• M.G. Hoffmann, M. Hoffmann, F. Ludwig, P. Morozov, Ch. Schmidt
  DESY, Hamburg

For future FELs (Free-Electron-Lasers) a 3rd harmonic system was proposed to increase the SASE intensity by linearization of the beam phase space after the first bunch compression section. At DESYs FLASH facility, a 3rd harmonic cavity system, consisting of four single cavities operating at 3.9GHz has been successfully tested at the module test stand. In this paper we present field regulation measurements using a step wised down converted field detector system and a model based designed LLRF field controller. First measurements showed a promising in loop vectorsum amplitude stability of about 2·10^-5 for pulse-to-pulse operation.

• 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.