compensate for their energy losses during their emission of radiation. This current of electron bunches slowly decays with time due to collisions between the electrons and any molecules contained within the ring; even with ultra-high vacuum conditions (typically 10-10 mbar) in the storage ring, the storage beam typically needs to be regenerated about every 24 hours. A very basic plan for a synchrotron might be:
The brown lines denote the paths of the synchrotron radiation emitted as the bunches of electrons pass through the dipole magnets; this produces a "Catherine wheel" effect. The properties of synchrotron radiation are considered in the next part.

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© Copyright 1997-2006.  Birkbeck College, University of London.
Author(s): Paul Barnes
Jeremy Karl Cockcroft
Simon Jacques
Martin Vickers