• A. Meseck, M. Abo-Bakr, J. Bahrdt, B.C. Kuske
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

BESSY proposes a soft X-ray free electron laser (FEL) multi-user facility. It will consist of three undulator lines, each based on cascaded High-Gain Harmonic-Generation (HGHG) scheme delivering photons in energy range of 24 eV to 1 keV. Start-to-end Simulations including error sources from the injector, and linac structure have been performed to provide realistic information about the expected radiation field (B. Kuske, FEL2005). However, the beamline designer needs to know the exact location and the size of the photon beam waist to maximize the brightness at the sample. This information can be derived from results of longitudinal propagation of the electric field distribution, which can be extracted from simulation results using the code GENESIS. The results of the Start-to-End simulation are used for realistic prediction of the photon beam properties of the BESSY Soft X-ray FEL.

• I.S. Ko
  PAL, Pohang, Kyungbuk

Pohang Accelerator Laboratory (PAL) has recently launched a new XFEL project based on SASE technology. This PAL-XFEL will utilize the existing 2.5 GeV injection linac to the storage ring by upgrading its energy up to 3.7 GeV initially and possibly up to 4.5 GeV later on. The wavelength covers up to 0.18 nm when the electron beam energy is 4.5 GeV. In-vacuum undulator will be used to generate FEL lasing. Overall design philosophy and some details will be presented.

• J.-Y. Raguin, R.J. Bakker, K.S.B. Li, M. Pedrozzi
  PSI, Villigen

In the Low Emittance Gun (LEG) under development at PSI an extremely bright electron beam is produced from a field emission array and then rapidly accelerated in a diode configuration up to 1 MeV with gradients of the order of 250 to 500 MV/m. The electronic emission from such a cold cathode allows normalized intrinsic emittance below 0.1 mm.mrad well suited for X-ray FELs or linear collider applications. The diode is followed by an L-band RF-gun like cavity to further accelerate the beam. A third harmonic field is superposed to the fundamental [1] 1.5 GHz pi-mode field to minimize the RF emittance. We report here on the design of such a two-frequency RF cavity with some details on the RF coupling and possible tuning mechanisms. Beam dynamics studies, performed with PARMELA and the fully self-consistent code MAFIA, are presented and compared with the results obtained for an RF cavity excited with the fundamental frequency only.

[1] D.H. Dowell et al., Nucl. Instr. and Meth. A 528 (2004) 316.

• Y. Kim, K. Floettmann, S. Schreiber
  DESY, Hamburg
• D. Son
  CHEP, Daegu

Funding: for the TTF2 VUV-FEL Team

By the help of nonlinearity in the longitudinal phase space, the VUV-FEL at the TESLA Test Facility phase 2 (TTF2) is under operating in the femtosecond (fs) FEL mode which generates coherent and ultra-bright SASE source with photon pulse duration time of around 20 fs (FWHM) and wavelength of around 32 nm. For the fs FEL mode operation, bunch length of electron beams should be compressed by two bunch compressors to have a leading spike in the longitudinal beam density distribution or peak current. The required peak current at the spike is higher than about 1.0 kA, and the spike length is shorter than around 200 fs (FWHM). In this paper, we describe our commissioning experiences to optimize two TTF2 bunch compressors for the fs FEL mode operation.

• M. Roehrs, A. Bolzmann, M. Huening, H. Schlarb
  DESY, Hamburg
• K. Honkavaara
  Uni HH, Hamburg

Among the very critical parameters for the operation of the VUV-FEL at DESY are the slice-emmittance and beam optics matching of the current peak in the electron bunch. Conventional tools for measuring the beam size are sensitive to the projected properties of the bunch only and hence suffer from mixing of different parts of the bunch. A combination of streaking with a transverse deflecting rf structure (LOLA) and a quadrupole scan allowed to measure the spike separate from the rest of the bunch. Indeed significant differences in terms of emmittance and optical functions have been found.

• G. Penco, M. Trovo
  ELETTRA, Basovizza, Trieste
• S.M. Lidia
  LBNL, Berkeley, California

In the framework of the FERMI@ELETTRA project, aimed to build an x-ray FEL source based on laser-seeded harmonic generation, a crucial role is played by the electron source, which has to produce a very high quality beam, in terms of low emittance and uncorrelated energy spread. A very attractive solution is the SLAC/BNL/UCLA 1.6 cell s-band gun III based upon the demonstrated high performance of this design and its descendants. This paper describes the results of the optimization studies based on the gun III design and carried out with two space charge tracking codes (GPT and ASTRA) for nominal operating parameters. In particular two different bunch charge regimes has been explored: low (few hundreds of pC) and high (~1nC) . In the first case, the limited charge extracted from the photo-cathode allows to propagate a bunch with an initial higher density and to compress it along the linac down to a few hundreds of fs, attaining a high peak current bunch with a very low slice emittance. The second case has been investigated in order to verify the possibility to produce a "1 ps plateau" bunch with acceptable peak current and a slice emittance lower than 2 mm mrad. We present simulation results for both cases.

• H. Loos, D. Dowell
  SLAC, Menlo Park, California
• M. Boscolo, M. Ferrario, C. Vicario
  INFN/LNF, Frascati (Roma)
• M. Petrarca
  INFN-Roma, Roma
• L. Serafini
  INFN-Milano, Milano
• B. Sheehy, Y. Shen, T. Tsang, X.J. Wang
  BNL, Upton, Long Island, New York

Funding: Work supported by DOE contracts DE-AC02-76SF00515 and DE-AC02-98CH10886

The photoinjectors for future short wavelength high brightness accelerator driven light sources need to produce an electron beam with ultra-low emittance. At the DUV-FEL facility at BNL, we studied the effect of longitudinally shaping the photocathode laser pulses on the electron beam dynamics. We report on measurements of transverse and longitudinal electron beam emittance and comparisons of the experimental results with simulations.