• 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.