Appendix

Appendix H: Steps toward a muon collider

This section lists the steps required to demonstrate the viability of a muon collider, and current activities.

End-to-end conceptual design

The "front end" of a muon collider and that of a neutrino factory have much in common. As neutrino factory work to date has shown, it is useful to develop an end-to-end design to illuminate the further simulation, design and hardware R&D needed for development of a facility; to derive early cost estimates; and to evaluate viability. Such exercises have been carried out.

Proton driver

To achieve luminosities of O(10³⁴cm^-2s^-1) requires proton power on target of about 4 MW in the form of approximately 3 ns-long bunches each with of O(10¹⁴) protons. This driver would be an upgrade of Project X. Some accumulator, from an appropriate source, with fast extraction would need development.

Targeting, capture and phase rotation

While several multimegawatt target developments have been carried out, each has special features, and the muon collider target is no exception. An international experiment is now underway using a mercury jet and the requisite peak proton intensity. Other target schemes need further investigation. Capture and phase rotation require very-high-field solenoids and low-frequency cavities or induction accelerator units that can operate in magnetic fields, all of which need R&D.

6D ionization cooling

Ionization cooling is a key process for both the neutrino factory and muon collider. The neutrino factory requires only transverse cooling (4D) by about a factor of 100 in the phase space area to produce a useful neutrino beam. A muon collider, however, requires a 6D phase space volume reduction of 10⁶. So far, neither has been demonstrated, although a 4D cooling experiment is now about three years from data taking. Current ideas envision three different configurations for performing the 6D cooling, but no complete experiment testing any of them is yet designed. All schemes use high magnetic fields and high-gradient cavities, preferably immersed in high-magnetic fields together with energy loss-cells either separate or incorporated into the reaccelerating cavities. All of these items require performance well beyond the current state of the art.

Reacceleration

After cooling, the muons must be rapidly accelerated to the full collision energy. Schemes using linacs, recirculating linacs, fixed-field alternating-gradient accelerators, pulsed synchrotrons and combinations of these have been suggested. High-gradient, relatively low-frequency superconducting cavities and other accelerator technology beyond today's practice require design and development in an iterative cycle with system design to understand the optimum approach and cost for a given target luminosity.

Collider ring

Maximizing the luminosity requires a very-high-magnetic-field storage ring formed of magnets with great radiation tolerance. Both conditions are far from current practice and would require a concerted design and development program for feasibility and economic assessment. The design of the focusing lattice is also very challenging in its demand for low-momentum compaction and high-momentum acceptance.

Detector

Besides the challenges of detection in a high-luminosity lepton environment, a muon collider detector must deal successfully with a very high radiation background caused by the muon decay electrons. This problem has received some consideration in the past, but the advances of detector technology require continuing reevaluation.

Program elements

In addition to the several technology R&D matters that require resolution for an evaluation of muon collider viability, extensive simulation and design activities are required. Some technology R&D items are

• high-field magnets, including solenoids, dipoles and quadrupoles, with the development of accompanying superconducting materials, including high-temperature superconductor,
• high-gradient RF cavities, both normal and superconducting, of various frequencies with normal-conducting cavities immersed in magnetic fields,
• liquid or high pressure gaseous hydrogen or LiH dE/dx cells,
• auxiliary technologies.

The simulation and design work that is required across the board is often neglected in evaluating needed resources.

Current activities

A worldwide collaboration currently looking at neutrino factories expects to issue a report in 2012 reviewing the physics as it appears then and presenting possibilities for discovery. Currently the international MERIT experiment at CERN is exploring the mercury jet production target at the needed peak power level. In the U.S., the Neutrino Factory and Muon Collider Collaboration of laboratory and university scientists, together with international partners, is performing the MUCOOL activities at Fermilab to develop muon cooling technologies. NFMCC is coordinating U.S. participation in the Muon Ionization Cooling Experiment at Rutherford Appleton Laboratory to carry out a 4D ionization cooling and demonstration project. In addition, Fermilab has commissioned a Muon Collider Task Force to explore long-term prospects of a muon collider.