Radiation damping
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Radiation damping in accelerator physics is a way of reducing the beam emittance of a beam of accelerated charged particles.
There are two main ways of using radiation damping to reduce the emittance of a particle beam -- damping rings and undulators -- and both rely on the same principle. That is, they extract momentum from the particles in the random direction they are moving, and replace that with momentum in the desired direction of motion.
[edit] Damping Rings
Here the particles are made to move around in a closed loop (not necessarily circular). As they are steered around the loop, they will give off energy in the form of synchrotron radiation, and the direction of propagation of the radiation will be concentrated in the direction of the particles motion. Thus the momentum vector of the particle is reduced in size, but its direction remains the same (ignoring quantum effects for the moment).
Included around the ring are accelerating sections, and these will replace the energy lost to the synchrotron radiation. The important point is that this momentum replacement will be entirely in the required direction of motion of the beam. In other words, the momentum vector of the particle will be restored to its original size, but will lie more closely in the correct direction.
Thus the emittance is decreased.
[edit] Undulators and Wigglers
These operate using the same principle as damping rings. They are devices with a periodic magnet field that causes the particles to oscillate transversely. This oscillation causes the charged particles to emit synchrotron radiation, which is then replaced in accelerating sections. Therefore, the emittance of the beam is decreased.
Quite often damping rings will be designed with long straight sections in order to increase the overall damping of the ring.
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