A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.

We describe the magnet challenges for a Muon Collider, an exciting option considered for the future of particle physics at the energy frontier. Starting from the comprehensive work performed by the US Muon Accelerator Program, we have reviewed the performance specifications dictated by beam physics and the operating conditions to satisfy the accelerator needs. Among the many magnets that make up a muon collider, we have identified four systems that represent well the envelope of challenges: the target and capture solenoid, the final cooling solenoid, the accelerator dipoles and the collider dipoles. These systems provide focus for the development of novel concepts, largely based on HTS for reasons of performance, cost and sustainability. After giving a consolidated overview of the needs for the magnet systems, we describe here the basic technology options considered, and the plan for design and development activities.

The High-Luminosity Large Hadron Collider (HL-LHC) project aims at extending the operability of the LHC by another decade and increasing by more than a factor of ten the integrated luminosity that the LHC will have collected by the end of Run 3. This will require doubling the beam intensity and reducing the transverse beam size compared to those of the LHC design. The higher beam brightness poses new challenges for machine safety, due to the large energy of 700 MJ stored in the beams, and for beam stability, mainly due to the collimator contribution to the total LHC beam coupling impedance. A rich research program was therefore started to identify suitable materials and collimator designs, not only fulfilling impedance reduction requirements but also granting adequate beam-cleaning and robustness against failures. The use of thin molybdenum coatings on a molybdenum–graphite substrate has been identified as the most promising solution to meet both collimation and impedance requirements, and it is now the baseline choice of the HL-LHC project. In this work we present the main results of the coating characterization, in particular addressing the impact of coating microstructure on the electrical resistivity with different techniques, from Direct Current (DC) to GHz frequency range.

The international particle physics community considers a Muon Collider (MC) as a possible option for the successor of the Large Hadron Collider (LHC) at CERN. An international collaboration has recently been set up to produce a conceptual design study of a Muon Collider. One of the main challenges is the need for an ultra-high magnetic field solenoid for the final cooling of the muons. This magnet must have a bore aperture of about 5 cm and a 1% magnetic field homogeneity over 0.5 m of length. CERN is exploring the possibility of developing such a magnet by only using a stack of Rare-earth Barium Copper Oxide (ReBCO) tapes as a conductor. The study's main idea is to produce a modular compact magnet constituted by an assembly of identical pancakes electrically connected in series. Quench protection and stress management are the biggest design challenges. To cope with them, we are investigating the option of Non/Metal-Insulated (N/M-I) pancakes, each made of a single coil (i.e., discarding concentric, nested coils), inserted in a stiff outer ring that provides a sufficient precompression to the coil (∼200 MPa). To protect the magnet after a quench and to ramp up the field sufficiently fast, the N/M-I coil interlayer electrical resistance must be optimized and controlled. In this paper, we present a preliminary design of this concept. If successful, this type of design will contribute to developing high-field solenoids for particle accelerators and promote the use of ReBCO tapes in compact windings needed for different applications, such as electrical machines and fusion reactors based on magnetic confinement.

The “target and capture channel” is one of the crucial systems in a Muon Collider, whose function is to create an intense muon beam from the decay of the pions generated by a multi-MW proton beam impinging on a particle-producing target. To guarantee the capture of the pions and resulting muons, the channel consists of a sequence of solenoids, providing a well-defined profile on axis, with 20 T peak at the target location and an adiabatic decrease along the channel length. The solenoids of the channel need substantial shielding from the radiation heat load and dose originating from the nuclear interactions at the target, which results in large bore in excess of 1 m. In this paper we describe a concept for the target and capture solenoid, built with modules whose geometry is optimized to produce the required field profile while minimizing the stored magnetic energy of the system. All solenoids are wound with an internally cooled HTS cable. We focus here on the magnetic and mechanical design of the magnets.