Conceptual Design of the Low-Power and High-Power SPL: A Superconducting H$^-$ Linac at CERN

The potential for a superconducting proton linac (SPL) at CERN started to be seriously considered at the end of the 1990s. In the first conceptual design report (CDR), published in 2000 [1], most of the 352 MHz RF equipment from LEP was re-used in an 800 m long linac, and the proton beam energy was limited to 2.2 GeV. During the following years, the design was revisited and optimized to better match the needs of a high-power proton driver for neutrino physics. The result was a more compact (470 m long) accelerator capable of delivering 5 MW of beam power at 3.5 GeV, using state-of-the-art superconducting RF cavities at 704 MHz. It was described in a second CDR, published in 2006 [2]. Soon afterwards, when preparation for increasing the luminosity of the LHC by an order of magnitude beyond nominal became an important concern, a low-power SPL (LP-SPL) was studied as a key component in the renovation of the LHC injector complex. The combination of a 4 GeV LP-SPL injecting into a new 50 GeV synchrotron (PS2) was proposed to replace the ageing Linac2, PSB, and PS. In a later stage, if necessary for future physics programmes (neutrino production or generation of radioactive ion beams), the linac could be brought up to multimegawatt beam power by upgrading the cooling, the electrical infrastructure, and the power supplies. The construction of the low-energy front end of the LP-SPL started in 2008 as the Linac4 project [3], aimed at the replacement of Linac2, with the potential for being adapted later to the needs of a low-power or high-power SPL (HP-SPL). In parallel, the R&D on the superconducting linac continued, initially to refine the design of the LP-SPL and afterwards that of the HP-SPL. This report presents the design of the LP-SPL and the work accomplished in preparing a proposal for new LHC injectors with the support of the European Commission under the Framework Programmes 6 and 7 and in collaboration with multiple laboratories and institutions worldwide. The status of the R&D towards an HP-SPL is summarized