Wolfgang Freund

A case study of novel X-ray Optics for FEL sources

We suggest optical schemes for the European X-ray Free Electron Laser facility (XFEL.EU) in Hamburg: a single element X-ray spectrometer on the basis of a reflection zone plate (RZP) for single-shot diagnostics; and a two-element soft X-ray spectrometer on the basis of two RZPs to carry out Resonant Inelastic X-ray Scattering (RIXS) experiments. With this setup, a full map of the sample spectrum is obtainable in a single measurement. The main advantage of using zone plates is the possibility to enable dispersion and focusing in one step. Moreover, highest possible X-ray transmission is achieved by using the minimum number of optical elements. Taking into account the European XFEL beam parameters, our simulations, concerning the RIXS experiment, produced very promising results, reaching an energy resolution (E/ΔE) of up to 30,000 at photon energy of 1 keV. When applied as a single shot spectrometer the energy resolution for RZP is of the same order of magnitude.

Simulations of diagnostic spectrometers for the European XFEL using the ray-trace tool RAY

This paper presents the outcome of ray tracing simulations for different optical schemes to be setup at the European X-ray Free Electron Laser facility (XFEL.EU), Germany: one- or two- channel (cut) crystal X-ray monochromators (K-Mono; using spontaneous radiation) are planned and designed mainly for photon beam based alignment, which is gap tuning of the undulator segments and phase tuning of the phase shifters during commissioning and maintenance of the undulators. The coherent SASE (Self Amplified Spontaneous Emission) radiation will be monitored pulse-resolved by single-shot spectrometers of which two types are investigated: i) a three element spectrometer, design proposed by Yabashi et al., which consists of a curved focusing mirror, followed by a flat analyzer crystal and a 2D-detector.ii) a two element spectrometer based on a reflection zone plate that reflects and focuses in one step, and a 2D-detector (currently under development).

Development status of the X-ray beam diagnostics devices for the commissioning and user operation of the European XFEL

X-ray Free-Electron-Lasers (XFEL) as the Linac Coherent Light Source (LCLS) in the USA, SACLA in Japan, and the European XFEL under construction in Germany are 4th generation light sources which allow research of at the same time extremely small structures (Angstrom resolution) and extremely fast phenomena (femtosecond resolution). Unlike the pulses from a conventional optical laser, the radiation in these sources is created by the Self-Amplified Spontaneous Emission (SASE) process when electron bunches pass through very long segmented undulators. The shot noise at the origin of this process leads to significant pulse-to-pulse variations of pulse intensity, spectrum, wavefront, temporal properties etc. so that for user experiments an online monitoring of these properties is mandatory. Also, the adjustment of the long segmented undulators requires dedicated diagnostics such as an undulator commissioning spectrometer and spontaneous radiation analysis. The extremely high brilliance and resulting single-shot damage issue are difficult to handle for any XFEL diagnostics. Apart from the large energy range of operation of the facility from 280 eV to 25 keV in FEL fundamental, the particular challenge for the European XFEL diagnostics is the high intra bunch train photon pulse repetition rate of 4.5 MHz, potentially causing additional damage by high heat loads and making shot-to-shot diagnostics very demanding. This contribution reports on the facility concepts, recent progress in instrumentation development, and the optimization of diagnostics performance with respect to resolution/accuracy, shot-to-shot capabilities and energy range.

Time-of-flight photoemission spectroscopy from rare gases fornon-invasive, pulse-to-pulse x-ray photon diagnostics at theEuropean XFEL

The European X-ray Free Electron Laser (XFEL.EU) under construction will provide highly brilliant soft to hard X-ray (<280 eV - <20 keV) radiation with an intra-bunch train repetition rate of 4.5 MHz by employing the self-amplified spontaneous emission process. The resulting statistical fluctuations of important beam characteristics makes pulse-to-pulse diagnostics data of the photon beam a mandatory reference during user experiments. We present our concepts of analysing the photoemission from rare gases with a time-of-flight spectrometer for non-invasive, pulse-to-pulse measurements of the photon spectrum and polarization with a special emphasis on real-time processing with a low latency of ≤ 10−5 s.

Radiation detectors based on microchannel plates for free-electron lasers

Detectors based on microchannel plates are used to detect the radiation of free-electron lasers operating in short-wavelength ranges. We present descriptions of radiation detectors for the FLASH free-electron laser (DESY, Hamburg) that operates in vacuum ultraviolet and soft X-ray wavelength ranges (4–100 nm) and detectors for the European X-ray free electron laser that is being constructed in Hamburg and is designed to operate in the X-ray wavelength range from 0.05 to 4.3 nm.

