K. Tiedtke

Measurement of gigawatt radiation pulses from a vacuum and extreme ultraviolet free-electron laser

In order to measure the photon flux of highly intense and extremely pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) radiation in absolute terms, we have developed a gas-monitor detector which is based on the atomic photoionization of a rare gas at low particle density. The device is indestructible and almost transparent. By first pulse-resolved measurements of VUV free-electron laser radiation at the TESLA test facility in Hamburg, a peak power of more than 100 MW was detected. Moreover, the extended dynamic range of the detector allowed its accurate calibration using spectrally dispersed synchrotron radiation at much lower photon intensities.

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.

Multi-photon ionization of molecular nitrogen by femtosecond soft x-ray FEL pulses

At the new free-electron laser (FEL) for vacuum ultraviolet (VUV) and soft x-ray radiation FLASH of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, multi-photon double ionization of molecular nitrogen has been observed and studied by ion time-of-flight spectroscopy. The experiments have been performed at the microfocus beamline BL2 with photon pulses of 25 fs duration and irradiance levels up to 2 × 1013 W cm−2 at a photon energy of 38 eV, i.e. well above the first ionization/dissociation thresholds of the target. A new and important aspect of our experiments has been the reliable determination of absolute photon numbers per pulse with the help of a calibrated setup allowing the investigation of nonlinear effects by quantitative measurements. Results obtained are discussed in terms of a sequential two-photon ionization scheme.

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.

Multiphoton ionization of atoms with soft x-ray pulses

The new soft x-ray free-electron laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometre and femtosecond scales. However, the mechanisms of photon–matter interaction are not well understood under the conditions of ultra-high photon intensities in conjunction with short wavelengths. In this context, we have quantitatively investigated nonlinear photoionization of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionization detectors for the characterization of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 1013 W cm−2 at about 40 eV photon energy and up to 1016 W cm−2 in the extreme ultraviolet at about 90 eV. Here, surprisingly high degrees of photoionization were observed on Xe atoms. By comparison with other rare gas targets, it emerged that the excitation of inner-shell resonances might play a significant role in the degree of atomic perturbation by the radiation field.

Photoelectron spectroscopy as a non-invasive method to monitor SASE-FEL spectra

Since the summer of 2005, the vacuum ultra-violet Free-electron LASer in Hamburg (FLASH) has operated as a user facility at the Deutsches Elektronen-Synchrotron (DESY), delivering ultra-short laser pulses of tens of femtosecond duration with a high peak brilliance of up to 1028 photons/(s mm2 mrad2 0.1% bandwidth). Due to the statistics of the Self-Amplified Spontaneous Emission (SASE) process, each photon pulse differs from the previous one in the number of modes per pulse, the wavelength (0.5% fluctuations) and the intensity, making experiments more complicated. Thus, for certain experiments the detailed knowledge of the beam properties on a shot-to-shot basis is mandatory. In this paper we describe an online method to gain spectral information about the individual Free-Electron Laser (FEL) pulses that is based on rare-gas photoionization and photoelectron spectroscopy.

Shot-to-shot and average absolute photon flux measurements of a femtosecond laser high-order harmonic photon source

The absolute flux of a femtosecond vacuum-ultraviolet (VUV) photon source based on the high-order harmonic generation of a femtosecond Ti:sapphire laser and monochromatized with a grating monochromator is determined both on a shot-to-shot basis and averaged over seconds by a calibrated gas monitor detector. The average flux is compared with the average flux as determined with a calibrated GaAsP semiconductor photodiode. We found that the photodiode is a reliable and easy-to-use tool for estimating the order of magnitude of the average photon flux but that, due to saturation losses, it underestimates the average flux by up to −15%.

Method based on atomic photoionization for spot-size measurement on focused soft x-ray free-electron laser beams

A method has been developed and applied to measure the beam waist and spot size of a focused soft x-ray beam at the free-electron laser FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. The method is based on a saturation effect upon atomic photoionization and represents an indestructible tool for the characterization of powerful beams of ionizing electromagnetic radiation. At the microfocus beamline BL2 at FLASH, a full width at half maximum focus diameter of (15±2)μm was determined.

Gas‐Monitor Detector for Intense and Pulsed VUV/EUV Free‐Electron Laser Radiation

In the framework of current developments of new powerful VUV and EUV radiation sources, like VUV free‐electron‐lasers or EUV plasma sources for 13‐nm lithography, we developed a gas‐monitor detector in order to measure the photon flux of highly intense and extremely pulsed VUV and EUV radiation in absolute terms. The device is based on atomic photoionization of a rare gas at low particle density. Therefore, it is free of degradation and almost transparent, which allows the detector to be used as a continuously working beam‐intensity monitor. The extended dynamic range of the detector allowed its calibration with relative standard uncertainties of 4% in the Radiometry Laboratory of the Physikalisch‐Technische Bundesanstalt at the electron‐storage ring BESSY II in Berlin using spectrally dispersed synchrotron radiation at low photon intensities and its utilization for absolute photon flux measurements of high power sources. In the present contribution, we describe the design of the detector and its applicat...

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