Bart Faatz

Pulsed operation of a high average power Yb:YAG thin-disk multipass amplifier

An Yb:YAG thin-disk multipass laser amplifier system was developed operating in a 10 Hz burst operation mode with 800 µs burst duration and 100 kHz intra-burst repetition rate. Methods for the suppression of parasitic amplified spontaneous emission are presented. The average output pulse energy is up to 44.5 mJ and 820 fs compressed pulse duration. The average power of 4.45 kW during the burst is the highest reported for this type of amplifier.

Digital In-line Holography with femtosecond VUV radiation provided by the free-electron laser FLASH

Femtosecond vacuum ultraviolet (VUV) radiation provided by the free-electron laser FLASH was used for digital in-line holographic microscopy and applied to image particles, diatoms and critical point dried fibroblast cells. To realize the classical in-line Gabor geometry, a 1 microm pinhole was used as spatial filter to generate a divergent light cone with excellent pointing stability. At a fundamental wavelength of 8 nm test objects such as particles and diatoms were imaged at a spatial resolution of 620 nm. In order to demonstrate the applicability to biologically relevant systems, critical point dried rat embryonic fibroblast cells were for the first time imaged with free-electron laser radiation.

Regenerative FEL amplifier at the TESLA test facility at DESY

Abstract This paper presents a conceptual design of a regenerative FEL amplifier (RAFEL) as an extension of the single-pass free electron laser project at the TESLA test facility (TTF) at DESY. The proposed scheme requires the additional installation of only two optical components for a narrow-band feedback system and is fully compatible with the present design and the infrastructure developed for the TTF FEL project. It would allow to construct a tunable VUV laser with a minimum wavelength around 60 nm, a pulse duration of about 1 ps, a peak power of about 300 MW and an average power of about 25 W. The output radiation of the regenerative FEL amplifier would possess all the features which are usually associated with laser radiation: full transverse and longitudinal coherence and shot-to-shot stability of the output power. The degeneracy parameter of the output radiation would be about 10 14 and thus have the same order of magnitude as that of a quantum laser operating in the visible.

Radiation exposure and magnetic performance of the undulator system for the VUV FEL at the TESLA Test Facility Phase-1 after 3 years of operation

Radiation damage to undulator systems made of permanent magnet materials like NdFeB or SmCo is a critical issue in SASE projects. It is of even more interest in high-duty cycle machines using superconducting accelerators such as the TESLA Test Facility (TTF), which is a prototype for the future X-FEL at TESLA. This paper reports on experience on the undulator system of the VUV-FEL of TTF-1, which ended on May 6, 2002 after nearly 3 years of operation. The radiation exposure of the undulator system was recorded continuously and meticulously over the whole installation period in the TTF linac. Results of these dosage measurements as well as magnetic measurements after de-installation are reported. A comparison with magnetic field data taken prior to installation is given.

PARAMETER STUDY ON PHASE I OF THE VUV-FEL AT THE TESLA TEST FACILITY

Abstract Currently, a SASE-FEL in the VUV, driven by the TESLA test facility accelerator, is under construction at DESY. As a first phase of the project, three of the eight TTF accelerator modules will be installed to deliver an electron beam with a nominal energy of approximately 380 MeV with a peak current of 500 A. With this energy, employing an undulator with the same parameters as for the final design, the resonant wavelength is around 44 nm. For this wavelength, the transverse beam emittance and energy spread conditions can be relaxed as compared to the design wavelength of 6 nm in the final stage. The proposed length of the planar undulator with the integrated FODO lattice is about 15 m. In this paper, we present a detailed study on how the performance around 300 MeV depends on the electron beam and undulator parameters. Also, the status of the project is briefly sketched.

Development of a pump-probe facility combining a far-infrared source with laser-like characteristics and a VUV free electron laser

Abstract The TESLA Test Facility (TTF) at DESY is a facility producing sub-picosecond electron pulses for the generation of VUV or soft X-ray radiation in a free electron laser (FEL). The same electron pulses would also allow the direct production of high-power coherent radiation by passing the electron beam through an undulator. Intense, coherent far-infrared (FIR) undulator radiation can be produced from electron bunches at wavelengths longer than or equal to the bunch length. The source described in this paper provides, in the wavelength range 50– 300 μm , a train of about 1– 10 ps long radiation pulses, with about 1 mJ of optical energy per pulse radiated into the central cone. The average output power can exceed 50 W . In this conceptual design, we intend to use a conventional electromagnetic undulator with a 60 cm period length and a maximum field of 1.5 T . The FIR source will use the spent electron beam coming from the VUV FEL which allows one to significantly extend the scientific potential of the TTF without interfering with the main option of the TTF FEL operation. The pulses of the coherent FIR radiation are naturally synchronized with the VUV pulses from the main TTF FEL, enabling pump-probe techniques using either the FEL pulse as a pump or the FIR pulse as a probe, or vice versa.

Undulator system for the VUV FEL at the TESLA test facility phase-2

The Phase-1 of the VUV Free Electron Laser at the TESLA Test Facility finishes in fall 2002. Phase-2, an extension of Phase-1 towards shorter wavelengths is under construction and will be ready for operation in 2003. A radiation wavelength as low as 6 nm will be obtained by raising the electron energy to 1 GeV. There will be only minor changes to the undulator system. Compared to Phase-1, six instead of three undulator segments will be installed. The integrated focusing system will be replaced by an electromagnetic doublet structure. We report about the changes of the undulator, the undulator vacuum system, the separated quadrupoles including a stretched wire alignment systems and the modifications to the beam diagnostic system consisting of pick up monitors and wire scanners.

Development of a femtosecond soft X-ray SASE FEL at DESY

Abstract 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 1012 photon/pulse.

Sructural changes at solid surfaces irradiated with femtosecond, intense XUV pulses generated by TTF-FEL

Abstract Interaction of ultrafast (∼ 50 fs), high intensity (up to 1013 W/cm2) XUV (λ ∼ 85 nm) FEL beam with solids has been studied at the TTF-FEL facility. Damaged surfaces have been investigated using light, electron, and atomic force microscopy. Influence of the FEL radiation intensity on the structural changes at the surfaces has been investigated. Results obtained for different materials, i. e. metals (Au), semiconductors (Si), inorganic insulators (Ce:YAG, BaF2, SiO2), and organic polymers (polymethylmethacrylate - PMMA), are compared. Laser-induced periodic surface structures (LIPSS) were observed at amorphous carbon (a-C) layers.

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 Mhz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm2. Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm2) and liquid state (54 mJ/cm2), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes.