X-ray laser | EPFL Graph Search

An X-ray laser is a device that uses stimulated emission to generate or amplify electromagnetic radiation in the near X-ray or extreme ultraviolet region of the spectrum, that is, usually on the order of several tens of nanometers (nm) wavelength. Because of high gain in the lasing medium, short upper-state lifetimes (1–100 ps), and problems associated with construction of mirrors that could reflect X-rays, X-ray lasers usually operate without mirrors; the beam of X-rays is generated by a single pass through the gain medium. The emitted radiation, based on amplified spontaneous emission, has relatively low spatial coherence. The line is mostly Doppler broadened, which depends on the ions' temperature. As the common visible-light laser transitions between electronic or vibrational states correspond to energies up to only about 10 eV, different active media are needed for X-ray lasers. Between 1978 and 1988 in Project Excalibur the U.S. military attempted to develop a nuc

Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility

Bennie Smit

The SwissFEL (Swiss Free Electron Laser) is a free electron laser which is being built at the Paul Scherrer Institute. The SwissFEL will be a user facility to study processes on unexplored scales of space and time. In order to achieve these aims, X-ray pulses on the order of femtoseconds must be generated. For such short X-ray pulses, the electron bunch length must be comparable to the final X-ray pulse length. In order to achieve such a short electron bunch length, it must be monitored at every compression stage. This work proposes two novel methods to measure such electron bunches, namely a novel spectral method that analyses the frequency content of electron bunches that becomes easier with shorter bunch length and a transverse deflecting cavity method which can conceptually measure a zero bunch length. Because the SwissFEL was not built at the time of the PhD, all experiments were performed at the SwissFEL Injector Test Facility (SITF). The SITF is a 200 MeV test bed linear electron accelerator. Theoretically the bunch length is limited due to the uncorrelated energy spread and the higher order components of the bunch compressor. However, due to the limited electron energy in combination with the short electron bunches, space charge effects dominate the beam in the transverse phase space as well as in the longitudinal phase space. A transverse deflecting cavity (TDC) measurement was used to measure a reference bunch length to which the spectral fluctuations could be correlated. In a TDC the electric fields are oriented perpendicular to the beam direction and streaks the head and tail in opposite directions onto a screen. In a conventional TDC measurement, the projected beam size onto the temporal axis is the current profile, provided that the intrinsic beam size is small. However, in the SITF the intrinsic beam size was large due to the aforementioned space charge effects. This led to the development of a novel bunch length analysis method in a space charge dominated beam. In the proposed analysis method, a dispersive section in combination with a TDC was used to visualise the longitudinal phase space. The longitudinal phase space was sliced in the momentum domain to reveal the difference between the intrinsic and the streaked beam in a momentum slice. The bunch length could be extracted by recording the times for all slices. Both the simulations and experiments suggest the method enables the measurement of bunch length in a space charge dominated beam in which the transverse intrinsic beam size is large. The measured minimal bunch length from simulations and experiments are of the same order, 19.5 fs to 24.5 fs, respectively. Since the TDC at PSI has a specified resolution of 20 fs at 200 pC, this method may be used to reduce the resolution. The spectral content of femtosecond electron bunches contains electron bunch length information. As an electron beam collides with an optical transition radiation (OTR) foil, radiation is emitted. The radiation is collected by means of a mirror setup into an optical fibre and into an optical spectrometer. The spectral range of the spectrometer is 316 THz (950 nm) to 1364 THz (220 nm). From OTR theory a flat OTR spectrum was expected and measured. As the electron bunch length was reduced below 70 fs, periodic fluctuations started to appear on the spectra. These fluctuations broaden with decreasing bunch length and show a good agreement with the bunch length measured with the TDC.

EPFL2015

Multispectroscopic Study of Single Xe Clusters Using XFEL Pulses

Christoph Bostedt, Christophe Nicolas

X-ray free-electron lasers (XFELs) deliver ultrashort coherent laser pulses in the X-ray spectral regime, enabling novel investigations into the structure of individual nanoscale samples. In this work, we demonstrate how single-shot small-angle X-ray scattering (SAXS) measurements combined with fluorescence and ion time-of-flight (TOF) spectroscopy can be used to obtain size- and structure-selective evaluation of the light-matter interaction processes on the nanoscale. We recorded the SAXS images of single xenon clusters using XFEL pulses provided by the SPring-8 Angstrom compact free-electron laser (SACLA). The XFEL fluences and the radii of the clusters at the reaction point were evaluated and the ion TOF spectra and fluorescence spectra were sorted accordingly. We found that the XFEL fluence and cluster size extracted from the diffraction patterns showed a clear correlation with the fluorescence and ion TOF spectra. Our results demonstrate the effectiveness of the multispectroscopic approach for exploring laser–matter interaction in the X-ray regime without the influence of the size distribution of samples and the fluence distribution of the incident XFEL pulses.

2019

Refinement for single-nanoparticle structure determination from low-quality single-shot coherent diffraction data

Christoph Bostedt, Christophe Nicolas

With the emergence of X-ray free-electron lasers, it is possible to investigate the structure of nanoscale samples by employing coherent diffractive imaging in the X-ray spectral regime. In this work, we developed a refinement method for structure reconstruction applicable to low-quality coherent diffraction data. The method is based on the gradient search method and considers the missing region of a diffraction pattern and the small number of detected photons. We introduced an initial estimate of the structure in the method to improve the convergence. The present method is applied to an experimental diffraction pattern of an Xe cluster obtained in an X-ray scattering experiment at the SPring-8 Angstrom Compact free-electron LAser (SACLA) facility. It is found that the electron density is successfully reconstructed from the diffraction pattern with a large missing region, with a good initial estimate of the structure. The diffraction pattern calculated from the reconstructed electron density reproduced the observed diffraction pattern well, including the characteristic intensity modulation in each ring. Our refinement method enables structure reconstruction from diffraction patterns under difficulties such as missing areas and low diffraction intensity, and it is potentially applicable to the structure determination of samples that have low scattering power.

INT UNION CRYSTALLOGRAPHY2020

Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility

Bennie Smit

EPFL2015