• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Recent theoretical and experimental studies have shown that SASE FEL with a planar undulator holds a potential for generation of relatively strong coherent radiation at the third harmonic of the fundamental frequency. Here we present detailed study of the nonlinear harmonic generation in SASE FEL obtained with time-dependent FEL simulation code FAST. Using similarity techniques we present universal dependencies for temporal, spectral, and statistical properties of the third harmonic radiation from SASE FEL.

• O. Grimm, J. Feldhaus, J. Rossbach, E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Funding: University of Hamburg

A special electromagnetic wiggler generating infrared radiation in the range 1-200 microns is planned to be installed at the DESY VUV-FEL in Hamburg by autumn 2006. The device is located after the FEL undulators, using the spent electron beam. The purpose is two-fold: first, it will serve longitudinal electron beam diagnostics, similar to other methods currently investigated using the coherent emission of radiation at wavelengths similar to the bunch length, and second it will be used as a powerful (100 MW peak) source for short (few ps) infrared radiation pulses. The natural, perfect synchronization with the VUV pulses will allow for pump-probe experiments with high timing precision. This paper will give an overview of the project, including the infrared beam transport line.

• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Present bunch compression scheme at the VUV FEL is essentially nonlinear and naturally results in a formation of a short high-current leading peak (spike) in the density distribution that produces FEL radiation. The main feature of the considered mode of operation is the production of short (15-50 fs FWHM) radiation pulses with GW-level peak power that are attractive for many users. In this paper we present main parameters of the SASE FEL radiation: temporal and spectral characteristics, intensity distributions, etc. We also analyze statistical properties of the radiation from a SASE FEL driven by short electron bunches. In the linear regime the radiation from a SASE FEL is a Gaussian random process. When approaching saturation point, statistical properties of the radiation change drastically on a scale of one field gain length. Particular attention is devoted to the analysis of fluctuations of total energy in the radiation pulse and after a narrow-band monochromator. It is shown that fluctuations at saturation are significantly suppressed when electron pulse length becomes comparable with cooperation length.

• G. Geloni, E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Nonlinear generation of coherent harmonic radiation is an important option in the operation of a X-ray FEL facility since it broadens the spectral range of the facility itself, thus allowing for a wider scope of experimental applications. We found that up-to-date theoretical understanding of second harmonic generation is incorrect. Derivation of correct radiation characteristics will follow our criticism.

• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Influence of a linear energy chirp, imposed on the electron beam, on SASE process is studied analytically and numerically.