X-ray photon diagnostics devices for the European XFEL

X-ray Free-Electron-Laser (XFEL) facilities like the Linac Coherent Light Source (LCLS) in the USA, SACLA in Japan, and the European XFEL under construction in Germany are 4th generation light sources which allow research of at the same time extremely small structures (Ångström resolution) and extremely fast phenomena (femtosecond resolution). Unlike the pulses from a conventional optical laser, the radiation in these sources is created by the Self-Amplified Spontaneous Emission (SASE) process when electron bunches pass through very long segmented undulators. The shot noise at the origin of this process leads to significant pulse-to-pulse variations of pulse intensity, spectrum, wavefront, temporal properties etc. so that for user experiments an online monitoring of these properties is mandatory. Additionally, the adjustment of the long segmented undulators requires dedicated diagnostics such as an undulator commissioning spectrometer and spontaneous radiation analysis. The extreme brilliance and resulting single-shot damage potential are difficult to handle for any XFEL diagnostics. Apart from the large energy range of operation of the facility from 280eV to 25keV in FEL fundamental, the particular challenge for the European XFEL diagnostics is the high intra bunch train photon pulse repetition rate of 4.5MHz, potentially causing additional damage by high heat loads and making shot-to-shot diagnostics very demanding. This presentation reports on the facility concepts, recent progress in instrumentation development, and the choices to compromise diagnostics performance between resolution/accuracy on one hand and shot-to-shot capabilities and energy range on the other.

Initial Evaluation of the European XFEL Undulator Commissioning Spectrometer with a Single Channel-Cut Crystal

At the European X-ray Free Electron Laser facility one‐ or two‐ Si(111) channel (cut) crystal X‐ray monochromator (Kmonochromator) are planned for photon beam based alignment: gap tuning of the undulator segments and phase tuning of the phase shifters during commissioning and maintenance of the undulators. A prototype device has been built using a single channel-cut crystal and was characterized at PETRAIII synchrotron (at P01, which is the only beamline with two undulator segments) by applying different undulator adjustment methods, intended for the European XFEL, that use imaging and intensity detection. This paper presents the setup and the first results from the experimental qualification of the K-monochromator prototype.

Pulse-resolved intensity measurements at a hard X-ray FEL using semi-transparent diamond detectors

Solid-state ionization chambers are presented based on thin diamond crystals that allow pulse-resolved intensity measurements at a hard X-ray free-electron laser (FEL), up to the 4.5 MHz repetition rate that will become available at the European XFEL. Due to the small X-ray absorption of diamond the thin detectors are semi-transparent which eases their use as non-invasive monitoring devices in the beam. FELs are characterized by strong pulse-to-pulse intensity fluctuations due to the self-amplified spontaneous emission (SASE) process and in many experiments it is mandatory to monitor the intensity of each individual pulse. Two diamond detectors with different electrode materials, beryllium and graphite, were tested as intensity monitors at the XCS endstation of the Linac Coherent Light Source (LCLS) using the pink SASE beam at 9 keV. The performance is compared with LCLS standard monitors that detect X-rays backscattered from thin SiN foils placed in the beam. The graphite detector can also be used as a beam position monitor although with rather coarse resolution.

Design and initial characterisation of X-ray beam diagnostic imagers for the European XFEL

The European X-Ray Free-Electron-Laser facility requires diagnostics of its x-ray photon beam. Besides other diagnostic components, imaging stations will be employed for the characterisation of beam properties like position, profile, and pointing, before and after different types of mirrors, slits and monochromators. In combination with soft x-ray grating monochromators or other dispersive devices, imagers can also deliver spectral information. The imagers will usually absorb the beam (invasive devices), however, for some applications they will be partially transmissive to allow for beam pointing monitoring together with a second imaging unit further downstream. For the first commissioning 25 diagnostic imagers are planned at various positions in the photon beam tunnels. Further similar devices are under development for monitoring the beam properties at the experimental stations. The design of theses imaging stations will be described. Initial testing has started and the optimization of some components will be reported. The main components of these imaging stations are: retractable scintillators for conversion of x-rays to visible light, mirrors, optics and CCD / CMOS cameras for image recording, an ultra-high vacuum (UHV) chamber, and the associated control electronics and software. Scintillators and mirrors will be the only components in an ultra-high vacuum chamber. Performance characteristics are addressed, especially mechanical stability, spatial resolution, signal-to-noise properties, and radiation hardness. The challenge in the design is to deal with a wide range of beam properties: photon energies from 0.26 – 25 keV, beam sizes from several 100 μm to several mm, large beam position shifts of up to 120 mm, pulse durations of 10 fs and pulse energies up to 10 mJ which may destroy materials by a single pulse.