Design and implementation of a flexible beamline for fs electron diffraction experiments

Ultrafast Electron Diffraction (UED) has been widely used to investigate the structural dynamics of molecules and materials. Femtosecond (fs) electron bunches are used to obtain diffraction images of a specimen upon photo-excitation by a temporally delayed light pulse. The high cross-section of electrons makes it a very flexible tool for the study of light elements, monolayers and surfaces; at the same time, electrons can travel down to few nanometers (nm) and structural information from the bulk can also be retrieved. In this article, we discuss the design and implementation of a flexible beamline for fs electron diffraction experiments in transmission or reflection geometry. By the use of a radiofrequency (RF) compression cavity synchronized to our laser system, in combination with a set of electron optics, we demonstrate that we can control the beam properties in terms of charge per pulse, transverse spot-size on the sample and temporal duration of the bunches. The characterization of the beam is performed via a light-electrons cross-correlation experiment and we demonstrate an overall temporal resolution around 300 fs for bunches containing up to 10(5) electrons at a repetition rate of 20 kHz. (c) 2012 Elsevier B.V. All rights reserved.

Design and implementation of an optimal laser pulse front tilting scheme for ultrafast electron diffraction in reflection geometry with high temporal resolution

Fabrizio Carbone, Rajeswari Jayaraman, Giulia Fulvia Mancini, Malte Oppermann, Francesco Pennacchio, Giovanni Maria Vanacore

Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution. However, in the reported optical scheme, the tilted pulse is characterized by a temporal chirp of about 1 ps at 1mm away from the centre of the beam, which limits the investigation of surface dynamics in large crystals. In this paper, we propose an optimal tilting scheme designed for a radio-frequency-compressed ultrafast electron diffraction setup working in reflection geometry with 30 keV electron pulses containing up to 10(5) electrons/pulse. To characterize our scheme, we performed optical cross-correlation measurements, obtaining an average temporal width of the tilted pulse lower than 250 fs. The calibration of the electron-laser temporal overlap was obtained by monitoring the spatial profile of the electron beam when interacting with the plasma optically induced at the apex of a copper needle (plasma lensing effect). Finally, we report the first time-resolved results obtained on graphite, where the electron-phonon coupling dynamics is observed, showing an overall temporal resolution in the sub-500 fs regime. The successful implementation of this configuration opens the way to directly probe structural dynamics of low-dimensional systems in the sub-picosecond regime, with pulsed electrons. (C) 2017 Author(s).

Amer Inst Physics2017

Design and implementation of a fs-resolved transmission electron microscope based on thermionic gun technology

Brett Eugene Barwick, Fabrizio Carbone, Luca Piazza

In this paper, the design and implementation of a femtosecond-resolved ultrafast transmission electron microscope is presented, based on a thermionic gun geometry. Utilizing an additional magnetic lens between the electron acceleration and the nominal condenser lens system, a larger percentage of the electrons created at the cathode are delivered to the specimen without degrading temporal, spatial and energy resolution significantly, while at the same time maintaining the femtosecond temporal resolution. Using the photon-induced near field electron microscopy effect (PINEM) on silver nanowires the cross-correlation between the light and electron pulses was measured, showing the impact of the gun settings and initiating laser pulse duration on the electron bunch properties. Tuneable electron pulses between 300 fs and several ps can be obtained, and an overall energy resolution around 1 eV was achieved. (C) 2013 Elsevier B.V. All rights reserved.

Elsevier Science Bv2013

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.