Cooling simulations for the Muon Collider
In a muon collider design the muon beam six-dimensional phase space volume must be reduced in order to be able to further accelerate it and inject into the storage ring. Ionization cooling is currently the only feasible option for cooling the beam within the muon lifetime. The RFOFO ring is one of the promising options currently under active investigation along with other designs, such as the helical cooling channel, or the RFOFO snake. The RFOFO ring provides an impressive reduction in the six-dimensional emittance in a small number of turns with a relatively low particle loss factor. However, the design of the injection and extraction channels and kickers is very challenging, and this ring could not be used as is, because the bunch train is too long to fit in the ring.
Both problems would be removed in the RFOFO helix, also known as the Guggenheim channel. In addition, utilizing the helix solves another important problem, namely, the overheating in the absorbers. Also, since the injection and extraction come naturally with the Guggenheim design, full use of all 12 cells for cooling is possible as opposed to the RFOFO ring in which RF cavities and absorbers in two of the 12 cells are removed for the purposes of installing utility insertions.
Both the RFOFO ring and the Guggenheim cooling channel are very compact by design due to the short lifetime of muons, which inevitably leads to the overlap of magnetic fields in the solenoids providing focusing and bending and electric fields in the RF cavities. Various studies suggest that the presence of the magnetic field might disrupt the performance of RF cavities by causing breakdown. Thus, an alternative layout of the cooling channel is being considered, the so-called magnetically insulated scheme. The concept implies moving the solenoidal coils from over the RF cavities into the irises and shaping the RF cavities such that the walls of the cavities are predominantly parallel to the magnetic field lines. The results of the corresponding simulations are provided.
PACS numbers: 29.20.db, 29.27.-a, 29.27.Bd, 41.75.Lx