• K. Hanke, C. Carli, R. Garoby, F. Gerigk, A.M. Lombardi, S. Maury, C. Rossi, M. Vretenar
  CERN, Geneva

The construction of Linac4, a 160 MeV H^- Linac, is the first step in upgrading the LHC injector chain. Unlike CERN's present injector linac, Linac4 will inject into the subsequent synchrotron via charge exchange injection. In a first stage, it will inject into the existing CERN PS Booster. At a later stage, Linac4 has the option to be extended by a superconducting linac (SPL) which could then inject into a new synchrotron (PS2). Construction of Linac4 has started in 2008, and beam operation is presently planned for 2014. An overview of the Linac4 main parameters and design choices is given, and the status of the construction reported.

• M. Schuh, F. Gerigk, J. Tuckmantel
  CERN, Geneva
• M. Schuh
  MPI-K, Heidelberg
• C.P. Welsch
  Cockcroft Institute, Warrington, Cheshire

Higher Order Modes (HOMs) can severely limit the operation of superconducting cavities in a linac with high beam current, high duty factor and complex pulse structure. The full HOM spectrum has to be analyzed in order to identify potentially dangerous modes already during the design phase and to define their damping requirements. For this purpose a dedicated beam simulation code focused on beam-HOM interaction was developed, taking into account important effects like the HOM frequency spread, beam input jitter, different chopping patterns, as well as klystron and alignment errors. Here this code is used to investigate in detail the HOM properties of the cavities foreseen in the Superconducting Proton Linac (SPL) at CERN and their potential to drive beam instabilities. A special focus is set to HOM excitation by chopped pulses with high repetition rate and on the influence of HOMs on recirculating electron beams in the high-energy part of the SPL. Finally, the HOM characteristics of similar linac designs are presented and compared to the SPL.