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The High-Luminosity upgrade of the Large Hadron Collider (LHC) (Todesco et al., 2021) foresees the replacement of the magnets around the interaction points of the ATLAS and CMS experiments at CERN. One of the new magnets is the separation-recombination dipole, also called Main Bending Recombination Dipole (MBRD)(Caiffi et al., 2021), (Levi et al., 2022) in the D2 cryomagnet assembly. It consists of a cosine-theta, double aperture with same polarity, winded with a Nb-Ti Rutherford cable, with a central field of 4.5 T and a magnetic length of 7.78 m, which generates an integrated magnetic field of 35 Tm, inside of a bore 105 mm in diameter. The magnet is a collaborative effort with a design and production follow up by INFN Genova, production by ASG Superconductors, cold mass assembly and test by CERN. One 1.6 m short model and a full-scale prototype have been fully tested, while four series magnets and 2 spares are currently being built. To cope with a stored energy of 2.2 MJ in the single layer magnet, the quench heater technology has been optimized and dimensioned to limit the hot spot temperature to 300 K. This paper will describe the design choices for the tested protection scheme and the results obtained by the test, including failure case studies. The data will be compared to the STEAM-LEDET (Ravaioli et al., 2016) simulations and used to further validate the modeling parameters. With the obtained results from the tests and from modeling, the protection scheme has been validated and will be applied in the series magnet production.

Following the recommendation of the Updated European Strategy for Particle Physics, an International Muon Collider Collaboration has been formed and is currently studying the feasibility of a 10 TeV center-of-mass energy muon collider facility. Several technical challenges must be faced, mainly due to the limited muon lifetime at rest, 2.2 $\mu$s. This extreme condition requires the use of ambitious magnets, RF systems, targets, shielding, and cooling. To avoid collimated neutrino beams from muon decay and remain below the natural radiation background that affects the area surrounding the facility, the straight lengths in the collider ring should be kept to an absolute minimum. To achieve this goal, the beam optics quadrupoles should be combined with the bending dipoles, featuring a high magnetic field ($>$10 T) and gradient ($>$100 T/m) in a large aperture ($\sim$150 mm). The need for a high field derives from the compactness requirement to achieve high luminosity via high crossing frequency. The large aperture is fundamental to allocate a radiation (W) beam screen, which will protect the superconductors from the muon decay products (a radiation heat load of 500 W/m due to electrons, positrons, and their synchrotron photons). All these constraints require cutting-edge technologies for the material choices, the mechanical layout, the quench protection, and the cooling. In this contribution, we show the performance limits of the possible candidate materials for such magnets (namely the LTS NbTi, Nb$_{3}$Sn, and the HTS ReBCO). The analysis is focused on dipoles, obtaining a relationship between maximum aperture and bore field determined by constraints including cost, critical current density, mechanical stress, and quench protection.

As part of the high-luminosity upgrade of CERN LHC accelerator project, the National Institute of Nuclear Physics (INFN) in Genoa, Italy, has developed the MBRD separation-recombination dipole, also known as D2, whose function is to bring beams into collision before and after the interaction regions of the CMS and ATLAS experiments. It is a NbTi cos-theta double aperture dipole that generates a 4.5 T field in a 105 mm aperture, with a magnetic length of 7.78 m, and has the specific feature that the magnetic field in the two apertures is oriented in the same direction. The agreements between INFN and CERN, signed in 2016 and 2020, called for the construction of a short model, 1.6 m long, a prototype of final size, and the six series magnets, four of which are to be installed in the tunnel and two spare. After an international tender, the construction of all magnets was awarded to ASG Superconductors. The short model was successfully tested at CERN in a vertical cryostat in August 2020, reaching nominal current after three quenches in the second thermal cycle, validating most of the mechanical, thermal, and electrical design and providing important insights into the improvements that were implemented in the prototype. Testing of the D2 cold mass prototype was performed in October 2022. Its performance was found to be extremely good, with no quenches below nominal current even in the first thermal cycle and showing excellent operating margin in terms of current, ramp rate, and temperature. Although the series magnets were designed to be identical to the prototype, some modifications and tuning improvements, including a small cross-sectional refinement, were implemented and assessed with the construction of the first magnet in the series. This contribution reports all the activities that, based on the short model and prototype experience, led us to the construction of the first series magnet.

The MBRD (Main Bending Recombination Dipole, or D2) is one of the magnets in the interaction region of the LHC that needs to be replaced in order to fulfill the requirements of the High-Luminosity upgrade. The magnetic field must be increased up to 4.5 T in an aperture with a diameter of 105 mm and a total length of about 8 m, to reach the target integrated field of 35 T$\cdot$m. The increase of the magnetic field and, consequently, of the cross talk between the two apertures having the same polarity required a brand-new design characterized by asymmetric left-right coils and aluminum rings around the two apertures in order to compensate the repulsive Lorentz forces arising between them. The design was carried out at INFN, while the construction is taking place at the ASG Superconductors (Genova, Italy). The development plan was to initially build a 1.6 m long model, completed and cold tested in summer 2020, a full scale prototype, whose cold tests are now ongoing, followed by a series of six magnets, presently under construction. The test campaign performed on the short model and the one currently underway on the prototype provide very useful information for the magnets of the series, such as the behavior of the protection circuit in case of quenches. During the measurements performed at CERN, several quenches induced by the quench heaters were provoked, and the magnet behavior was recorded for different initial currents. This contribution will focus on the results of such tests and their comparison with the predictions obtained through the simulations with two different codes, Roxie and STEAM-LEDET.

The Future Circular Collider in hadron-hadron configuration (FCC-hh) is one of the main options for the post-LHC era of particle accelerator. The project led by CERN aims to build a 100 km-long collider with 100 TeV of center-of-mass energy, using bending dipoles able to achieve a bore field $>$14 T. The Istituto Nazionale di Fisica Nucleare (INFN) and CERN are collaborating to construct a Nb$_{3}$Sn cos-theta dipole model, to increase the R&D efforts for high-field superconducting magnets technology, according to the updated European Strategy for Particle Physics. Here we give an update on the intense design activity, and we report the results of the first winding trials performed with a dummy conductor, to experimentally validate the magnet design and start the iterative process to optimize the manufacturing method. The results of the first winding tests are shown in the paper, focusing on the impact of the coiling procedure on the mechanical behavior of the unreacted Nb$_{3}$Sn Rutherford cable.

The recombination dipoles MBRD for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN are double-aperture superconducting magnets generating a central magnetic field of 4.5 T in a 105 mm diameter bore, directed in the same direction in both apertures. The integrated magnetic field is 35 T-m in a magnetic length of 7.78 m: with respect to the corresponding magnet presently installed in LHC, the aperture is larger, the length is smaller and the central field is higher. The project, currently underway, includes the fabrication by ASG Superconductors and testing at CERN of a short model (1.6 m long), a prototype, and a series of 4 + 2 spare dipoles. The short model was delivered to CERN and successfully tested in a vertical cryostat in summer 2020, reaching nominal current after three quenches in the second thermal cycle, validating most of the mechanical, thermal and electrical design and giving indications for improvements that have been implemented in the prototype, which was completed and delivered to CERN in October 2021. It was tested in October 2022 in its final cold mass, 14 m long, which also includes the two orbit correctors. This contribution reports on key results from the prototype tests and on the further activities related to the construction of the first magnets of the series.