• K. Yonehara, A. Lunin, A. Moretti, M. Popovic, G.V. Romanov
  Fermilab, Batavia
• R.P. Johnson, M.L. Neubauer
  Muons, Inc, Batavia
• L. Thorndahl
  CERN, Geneva

Funding: supported in part by USDOE STTR Grant DE-FG02-08ER86350

The great advantage of the helical ionization cooling channel (HCC) is its compact structure that enables the fast cooling of muon beam 6-dimensional phase space. This compact aspect requires a high average RF gradient, with few places that do not have cavities. Also, the muon beam is diffuse and requires an RF system with large transverse and longitudinal acceptance. A traveling wave system can address these requirements. First, the number of RF power coupling ports can be significantly reduced compared with our previous pillbox concept. Secondly, by adding a nose on the cell iris, the presence of thin metal foils traversed by the muons can possibly be avoided. We show simulations of the cooling performance of a traveling wave RF system in a HCC, including cavity geometries with inter-cell RF power couplers needed for power propagation.

• V.P. Yakovlev, I.G. Gonin, T.N. Khabiboulline, A. Latina, A. Lunin, V. Poloubotko, N. Solyak
  Fermilab, Batavia

Different couplers are described that allow the reduction of both transverse wake potential and RF kick in the SC acceleration structure of ILC. A simple rotation of the couplers reducing the RF kick and transverse wake kick is discussed for both the main linac and bunch compressors, along with possible limitations of this method. Designs of a coupler unit are presented which preserve axial symmetry of the structure, and provide reduced both the RF kick and transverse wake field.

• V.P. Yakovlev, A. Lunin, N. Solyak
  Fermilab, Batavia
• P.V. Avrakhov, A. Kanareykin
  Euclid TechLabs, LLC, Solon, Ohio
• S. Kazakov
  KEK, Ibaraki

The accelerating gradient required for the ILC project exceeds 30 MeV/m. With current technology the maximum acceleration gradient in SC structures is determined mainly by the value of the surface RF magnetic field. In order to increase the gradient, the RF magnetic field is distributed homogeneously over the cavity surface (low-loss structure), and coupling to the beam is improved by introducing aperture "noses" (re-entrant structure). These features allow gradients in excess of 50 MeV/m to be obtained for a singe-cell cavity. Further improvement of the coupling to the beam may be achieved by using a TW SC structure with small phase advance per cell. We have demonstrated that an additional gradient increase by up to 46% may be possible if a pi/2 TW SC structure is employed. However, a TW SC structure requires a SC feedback waveguide to return the few GW of circulating RF power from the structure output back to the structure input. Advantages and limitations of different techniques of exciting the traveling wave in this structure are considered, including an analysis of mechanical tolerances. We also report on investigations of transient processes in the SC TW structure.