• 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.

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

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

BESSY proposes a linac-based High-Gain Harmonic-Generation (HGHG) free electron laser (FEL) facility with three independent FEL lines. In the BESSY High-Energy-FEL (HE-FEL), a seed laser wavelength of 280nm is downconverted to 1.24nm by a cascade of four HGHG-stages. This procedure requires a high brightness electron beam and a high power seed laser. With the nominal set of beam parameters, radiation power in the range of GWs can be achieved. However, the signal to noise ratio degrades in each HGHG stage. This motivated intensive studies on the possibilities to further optimize the performance of the BESSY HE-FEL. In this paper, we report on three methods aiming to control the signal to noise ratio. They include simulation studies of new seeding schemes with HHG-lasers at shorter wavelengths and seeding with higher seed powers. Also, a concept for the integration of monochromators between two HGHG-stages has been worked out, see also [1]. All methods were studied extensively with regard to their influence on FEL output power, pulse duration and spectral bandwidth.

[1] M. Abo-Bakr et al., these Proceedings