• J. Bödewadt, A. Azima, F. Curbis, H. Delsim-Hashemi, M. Drescher, Th. Maltezopoulos, V. Miltchev, M. Mittenzwey, J. Roßbach, S. Schulz, R. Tarkeshian, M. Wieland
  Uni HH, Hamburg
• S. Düsterer, J. Feldhaus, T. Laarmann, H. Schlarb
  DESY, Hamburg
• R. Ischebeck
  PSI, Villigen
• S. Khan
  DELTA, Dortmund
• A. Meseck
  BESSY GmbH, Berlin

Funding: This work is supported by the Bundesministerium für Bildung und Forschung under contract 05 ES7GU1

A seeded free-electron laser operating in the soft X-ray (XUV) spectral range will be added to the SASE FEL facility FLASH. For this purpose, a 40 m long section upstream of the existing SASE undulator will be rebuilt during the shutdown in fall 2009. This includes the injection of the seed beam into a new 10 m variable-gap undulator, the out-coupling of the seeded FEL radiation and all diagnostics for photon- and electron beams. The XUV seed pulse is generated by high harmonics (HHG) from a near-infrared laser, optically synchronized with FLASH. After amplification within the undulators the XUV light will be guided towards diagnostic stations. Besides a proof-of-principle demonstration for seeding at short wavelength the purpose of this development is to provide future pump-probe experiments with a more stable FEL source in terms of spectral properties and timing.

• A. Falone, H. Fitze, R. Ischebeck, Y. Kim, M. Pedrozzi, V. Schlott, B. Steffen, L. Stingelin
  PSI, Villigen
• D. Alesini, L. Ficcadenti, L. Palumbo
  INFN/LNF, Frascati (Roma)

RF deflectors are crucial diagnostic tools for bunch length and slice emittance measurements with sub-picosecond resolution. Their use is essential in commissioning and operation of VUV and X-ray FELs. The 250MeV FEL injector, under construction at PSI, will use two of them. The first one will be installed after the gun at low energy (~7MeV), the second one at the end of the Linac at high energy (250MeV). The first RF deflector consists of a single cell standing wave cavity working on the TM110 deflecting mode, and tuned at 2997.912 MHz (frequency of the linac structures). In this note we report the motivation of this measurement, beam dynamics and beam diagnostics considerations and the RF design and simulations of this cavity.

• R.J. England, E.R. Colby, C. McGuinness, R.J. Noble, R. Siemann, J.E. Spencer, D.R. Walz
  SLAC, Menlo Park, California
• R. Ischebeck
  PSI, Villigen
• T. Plettner
  Stanford University, Stanford, California
• C.M.S. Sears
  MPQ, Garching, Munich

Funding: DOE Grants DE-AC02-76SF00515, DE-FG06-97ER41276

An experimental effort is currently underway at the SLAC National Accelerator Laboratory to focus a 50pC, 60 MeV electron beam into the hollow core of a commercial photonic bandgap fiber. The wakefield radiation produced in the fiber will be spectrally analyzed using a spectrograph in order to detect the frequency signatures of fiber modes that could be used as accelerating modes in a laser-driven fiber-based accelerator scheme. We discuss the current status of the experiment, including efforts to successfully focus the electron beam through the fiber aperture and to collect the produced wakefield radiation.

• I. Blumenfeld, F.-J. Decker, M.J. Hogan, R. Ischebeck, R.H. Iverson, N.A. Kirby, R. Siemann, D.R. Walz
  SLAC, Menlo Park, California
• C.E. Clayton, C. Huang, C. Joshi, W. Lu, K.A. Marsh, W.B. Mori, M. Zhou
  UCLA, Los Angeles, California
• T.C. Katsouleas, P. Muggli, E. Oz
  USC, Los Angeles, California

High gradient acceleration of electrons has recently been achieved in meter scale plasmas at SLAC. Results from these experiments show that the wakefield is sensitive to parameters in the electron beam which drives it. In the experiment the bunch lengths were varied systematically at constant charge. Here we investigate the correlation of peak beam current to the wake amplitude. The effect of beam head erosion will be discussed and an experimental limit on the transformer ratio set. The results are compared to simulation.