• 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.

• I.R.R. Shinton, R.M. Jones
  UMAN, Manchester

It is necessity to be able to accurately predict the performance of the any proposed baseline accelerator design in which the effects of couplers, trapped modes, Wakefields, realistic machining and alignment errors as well as numerous other important effects have been taken into consideration. Traditionally used numerical schemes (such as Finite element and Finite difference) require vast resources and time, not only that but the inclusion of realistic defects and misalignments into the baseline configuration will prove time consuming as it will potentially require remeshing of the problem. Here we present a mode matching scheme which utilises a globalised scattering matrix approach that allows large scale electromagnetic field calculations to be obtained rapidly and efficiently. The scalar product of all the S matrices used within this paper has been determined analytically and is calculated only once per transition, adding to the efficiency of the calculation. The influence of cell misalignments and cavity perturbations on the main accelerating linacs of XFEL and CLIC are exhibited. The wake-fields in super-structures and segments of entire modules are also presented

• I.R.R. Shinton, R.M. Jones, N. Juntong
  UMAN, Manchester

The main linacs of the ILC consist of nine-cell cavities based on the TESLA design. In order to facilitate reaching higher gradients we have re-designed the cavity shape. This leads to a reduction, comparable to several current designs, in both the ratio of the surface electric field to the accelerating field (Es/Ea) and the magnetic field to the accelerating field (Bs/Ea). The bandwidth of the accelerating mode is also optimized. This new shape, which we refer to as the New Low Surface Field (NLSF) design, bears comparison with the Ichiro, Re-entrant and LSF designs.

• M.A. Fraser, R.M. Jones
  UMAN, Manchester
• M. Lindroos, M. Pasini
  CERN, Geneva

The upgrade of the normal conducting REX-ISOLDE heavy ion accelerator at CERN, under the HIE-ISOLDE framework, proposes the use of superconducting (SC) quarter-wave resonators (QWRs) to increase the energy capability of the facility from 3 MeV/u to beyond 10 MeV/u. A beam dynamics study of a lattice design comprising SC QWRs and SC solenoids has confirmed the design's ability to accelerate ions, with a mass-to-charge ratio in the range 2.5 < A/q < 4.5, to the target energy with a minimal emittance increase. We report on the development of this study to include the implementation of realistic fields within the QWRs and solenoids. A preliminary error study is presented in order to constrain tolerances on the manufacturing and alignment of the linac.