SIMULATION AND MEASUREMENTS OF CRAB CAVITY HOMS AND HOM COUPLERS FOR HL-LHC * J. A. Mitchell † 1, 2 , R. Apsimon 2 , G. Burt 2 , T. Jones 2 , N. Shipman 1, 2 Lancaster University, Lancaster, UK I. Ben-Zvi, S.Verdú-Andrés, B. Xiao, BNL, Upton, NY 11973, USA R. Calaga, A. Castilla, A. Macpherson, A. Zwozniak, CERN, Geneva, Switzerland T. Powers, H. Wang, JLab, Newport News, VA 23606, USA 1 also at CERN, Geneva, Switzerland 2 also at the Cockcroft Institute, Daresbury, UK Abstract Two Superconducting Radio-Frequency (SRF) crab cavi- ties are foreseen for the High Luminosity LHC (HL-LHC) upgrade. Preliminary beam tests of the Double Quarter Wave (DQW) crab cavity will take place in the Super Proton Synchrotron (SPS) in 2018. For damping of the cavity’s Higher Order Modes (HOMs) the DQW has three identi- cal on-cell, superconducting HOM couplers. The couplers are actively cooled by liquid heluim. In this paper, elec- tromagnetic simulations of the HOMs and HOM couplers are presented. A novel approach to pre-installation spectral analysis of the HOM couplers is then presented, detailing both simulated and measured data. Measurements of the cavity HOMs at warm and in Vertical Test Facilities (VTFs) at both JLAB and CERN are detailed, comparing the mea- sured characteristics of each mode to that of the simulated data-sets. Finally, the measured cavity data is compared with the test box measurements to see by what extent any reduction in damping can be predicted. INTRODUCTION To damp the HOMs in the DQW crab cavity, the trans- mission response of the three identical HOM couplers was deigned to provide high transmission at the frequencies of high impedance modes, whilst rejecting the fundamental mode at ∼ 400 MHz. The high transmission peaks are no- tated as the ‘filter interaction regions’ and the rejection of the fundamental mode is achieved with a band-stop response at this frequency. with a band-stop response centred around this frequency. The HOM coupler designed for the SPS DQW crab cavity [1] is shown in Fig. 1 and it’s S 21 response is plotted in Fig. 2. HOM COUPLER TEST BOXES Complex HOM coupler geometries could result in a de- viation from RF performance criteria due to inaccuracies in machining processes. From this, a motivation for test- stands capable of accurately defining the coupler’s spectral response produced two ‘test-boxes’ [2]. The manufactured test-boxes can be seen in Fig. 3. * This work is supported by the HL-LHC project, Lancaster University and the Cockcroft core grant. † j.a.mitchell@lancaster.ac.uk Figure 1: CAD model (left) and photograph (right) of the HOM coupler for the SPS DQW crab cavity. Figure 2: Transmission characteristics for the SPS HOM coupler. Relative amplitude is used as a waveguide port is located on open vacuum. Although the test boxes were designed to give an accu- rate representation of the coupler’s spectral response, minor differences between the test-box and coupler response were still present. This difference was optimised to best represent Figure 3: Test box designs for the DQW crab cavity HOM couplers. The designs are denoted the L-bend transmission (left) and coaxial chamber (right) test-boxes. THPB059 Proceedings of SRF2017, Lanzhou, China - Pre-Release Snapshot 21-July-2017 10:00 ISBN 978-3-95450-191-5 0 Copyright © 2017 CC-BY-3.0 and by the respective authors - Pre-Release Snapshot 21-July-2017 10:00 Fundamental SRF R&D Other than bulk Nb