Higher order modes and dampers for the LHC double quarter wave crab cavity

Abstract

This thesis initially investigates the DQW crab cavity HOM couplers manufactured for tests in the Super Proton Synchrotron (SPS) at CERN and proposed for the HL-LHC upgrade. The coupler’s are modelled using equivalent circuit analysis and ‘test-boxes’ are used to measured each of the six HOM couplers for the SPS test. The DQW crab cavity’s impedance spectra and resulting HOM power generation as a result of excitation by the high-current HL-LHC beam is quantified. Following this, HOM measurements are used to characterise the deviations from the simulated case. Evaluation of the DQW’s impedance spectra and stochastic analysis of the foreseeable HOM power generation in the HL-LHC highlights potential issues with the HOMs. To mitigate the issues, a re-design of the HOM coupler is presented; The impedance and HOM power are quantified and benchmarked. In parallel, a new ‘quarter wave’ rejection filter is designed and evaluated. The operation of the HOM couplers in the presence of the operating electromagnetic field is assessed, quantifying thermal and multipacting behaviour. Finally, first measurements of crab cavity HOMs with proton bunches are presented and compared with simulations. The measurements are used to evaluate the accuracy of the methods used to calculate impedance and HOM power. Several conclusions feed-back into ancillary design and acceptance criteria for future manufacture.