• P.K. Ambattu, G. Burt, R.G. Carter, A.C. Dexter
  Cockcroft Institute, Lancaster University, Lancaster
• V.A. Dolgashev
  SLAC, Menlo Park, California
• R.M. Jones, V.F. Khan
  UMAN, Manchester

Funding: The work is supported by STFC

A crab cavity is required in the CLIC to allow effective head-on collision of bunches at the IP. A high operating frequency (X-band) for the crab cavity is preferred as the deflection voltage required and the RF phase tolerance are inversely proportional to the operating frequency. However, the strong inter-bunch wakefields deteriorate the quality of the colliding bunches. The short bunch spacing of the CLIC scheme and the crab cavity's high sensitivity to dipole kicks demands very high damping of the inter-bunch wakes. A crab cavity requires special attention to the damper design as its wakefield spectrum is entirely different from that of an accelerating cavity. In addition to the higher-order modes, the orthogonally polarised dipole mode (same order mode) and the fundamental monopole mode (lower order mode) also need to be damped, however their resonant frequencies make damping these modes complicated. The same order mode suppression requires the use of an azimuthally asymmetric damper. This paper investigates the nature of the wakefields in the CLIC crab cavity and the possibility of using choke-mode damping and various types of waveguide damping to suppress them effectively.

• V.F. Khan, R.M. Jones
  UMAN, Manchester

Here we present initial results on an alternate design for CLIC main accelerating linacs which is moderately damped and detuned structure. In order to suppress the wake-fields, we detune the lowest dipole modes as they have significant impact on the beam emittance compared to the other multipoles. In order to mitigate the reappearance of the wake-field of a detuned accelerator structure, we provide moderate damping by coupling cells to manifolds which run parallel to each accelerator structure. The manifolds are designed such that they are non-propagating at the acceleration mode frequency. The cell parameters are optimised by considering the r.f. breakdown, pulse surface heating and beam dynamics constraints.