• V.P. Yakovlev, T.N. Khabiboulline, N. Solyak, A. Vostrikov
  Fermilab, Batavia
• A. Saini
  University of Delhi, Delhi

Results of analysis are presented for the longitudinal and transverse effects of High-Order Mode (HOM) excitation in the acceleration RF system of the CW proton linac of the Project X facility. Necessity of HOM dampers in the SC cavities of the linac is discussed.

• I.R.R. Shinton, R.M. Jones, N. Juntong
  UMAN, Manchester
• N. Baboi
  DESY, Hamburg
• N. Eddy, T.N. Khabiboulline
  Fermilab, Batavia
• T. Flisgen, H.-W. Glock, U. van Rienen
  Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock

We analyse the higher order modes in the 3.9GHz bunch shaping cavities recently installed in the XFEL/FLASH facility at DESY. We report on recent experimental results on the frequency spectrum, both beam and probe based. These are compared to those predicted by finite element computer codes, globalised scattering matrix calculations and a two-band circuit model. This study is focused on the dipole component of the multiband expansion of the wakefield.

• I.G. Gonin, M.S. Champion, T.N. Khabiboulline, A. Lunin, N. Perunov, N. Solyak, V.P. Yakovlev
  Fermilab, Batavia

In the low-energy part of the Project X H-linac there families of 325 MHz SC single spoke cavities will be used, having beta = 0.11, 0.22 and 0.4. Two versions of the beta = 0.11 cavity were considered: low-beta single-spoke cavity and half-wave cavity. Results of detailed optimization of both versions are presented. Single spoke cavity was selected for the linac because of higher r/Q. Results of the beam dynamics optimization for initial stage of the linac with beta=0.11 single spoke cavity are presented as well.

• P.V. Avrakhov, A. Kanareykin
  Euclid TechLabs, LLC, Solon, Ohio
• M. Ge, I.G. Gonin, T.N. Khabiboulline, J.P. Ozelis, A.M. Rowe, N. Solyak, G. Wu, V.P. Yakovlev
  Fermilab, Batavia
• J. Rathke
  AES, Medford, NY

Use of a superconducting travelling wave accelerating (STWA) structure with a small phase advance per cell rather than a standing wave structure may provide a significant increase in the accelerating gradient in the ILC linac. For the same surface electric and magnetic fields the STWA achieves an accelerating gradient 1.2 larger than TESLA-like standing wave cavities. In addition, the STWA allows longer acceleration cavities, reducing the number of gaps between them. However, the STWA structure requires a SC feedback waveguide to return the few hundreds of MW of circulating RF power from the structure output to the structure input. A test single-cell cavity with feedback was designed and manufactured to demonstrate the possibility of a proper processing to achieve a high accelerating gradient. The first results of high-gradient tests of a prototype 1.3 GHz single-cell cavity with feedback waveguide will be presented.

• E. Vogel, C. Albrecht, N. Baboi, C. Behrens, T. Delfs, J. Eschke, C. Gerth, M.G. Hoffmann, M. Hoffmann, M. Hüning, R. Jonas, J. Kahl, D. Kostin, G. Kreps, F. Ludwig, W. Maschmann, C. Mueller, P. Nommensen, J. Rothenburg, H. Schlarb, Ch. Schmidt, J.K. Sekutowicz
  DESY, Hamburg
• H.T. Edwards, E.R. Harms, A. Hocker, T.N. Khabiboulline
  Fermilab, Batavia
• M. Kuhn
  Uni HH, Hamburg

Ultra short bunches with high peak current are required for efficient creation of high brilliance coherent light at the free electron laser FLASH. They are obtained by a two stage transverse magnetic chicane bunch compression scheme based on acceleration of the beam off the rf field crest. The deviation of the rf field's sine shape from a straight line leads to long bunch tails and reduces the peak current. This effect will be eliminated by adding the Fermilab-built third harmonic superconducting accelerating module operating at 3.9 GHz to linearize the rf field. The third harmonic module also allows for the creation of uniform intensity bunches of adjustable length that is needed for seeded operation. This paper summarizes the results from the first complete rf system test at the crymodule test bench at DESY and the first experience gained operating the system with beam in FLASH.