While the LHC accelerator at CERN has been operating smoothly, the High-Luminosity LHC (HL-LHC) Upgrade Project, which has aimed at attaining a total integrated luminosity of 3000 fb^-1 for about 10 years—a factor tenfold the design value of the present LHC—was planned in order to improve the statistical precision of physics results. To realize this ambitious target, a full upgrade of accelerator elements in the insertion regions for the ATLAS and CMS experiments is crucial. In particular, large-aperture superconducting magnets with a higher field and superconducting RF crab cavities hold the key to the success of the project. In this article, an overview of the HL-LHC and its relevant R&D efforts are introduced.

The large-aperture beam separation dipole (D1 magnet) needs to be developed for the high luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN. This magnet requires a field integral of 35 T·m at 12.0 kA and 1.9 K with a coil aperture of 150 mm. The technical challenges are controlling the pre-stress of the thin coils, achieving high radiation resistance and managing the iron saturation. In the framework of CERN-KEK collaboration, KEK is in charge of developing the D1 magnet and has been conducting design studies since 2011. A design including a single-layer coil based on Nb-Ti technology with a collared yoke structure was selected and fabrication of the first 2-m model started in 2015. In this paper, the overview and design studies of the D1 magnet are first introduced. Then, fabrication and test results of the first 2-m model are reported. Finally, future perspectives for the D1 production magnets are described.