• M. Abo-Bakr, R. Follath, A. Meseck
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministerium für Bildung und Forschung, the state of 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 a cascaded High-Gain Harmonic-Generation (HGHG) scheme. With a seed laser, tunable between 230 nm and 460 nm, the desired output radiation wavelength range from 1.24 nm to 51 nm can be covered. Signal to noise ratio and coherence of the HGHG FEL radiation degrades quadratically with the harmonic number. For the short-wavelength BESSY-FEL line, operating on the 225th harmonic of the seed, a cure to this effect and maintaining the coherence is to improve the spectral purity of the output radiation by implementation of a "non-dispersive double-monochromator" system between two HGHG stages. Layout and parameters of such a monochromator section are described. To separate the electron beam path from the optical devices a bypass section is needed. Its design is presented and influences on the electron beam dynamics are discussed. Simulations of the full cascaded HGHG FEL, using the restored seed radiation and the bypassed electron beam, are presented.

• M.-E. Couprie, M. LABAT
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• B. Carre, D. Garzella, G. Lambert
  CEA/Saclay, Gif-sur-Yvette
• O.V. Chubar, A. Loulergue, L. Nahon
  SOLEIL, Gif-sur-Yvette
• M. Jablonka, F. Meot, A. Mosnier
  CEA/DSM/DAPNIA, Gif-sur-Yvette
• J.-R. Marques
  LULI, Palaiseaux
• P. Monot
  CEA/DSM, Gif-sur-Yvette
• J.-M. Ortega
  LURE, Orsay
• A. Rousse
  LOA, Palaiseau

ARC-EN-CIEL (Accelerator-Radiation for Enhanced Coherent Intense Extended Light), the French project of a fourth generation light source aims at providing the user community with coherent femtosecond light pulses covering from UV to soft X ray. It is based on a CW 1 GeV superconducting linear accelerator delivering high charge, subpicosecond, low emittance electron bunches with a high repetition rate. The FEL is based on in the injection of High Harmonics in Gases in a High Gain Harmonic Generation scheme, leading to a rather compact solution. The produced radiation extending down to 0.8 nm with the Non Linear Harmonic reproduces the good longitudinal and transverse coherence of the harmonics in gas. Optional beam loops are foreseen to increase the beam current or the energy. They will accommodate fs synchrotron radiation sources in the IR, VUV and X ray ranges and a FEL oscillator in the 10 nm range. An important synergy is expected between accelerator and laser communities. Indeed, electron plasma acceleration will be tested for possible future compact electron beam sources for Xray FEL. Fs hard X ray can also be produced by Thomson Scattering. An overview of the user scientific case will also be given.

• H. Freund
  SAIC, McLean
• M. Abo-Bakr, K. Goldammer, D. Kraemer, B.C. Kuske, A. Meseck
  BESSY GmbH, Berlin
• S. Biedron
  ANL, Argonne, Illinois

The BESSY FEL is based on a seeded cascade of High Gain Harmonic Generation (HGHG) sections followed by an amplifier to produce coherent and stable short wavelength output. Here, we report on comparative design studies carried out using the MEDUSA [1], and GENESIS [2] simulation codes. These two codes have each been used to successfully predict a variety of FEL designs and have agreed well with a number of important experiments. In addition, they were included in a comparative study of FEL simulation [3] that reported substantial agreement between the codes for the specific configurations studied. However, these codes are based on different assumptions. GENESIS treats the particle dynamics using a wiggler-averaged orbit approximation, the transverse electromagnetic field is treated using a field solver, and harmonics are not included. MEDUSA does not use the wiggler-averaged orbit approximation to treat particle dynamics, the transverse fields are treated using a Gaussian modal superposition, and harmonics are included self-consistently. Hence, the comparative study for an HGHG cascade is important. We report the results where the parameters of each stage have been optimized.

[1] H.P. Freund et al., IEEE JQE 36, 275 (2000). [2] S. Reiche, NIMA 429, 243 (1999). [3] S.G. Biedron et al., NIMA 445, 110 (2000).

• T.V. Shaftan, S. Krinsky, D.F.L. Liu, J.B. Murphy, J. Rose, X.J. Wang, T. Watanabe, L.-H. Yu
  BNL, Upton, Long Island, New York
• H. Loos
  SLAC, Menlo Park, California

Funding: The manuscript has been authored by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH1-886 with the U.S. Department of Energy.

We discuss analysis of an experiment with High-Gain Harmonic Generation Free Electron Laser at BNL (DUV FEL). The tunability concept [1] of a seeded FEL with a fixed seed wavelength has been verified experimentally. During the experiment we recorded about 200 radiation spectra corresponding to different energy chirps in the electron beam. We have analyzed this set of spectral data to obtain properties of HGHG radiation. Correlations and trends in the radiation spectrum at 266 nm have been observed and studied.

[1] T. Shaftan and L.H. Yu, Phys. Rev. E 71, 046501 (2005)