• 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

• K. Goldammer, A. Meseck
  BESSY GmbH, Berlin

In the FEL process, bunching and coherent radiation is produced at the fundamental frequency as well as its higher harmonics. BESSY proposes a linac-based cascaded High-Gain Harmonic-Generation (HGHG) free electron laser (FEL) multi-user facility. The BESSY soft X-ray FEL will be seeded by three lasers spanning the spectral range of 230nm to 460nm. Two to four HGHG stages downconvert the seed wavelength to the desired radiation range of 1.24nm to 51nm using higher harmonic bunching. As a surplus, higher harmonic radiation is intrinsically produced in each FEL stage. Radiation on a higher harmonic of the FEL frequency is of high interest because it yields the possibility to reduce the number of FEL stages. This paper details extensive studies of the higher harmonic content of the BESSY FEL radiation. Important aspects of FEL interaction on higher harmonics as resulting from theory and from numerical simulations are discussed. For the case of the BESSY FEL, methods for improving the harmonic content are presented. These methods are examined as to their influence on FEL output power, pulse duration and spectral bandwidth. Focus is laid on the application of higher harmonic radiation to seeding.

Funded by the Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

• K. Goldammer
  BESSY GmbH, Berlin
• P. Emma, Z. Huang
  SLAC, Menlo Park, California

The Linac coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. With its nominal set of electron beam, focusing and undulator parameters, it is designed to achieve SASE saturation at an undulator length of about 100m with an average power of 10GW. In order to keep the electron beam focused in the undulators, a FODO lattice is integrated along the entire length of the undulators. Nominally, the quadrupoles strengths are chosen to produce nearly constant beta function and beam size along the undulator, optimized for the FEL interaction in the exponential growth regime. Since these quadrupoles are electromagnetic, it is possible to adjust the individual quadrupole strength to vary the beta function and the beam size along the undulator, tailoring the FEL interaction in the startup and the saturation regimes. In this paper, we present simulation studies of the tapered beta function in the LCLS undulator and discuss the generated x-ray properties.

• B.C. Kuske, M. Abo-Bakr, K. Goldammer, A. Meseck
  BESSY GmbH, Berlin

Funding: Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

Contrary to storage rings, where the electron bunch properties are damped to equilibrium values due to the repeated passages through identical structures, every bunch in a single pass FEL will show individual imprints of it's passage through the linac. Based on ASTRA and ELEGANT tracking studies, realistic bunches were tracked through the BESSY-FEL undulators; the effect of timing errors of the photo cathode laser, and phase and amplitude errors of the RF fields in the injector and the linac on the FEL radiation were studied. The fluctuations of the bunch parameter due to these errors are of the order of magnitude of their variation over the bunch length, reflecting the initial electron distribution and the impact of the passed optics. The unavoidable residual energy chirp in connection with the timing jitter is of concern. The expected shot to shot variations in the FEL output are discussed.

• 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.  Kamps, V. Duerr, K. Goldammer, D. Kraemer, P. Kuske, J. Kuszynski, D. Lipka, F. Marhauser, T. Quast, R. Richter
  BESSY GmbH, Berlin
• P. Evtushenko
  Jefferson Lab, Newport News, Virginia
• U. Lehnert, P. Michel, J. Teichert
  FZR, Dresden
• I. Will
  MBI, Berlin

Funding: Funded by the Bundesministerium für Bildung und Forschung, the State of Berlin and the Zukunftsfonds Berlin

A superconducting rf photo electron injector (SRF gun) is currently under construction by a collaboration between BESSY, DESY, FZR and MBI. The project aims at the design and setup of an CW SRF gun including a diagnostics beamline for the ELBE FEL and to address R&D issues on low emittance injectors for future light sources such as the BESSY FEL. Of critical importance for the injector performance is the control of the electron beam parameters. For this reason a compact diagnostics beamline is under development serving a multitude of operation settings ranging from low-charge (77pC), low-emittance (1 pi mm mrad) mode to high-charge (2.5nC) operation of the gun. For these operation modes beam dynamics simulations are resulting in boundary conditions for the beam instrumentation. Proven and mature technology is projected wherever possible, for example for current and beam position monitoring. The layout of the beam profile and emittance measurement systems is described. For the bunch length, which varies between 5 and 50 ps, two schemes using Electro-optical sampling and Cherenkov radiation are detailed. The beam energy and energy spread is measured with an especially designed 180 degree spectrometer.