Jan Grünert

Nanofocusing of hard X-ray free electron laser pulses using diamond based Fresnel zone plates

A growing number of X-ray sources based on the free-electron laser (XFEL) principle are presently under construction or have recently started operation. The intense, ultrashort pulses of these sources will enable new insights in many different fields of science. A key problem is to provide x-ray optical elements capable of collecting the largest possible fraction of the radiation and to focus into the smallest possible focus. As a key step towards this goal, we demonstrate here the first nanofocusing of hard XFEL pulses. We developed diamond based Fresnel zone plates capable of withstanding the full beam of the worlds most powerful x-ray laser. Using an imprint technique, we measured the focal spot size, which was limited to 320 nm FWHM by the spectral band width of the source. A peak power density in the focal spot of 4×1017 W/cm2 was obtained at 70 fs pulse length.

Exploring the wavefront of hard X-ray free-electron laser radiation

The high photon flux and femtosecond pulse duration of hard X-ray free-electron lasers have spurred a large variety of novel and fascinating experiments in physical, chemical and biological sciences. Many of these experiments depend fundamentally on a clean, well-defined wavefront. Here we explore the wavefront properties of hard X-ray free-electron laser radiation by means of a grating interferometer, from which we obtain shot-to-shot wavefront information with an excellent angular sensitivity on the order of ten nanoradian. The wavefront distortions introduced by optical elements are observed in-situ and under operational conditions. The source-point position and fluctuations are measured with unprecedented accuracy in longitudinal and lateral direction, both during nominal operation and as the X-ray free-electron laser is driven into saturation.

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.

Polarization measurement of free electron laser pulses in the VUV generated by the variable polarization source FERMI

FERMI, based at Elettra (Trieste, Italy) is the first free electron laser (FEL) facility operated for user experiments in seeded mode. Another unique property of FERMI, among other FEL sources, is to allow control of the polarization state of the radiation. Polarization dependence in the study of the interaction of coherent, high field, short-pulse ionizing radiation with matter, is a new frontier with potential in a wide range of research areas. The first measurement of the polarization-state of VUV light from a single-pass FEL was performed at FERMI FEL-1 operated in the 52 nm-26 nm range. Three different experimental techniques were used. The experiments were carried out at the end-station of two different beamlines to assess the impact of transport optics and provide polarization data for the end user. In this paper we summarize the results obtained from different setups. The results are consistent with each other and allow a general discussion about the viability of permanent diagnostics aimed at monitoring the polarization of FEL pulses.

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.

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.

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.

Advanced temporal characterization of free electron laser pulses at European XFEL by THz photoelectron spectroscopy

We report an on-going temporal characterization project and method on a shot-to-shot basis for free electron laser pulses by using THz photoelectron spectroscopy at the European XFEL facility. Shot to shot FEL time jitter and pulse profile information can be reconstructed by resolving the photoelectrons energy spectra in an external THz field. Laser based THz generation and optimization, photoelectron generation and detection are described. Further considerations of the temporal resolution based on proposed photoelectron lines are presented and intuitive simulations are made to demonstrate the feasibility of such technique.