• R. Tarkeshian, A. Azima, J. Bödewadt, H. Delsim-Hashemi, V. Miltchev, J. Roßbach, J. Rönsch-Schulenburg
  Uni HH, Hamburg
• R. Ischebeck
  PSI, Villigen
• B. Mukherjee
  Westdeutsches Protonentherapiezentrum, Essen
• E. Saldin, H. Schlarb, S. Schreiber
  DESY, Hamburg

sFLASH is a seeded FEL experiment at DESY, which uses a 38nm high harmonic gain (HHG)-based XUV-beam laser in tandem with FLASH electron bunches at the entrance of a 10m variable-gap undulator. The temporal overlap between the electron and HHG beams is critical to the seeding process. Use of a 3rd harmonic accelerating module provides a high current electron beam (at the kA level) with ~ 600fs FWHM bunch duration. The length of the HHG laser pulse will be ~30fs FWHM. The desired overlap is achieved in steps. First is the synchronization of the HHG drive laser (Ti: Sapphire, 800nm) and the incoherent spontaneous radiation from an upstream undulator. Next, the IFEL-modulated electron bunch will pass through a dispersive section, producing a density modulation in the beam. This in turn yields emission of coherent radiation from a downstream undulator or transition radiation screen when the longitudinal overlap of the two beams is achieved. The coherently enhanced light emitted will be then spectrally analyzed. The experimental layout, simulation results of generation and transport of both light pulses, and preliminary measurements are presented.

• H. Delsim-Hashemi, A. Azima, J. Bödewadt, F. Curbis, M. Drescher, Th. Maltezopoulos, V. Miltchev, M. Mittenzwey, J. Roßbach, J. Rönsch-Schulenburg, R. Tarkeshian, M. Wieland
  Uni HH, Hamburg
• S. Bajt, K. Honkavaara, T. Laarmann, H. Schlarb
  DESY, Hamburg
• R. Ischebeck
  PSI, Villigen
• S. Khan
  DELTA, Dortmund
• A. Meseck
  Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Elektronen-Speicherring BESSY II, Berlin

Recently, the free-electron laser in Hamburg (FLASH) at DESY has been upgraded considerably. Besides increasing the maximum energy to about 1.2 GeV and installation of a third harmonic rf cavity linearizing the longitudinal phase space distribution of the electron bunch, an FEL seeding experiment at wavelengths of about 35 nm has been installed. The goal is to establish direct FEL seeding employing coherent VUV pulses produced from a powerful drive laser by high-harmonic generation (HHG) in a gas cell. The project, called sFLASH, includes generation of the required HHG pulses, transporting it to the undulator entrance of a newly installed FEL-amplifier, controlling spatial, temporal and energy overlap with the electron bunches and setting up a pump-probe pilot experiment. Sophisticated diagnostics is installed to characterize both HHG and seeded FEL pulses, both in time and frequency domain. Compared to SASE-FEL pulses, almost perfect longitudinal coherence and improved synchronization possibilities for the user experiments are expected. In this paper the status of the experiment is presented.

• H. Delsim-Hashemi, J. Roßbach
  Uni HH, Hamburg
• U. Englisch, T. Mueller, A. Schöps, M. Tischer, P.V. Vagin
  DESY, Hamburg
• I. Vasserman
  ANL, Argonne

A seeded free-electron laser (FEL) experiment at VUV wavelengths, called sFLASH, is being prepared at the existing SASE FEL user facility FLASH. Seed pulses at wavelengths around 35 nm from high harmonic generation (HHG) will interact with the electron beam in sFLASH undulators upstream of the existing SASE undulator section. In this paper the tuning results and performance of the sFLASH undulators are presented.