Intense Muon Beam R&D
As the muon is an unstable particle, there are many new facets of accelerator physics and technology that need to be examined before construction of such a device could begin. In particular, there are several major technical challenges to be faced when using muons in an accelerator:
- They are hard to produce (requiring a high-power proton beam on target).
- They are created, through pion decay, into a diffuse phase space and, due to the short muon lifetime, this phase space cannot be reduced by conventional stochastic or radiation cooling. For a Neutrino Factory, the phase space can be sufficiently reduced by ionization cooling. This mechanism also works for a Muon Collider, but the parameters are more severe and cooling is considerably more challenging. Thus, the final emittance of the muon beam will likely remain larger than that possible in an electron-positron collider. Novel approaches to emittance reduction are being explored, and some of these may provide a solution.
- They decay in their rest frame with a lifetime of 2.2 microseconds. This problem is partially overcome by rapidly increasing the energy of the muons, and thus taking advantage of their relativistic gamma factor. At high energies, the muon lifetime suffices for roughly 500 decay ring turns. One consequence of the muon decay is that the decay products heat the magnets of the decay ring or collider, and also create backgrounds in the collider detector.
- For a collider, the luminosity requirements and the limits on muon production require the acceleration, storage, and collisions of intense bunches (2x1012 particles) in a bunch length of 3-10 mm and transverse dimensions of about 10 microns at the interaction point. For a Neutrino Factory, the bunch length and number of circulating bunches are not critical parameters, which greatly eases the beam preparation requirements.
Dealing with these challenges has been undertaken by the U.S. Neutrino Factory and Muon Collider Collaboration (NFMCC), including scientists from the sponsoring National Laboratories and a number of university groups.
In a muon accelerator complex, a high intensity proton source is bunch compressed and focused on a target. The pions generated are captured by a high-field solenoid and transferred to a solenoidal decay channel followed by an RF bunching and phase rotation section. The phase rotation serves to reduce the momentum spread of the muons into which the pions decay. Subsequently, the muons are cooled transversely by a sequence of ionization cooling stages. Each stage consists of energy loss, and acceleration. For a muon collider, it is also necessary to reduce the longitudinal phase space of the beam, which requires emittance exchange in energy-absorbing wedges in the presence of dispersion. Once they are cooled, the muons must be rapidly accelerated to minimize decay losses. This can be done in recirculating linear accelerators (e.g., like that at Jefferson Lab), in rapid-cycling synchrotrons, or in Fixed-Field Alternating Gradient (FFAG) rings.Finally, for a Neutrino Factory, the beam is injected into a decay ring with one or more long straight sections aimed at nearby detectors as well as those located thousands of kilometers away. Muon decays in the straight sections produce beams of neutrinos for the remote detectors.
Work on a Neutrino Factory can be separated into the following areas: Target and Capture Section, Phase Rotation and Bunching Section, Cooling Section, Acceleration Section, and Decay Ring. The first three areas, collectively referred to as the "Front End", represent the broad area that is presently the main focus of CBP activity. All the Front-End systems require high-gradient RF systems operating at relatively low frequency (a few hundred MHz) and superconducting solenoids operating at fields of about 5 T.
A Muon Collider would have unique technical and physics advantages when compared with either hadron or electron machines, and should thus be regarded as complementary. Of course, a Muon Collider is a new and untried concept, and our effort to understand it has just begun. Still, studies aimed toward an eventual Muon Collider have the potential to define a new and productive direction for high-energy physics at the energy frontier. LBNL scientists are continuing to look at the problems relevant to a Muon Collider.