Cooling techniques are important for the control the emittance of particle beams in particle accelerators. Usually, these techniques take advantage, in some way, of the electric or the magnetic fields present in the fast-moving bunches of particles in the accelerator in clever ways to damp the tendency of these particles to repel each other. It is accurate to think of these techniques as thermodynamic cooling, or the reduction of entropy, in much the same way that a refrigerator or an air conditioner cools its contents.
The main types of particle beam cooling are:
References
Particle beam cooling Wikipedia(Text) CC BY-SA