Proton colliders at the energy frontier

Since the first proton collisions at the CERN Intersecting Storage Rings (ISR) (Johnsen, 1973; Myers, 2010) [[1], [2]], hadron colliders have defined the energy frontier (Scandale, 2014) [3]. Noteworthy are the conversion of the Super Proton Synchrotron (SPS) (Hatton, 1991; Evans, 1988) [[4], [5]] into a proton-antiproton collider, the Tevatron proton-antiproton collider (Lebedev and Shiltsev, 2014) [6], as well as the abandoned SSC in the United States (Jackson et al., 1986; Wienands, 1997) [[7], [8]], and early forward-looking studies of even higher-energy colliders (Keil, 1992; Keil, 1997; Barletta and Leutz, 1994; The VLHC Design Study Grup (Ambrosio et al.) 2001) [[9], [10], [11], [12]]. Hadron colliders are likely to determine the pace of particle-physics progress also during the next hundred years. Discoveries at past hadron colliders were essential for establishing the so-called Standard Model of particle physics. The world's present flagship collider, the Large Hadron Collider (LHC) (Brüning et al., 2004) [13], including its high-luminosity upgrade (HL-LHC) (Apollinari et al., 2017) [14], is set to operate through the second half of the 2030's. Further increases of the energy reach during the 21st century require another, still more powerful hadron collider. Three options for a next hadron collider are presently under investigation. The Future Circular Collider (FCC) study, hosted by CERN, is designing a 100 TeV collider, to be installed inside a new 100 km tunnel in the Lake Geneva basin. A similar 100-km collider, called Super proton-proton Collider (SppC), is being pursued by CAS-IHEP in China. In either machine, for the first time in hadron storage rings, synchrotron radiation damping will be significant, with a damping time of the order of 1 h. In parallel, the synchrotron-radiation power emitted inside the cold magnets becomes an important design constraint. One important difference between FCC and SppC is the magnet technology. FCC uses 16 T magnets based on Nb3Sn superconductor, while SppC magnets shall be realized with cables made from iron-based high-temperature superconductor. Initially the SppC magnets are assumed to provide a more moderate dipole field of 12 T, but they can later be pushed to a final ultimate field of 24 T. A third collider presently under study is the High-Energy LHC (HE-LHC), which is a higher energy collider in the existing LHC tunnel, exploiting the FCC magnet technology in order to essentially double the LHC energy at significantly higher luminosity.