Ch. Gerth

Study of the transverse coherence at the TTF free electron laser

Double slits with different separations, crossed slits and circular apertures have been used to study the transverse coherence of the VUV light of the SASE Free Electron Laser at the TESLA Test facility at DESY. The resulting diffraction patterns are converted to visible light by a Ce:YAG crystal and imaged by a high-resolution CCD camera. The visibility of the diffraction patterns indicates a high degree of transverse coherence. Measurements have been taken at various operating modes and wavelengths of the FEL. A numeric FEL simulation code has been used to calculate the wavefronts of the light at the exit of the undulator. By propagating the wavefronts through the optical setup, the diffraction at the double slits is computed with the code GLAD. Good agreement with the measurements is found.

Study of the statistical properties of the radiation from a VUV SASE FEL operating in the femtosecond regime

The Free-Electron Laser (FEL) at the TESLA Test Facility at DESY operates in the self-amplified spontaneous emission mode and generates sub-100-fs radiation pulses in the vacuum ultraviolet spectral region. During operation in the saturation regime, radiation pulses with GW peak power are produced. The statistical properties of the FEL radiation have been studied for different amplification regimes as well as behind a narrow-band monochromator and found to be in good agreement with the results of numerical simulations. Information about the spectral and temporal structure of the FEL radiation has been deduced from the statistical properties. The pulse duration of the FEL radiation can be varied by tailoring the electron bunch that drives the FEL.

Photon diagnostics on the VUV FEL at DESY: First lasing around

Abstract A single-pass free electron laser (FEL) based on the principle of self-amplified spontaneous emission (SASE) is currently under test at the TESLA Test Facility at DESY. So far lasing has been observed in the vacuum ultraviolet (VUV) region between 80 and 180 nm . The photon beam has been characterized by different techniques in order to provide experimental evidence for SASE and to compare with numerical simulations. This paper presents some of the first results and summarizes the techniques used for the measurement of the photon beam intensity, its angular and spectral distribution, and its statistical properties.

Photon diagnostics for the study of electron beam properties of a VUV SASE-FEL

Abstract A single-pass free-electron laser operating in the self-amplified spontaneous-emission (SASE) mode at around 100 nm is currently under test at the TESLA Test Facility at DESY. After first observation of SASE in February 2000, the photon beam has been characterized by different techniques. We present the methods of VUV photon diagnostics that were used to measure the spectral and angular distribution of the photon beam and how these properties are affected by the electron beam energy and orbit in the undulator.

Alignment of the optical feedback system of VUV regenerative FEL amplifier at the TESLA test facility at DESY

In this paper, we describe optical feedback system of VUV Regenerative FEL Amplifier (RAFEL) at the TESLA test facility at DESY. The aim of the RAFEL experiment is to construct fully coherent, tunable VUV radiation source by means of applyingnarrow-band optical feedback in the VUV SASE FEL operatingcurrently at DESY. One of the problem of the realization of the RAFEL is severe requirements for the angular stability of the optical elements (about few microradians). This problem has been solved by means of installation of active alignment system with reference laser. Another problem is alignment of optical elements separated by 65 m within complicated experimental conditions connected with aperture limitations (down to 6 mmÞ: This problem has been solved in two steps. Preliminary alignment with an accuracy of about 80 mrad has been performed with laser alignment system and OTR screens used at the TTF accelerator for electron beam diagnostics. Final alignment has been performed with VUV SASE FEL radiation. Measured feedback coefficient is about 1 percent and is in agreement with the designed value. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr; 52.75.M; 42.62.Cf

Resonant 3p photoelectron spectroscopy of free Cu atoms

Excitation and decay of the Cu 3p core resonances have been studied by resonant photoelectron spectroscopy. In comparison with theoretical predictions the importance of electron correlation for the resonance energies and line shapes is verified.

Valence satellite and 3p photoelectron spectra of atomic Fe and Cu

The 3p photoelectron spectra of atomic Fe and Cu, and the Fe outer-shell satellites have been investigated. The complex multiline structure of the Fe spectra has been resolved using high-resolution electron spectroscopy. The thresholds of the direct 3p photoionization have been determined and are compared with experimental results from photoabsorption measurements and Auger spectra, and with the multiplet structures calculated taking into account intershell and configuration interaction. The stepwise decay of highly excited Fe states into has been verified by the detection of satellite lines with binding energies above the lowest ionization threshold.

Bunch length and phase stability measurements at the TESLA test facility

Abstract Electron bunch lengths and time jitters have been measured with sub-ps resolution at the TESLA Test Facility at DESY. Synchrotron radiation emitted from a dipole magnet reflects the longitudinal charge distribution of the electron bunch and can also be used as a timing reference of the radiation pulses produced by the free-electron laser. For sub-ps time resolution, dispersion effects of the spectrally broad-band synchrotron radiation have to be suppressed. The phase stability of the FEL pulses with respect to the radio-frequency master oscillator has been studied with a streak camera operating in synchroscan mode.

Electron bunch shape measurements at the TTF-FEL

The TESLA Test Facility linac has been operated in the first half of 2002 with two bunch compressors to drive the TTF-FEL free electron laser. During this running period, SASE radiation with a wavelength around 100 nm has been routinely delivered to experiments. The longitudinal shape of the electron bunches is a crucial property of the electron beam: a peak current in the order of 1 kA is required to drive the SASE process with the given undulator design, transverse emittance, and energy spread. We report on measurements of the bunch length and shape for different operating conditions of the two bunch compressors.

multiplet structure of atomic Lu studied by photoelectron spectroscopy

The direct 5p photoionization of atomic Lu has been investigated by photoelectron spectroscopy in combination with monochromatized synchrotron radiation. Binding energies of the multiplet states have been determined from the photoelectron spectra. For the interpretation of the multiplet structure, we performed Hartree-Fock calculations, where configuration interaction was taken into account. The main result is that the splitting of the multiplet structure is dominated by the spin-orbit interaction.