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Ionization cooling is a process by which the beam emittance of a beam of particles may be reduced . In ionization cooling, particles are passed through some material. The momentum of the particles is reduced as they ionize atomic electrons in the material. Thus the normalised beam emittance is reduced. By re-accelerating the beam, for example in an RF cavity, the longitudinal momentum may be restored without replacing transverse momentum. Thus overall the angular spread and hence the geometric emittance in the beam will be reduced.
Ionization cooling can be spoiled by stochastic physical processes. Multiple Coulomb scattering in muons as well as nuclear scattering in protons and ions can reduce the cooling or even lead to net heating transverse to the direction of beam motion. In addition, energy straggling can cause heating parallel to the direction of beam motion.
The primary use of ionization cooling is envisaged to be for cooling of muon beams. This is because ionization cooling is the only technique that works on the timescale of the muon lifetime. Ionization cooling channels have been designed for use in a Neutrino Factory and a Muon Collider. Muon ionization cooling is expected to be demonstrated for the first time by the Proof of Principle International Muon Ionization Cooling Experiment. Other PoP muon ionization cooling experiments have been devised.
Ionization cooling has also been proposed for use in low energy ion beams and proton beams.
^ G.I. Budker, in: Proceedings of 15th International Conference on High Energy Physics, Kiev, 1970 A.N. Skrinsky, Intersecting storage rings at Novosibirsk, in: Proceedings of Morges Seminar, 1971 Report CERN/D.PH II/YGC/mng