Development of HOM Couplers for the LHC Superconducting Crab Cavities

As part of the planned luminosity upgrade for the Large Hadron Collider at CERN the HiLumi project), compact crab cavities are under development. These cavities will rotate the hadron bunches at the interaction point to produce more effective collisions. To get the high gradients needed to meet the needs of the LHC main ring, superconducting RF has been chosen for these cavities. In collaboration with CERN and the US LHC Accelerator Research Program LARP), Niowave is currently building advanced versions of two crab cavity designs: the double quarter- wave designed at Brookhaven National Lab and the RF dipole design created at Old Dominion University. The current development plan involves installing cryomodules with crab cavities at the SPS ring at CERN for testing followed by installation into the main LHC ring. One of the main challenges in moving from the ring test in the Super Proton Synchrotron SPS) at CERN to operation in the main LHC ring is improvement of the higher-order-mode HOM) damping in the two cavity designs. Both cavities were designed with HOM impedances in mind, and the full performance can only be reached with a set of HOM couplers. HOM coupler designs have already been proposed for the two advanced cavity designs, and the cavities themselves, under construction at Niowave, include ports for HOM dampers. Further, a significant amount of progress has already been made on designs of antenna-based dampers to meet the needs of the SPS ring test. This fast track SBIR project will develop HOM couplers for the RF dipole crab cavity, which has been designed to allow waveguide dampers. The incorporation of waveguide elements can greatly increase the HOM power handling and will perform over a broad frequency range. The SBIR project will design and fabricate the first damping structures that meet the requirements for installation of crab-cavity cryomodules in the LHC main ring. This work will necessarily involve breakthrough R&D on the part of Niowave and its collaborators, as crab cavities with HOM damping have never been successfully implemented in a hadron ring, much less one as demanding as the LHC.