Radiation damping

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Radiation damping in accelerator physics is a way of reducing the beam emittance of a high-velocity beam of charged particles. More specifically, it reduces the momentum spread of the particles making up the beam.

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. They induce synchrotron radiation to reduce the particles' momentum, which reduces the momentum spread by an equal factor, then replace that with momentum in the desired direction of motion.

[edit] Damping Rings

As particles are steered around a closed loop (not necessarily circular), the lateral acceleration causes them to give off synchrotron radiation in their direction of motion, thereby losing momentum. Thus, each particle's momentum vector is reduced in size, but its direction remains the same (ignoring quantum effects for the moment). This reduces the momentum spread in the beam.

Also included around the ring are accelerating sections, to replace the energy lost to the synchrotron radiation. The important point is that this momentum replacement will be entirely in the desired direction of motion of the beam. In other words, the momentum vector of the particle will be restored to its original size, but will point more accurately in the correct direction.

Because tight corners enhance synchrotron radiation, damping rings are often small. When a long beam of particles is needed, to fill a large particle accelerator, the damping ring may be extended with long straight sections. The straight sections are convenient places to add momentum back, but do not contribute to the damping effect.

[edit] Undulators and Wigglers

When faster damping is required than can be provided by the turns inherent in a damping ring, it is common to add undulator or wiggler magnets to induce more synchrotron radiation. These are devices with periodic magnetic fields that cause the particles to oscillate transversely, equivalent to many small tight turns. 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.

The many small turns in an undulator have the advantage that the cone of synchrotron radiation is all in one direction, forward. This is easier to shield than the broad fan produced by a large turn.