In accelerator physics, ionization cooling is a physical process for reducing the beam emittance of a charged particle beam[1][2] ("cooling") by passing the particles through some material, reducing their momentum as they ionize atomic electrons in the material.
In addition, energy straggling can cause heating parallel to the direction of beam motion.
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.
The technique can be adapted to provide longitudinal as well as transverse cooling by using a dipole magnet as a dispersive prism to divide the particles by energy, and then passing the resultant "rainbow" beam though a tapered wedge of cooling material.