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Higher Order Modes and Dampers for the LHC Double Quarter Wave Crab Cavity

Research output: ThesisDoctoral Thesis

Published
  • James Mitchell
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Publication date10/10/2019
Number of pages273
QualificationPhD
Awarding Institution
Supervisors/Advisors
Publisher
  • Lancaster University
<mark>Original language</mark>English

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.