A multi-laboratory collaboration is studying the feasibility of building a muon collider, the first phase of which maybe a neutrino factory.
Ionization cooling is the base line method for muon cooling.
This scheme uses hydrogen absorbers and rf re-acceleration in a long series of magnetic focusing channels to cool the muons. At Fermilab two rf cavity types are under study to provide the required cooling rf re-acceleration. A 805 MHz high gradient cavity for the collider and a 201 MHz high gradient cavity for the neutrino factory. The 201 MHz cavity uses hollow thin metal tubes over the beam aperture to terminate the field in a pill-box type mode to increase its shunt impedance. This is possible because muons have little interactions with thin metal membranes.