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9 pp. (2002)., In this paper we consider a possible mechanism of strong distortions of longitudinal phase space due to collective effects in an electron bunch passing a bunch compressor. Analytical expressions are derived for the case of a linear compression. Estimates for the TESLA Test Facility at DESY are presented.

13 pp. (2002). doi:10.3204/PUBDB-2017-07167, A new design for a multistage High-Gain Harmonic Generation (HGHG) scheme is proposed. The main difference with previous HGHG schemes is that in our scheme the HGHG technique can be applied more than once in a HGHG chain (single bunch scheme). This is consequence of the fact that the growth of the energy spread due to the HGHG process in our case is much less than initial energy spread, and exponential growth rate in the main undulator is practically the same as without stage sequence. Problems relating to X-ray HGHG FEL are discussed. Our studies have shown that the frequency multiplication process produces a noise degradation proportional to at least the square of the multiplication ratio. This prevents operation of HGHG FEL at a very short wavelength range. The results presented in this paper have demonstrated that the HGHG FEL approach is quite adequate for the VUV coherent source, but not scalable to X-ray devices.

11 pp. (2002)., New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt . The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0 . This leads to an amplitude modulation of the beam density at the sidebands ω 0 ± ω opt . The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0 − ω opt . Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses.

4 pp. (2002)., Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung (BESSY) plans to construct a linac-based single-pass FEL as an addition to its existing third generation storage-ring-based light-source. The project aims to obtain an FEL-based user-facility that covers the VUV and soft X-ray spectral range (20 eV⩽ℏ ω ⩽1 keV). At present, the design stage is funded as a collaboration between BESSY, DESY, the Hahn–Meitner-Institute in Berlin, and the Max–Born-Institute in Berlin. This stage focuses on optimization of the FEL as a user light-source, both with respect to its capabilities and in its performance. Important issues are: stability, seeding options of the SASE FEL, wavelength-tunability, synchronization with external laser sources and, on a longer time-scale, the generation of ultra-short (

8 pp. (2002)., A collimation scheme for the protection of the undulators at the TESLA Test Facility phase 2 (TTF2) is discussed. The transverse and energy collimation, and the protection against off-energy and mis-steered bunches are proposed in a beam line which, besides the collimation elements, integrates the fast feedback system and matches the beam to the undulator entrance.

9 pp. (2002)., This paper presents an experimental study of the statistical properties of the radiation from a SASE FEL. The experiments were performed at the TESLA Test Facility VUV SASE FEL at DESY operating in a high-gain linear regime with a gain of about 10 6 . It is shown that fluctuations of the output radiation energy follows a gamma-distribution. We also measured for the first time the probability distribution of SASE radiation energy after a narrow-band monochromator. The experimental results are in good agreement with theoretical predictions, the energy fluctuations after the monochromator follow a negative exponential distribution.

5 pp. (2002)., This paper presents the progress of the Self-Amplified Spontaneous Emission and Regenerative Amplifier Free Electron Laser experiments at the TESLA Test Facility (TTF). In the second part, the layout of the soft X-ray user facility TTF2 and the status of construction work is described.

11 pp. (2002)., A large X-ray FEL lab is under consideration within the TESLA project and is supposed to be operated in parallel with the TESLA linear collider. There will be five SASE FELs and five conventional spontaneous undulators. A conceptual design study has been made for the undulator systems for these X-FELs. It includes segmentation into 6.1 m long undulator ‘cells’. Each consists of a 5 m long undulator ‘segment’, a separate quadrupole, one horizontal and one vertical corrector, and a phase shifter. These items are presented and discussed.

4 pp. (2002)., An experiment has been carried out at the TESLA Test Facility (TTF) linac to investigate the wake fields generated by picosecond electron bunches in narrow beam pipes with artificially roughened inner surface. Two methods were employed: (a) the energy structure imposed on the bunches by the wake fields was analysed with a magnetic spectrometer, and (b) the generated THz radiation was directly recorded in an interferometer equipped with Golay cell detectors. Both methods indicate the presence of strong wake field effects as expected from simulations in which the rough surface is modelled by a dielectric layer.

9 pp. (2002)., An X-ray diagnostic station will be installed for each of the XFEL undulator beamlines at TESLA. Primary purpose of the X-ray diagnostics is to provide an additional tool for alignment and commissioning of the numerous undulator cells along an XFEL beamline independently from electron-beam-based alignment procedures. Both methods will complement one another. The X-ray diagnostic station will be a sensitive instrument generating essential input for the undulator control system. The diagnostic station will be located about 120 m downstream from the last undulator cell. Total flux measurements will verify the XFEL's gain. Analysis of the spectral and spatial distribution of the spontaneous radiation of individual or several consecutive undulator segments will be used to optimize angle and position of the electron beam trajectory, to verify the magnetic gap, and to adjust the phase match between two undulator segments. The two latter purposes cannot be served by electron-beam-based alignment.

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10 pp. (2002)., In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The proposed scheme operates as follows. The first stage is a conventional FEL amplifier seeded by 523 nm external laser. A zero area optical pulse (i.e. the pulse with zero value of optical field in the central area of the pulse) is timed to overlap with the electron bunch. Radiation power is amplified up to the saturation level. Following the first stage the electron beam enters the main 6 nm SASE undulator. Large energy spread is induced in the significant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is able to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 10 22 photons / s 1 / mrad 2 / mm 2 / (0.1% BW). The average number of photons can exceed 10 12 photon/pulse.

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