Reverse Emittance Exchange for Muon Colliders

Muon collider luminosity depends on the number of muons in the storage ring and on the transverse size of the beams in collision. As currently envisioned, adequate luminosity cannot be provided without large muon intensities, due to insufficient ionization cooling of the beam. Unfortunately, the proton drivers needed to produce these large muon intensities are expensive, and the decay of the large number of muons in the storage ring present environmental radiation problem, making experiments difficult. To address these problems, six-dimensional cooling schemes are being developed that would allow smaller, high-frequency radio frequency (RF) cavities to be used for later cooling stages and for acceleration. However, the bunch length at collision energy becomes shorter than needed to match the interaction region beta function. This project will develop a new concept for shrinking transverse beam dimensions by lengthening each bunch (reverse emittance exchange) to achieve high luminosity in a muon collider with fewer muons. In Phase I, analytic expressions for the reverse-emittance exchange mechanism were derived, including a new resonant method of beam focusing. Correction schemes for the aberrations were explored, and a scheme was invented to coalesce smaller intensity bunches, after being cooled and accelerated, to high-energy intense bunches suitable for a muon collider. In Phase II, the two stages of reverse emittance exchange, along with the third coalescing stage, will be optimized. The optimization will involve determining the fraction of momentum space to be allocated to each stage, in order to maximize the peak and average luminosity. Commercial Applications and other Benefits as described by the awardee: The technology should allow the requirements for muon production rates to be relaxed enough so that existing or near future facilities could be modified for use as a muon collider. If the case for a muon collider as the next energy frontier machine can be made compelling, it becomes a candidate to be added to the other options for the High Energy Physics community.