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| Home > Publications database > Generation of Ultra-Short Electron Bunches and FEL Pulses, and Characterization of Their Longitudinal Properties at FLASH2 |
| Book/Dissertation / PhD Thesis | PUBDB-2019-04319 |
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2019
Verlag Deutsches Elektronen-Synchrotron
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2019-04319
Report No.: DESY-THESIS-2019-022
Abstract: The free-electron laser in Hamburg (FLASH) is a soft X-ray user facility, delivering radiation for photon science since 2005. Since 2014, a second beam line for user operation, FLASH2, has been in operation. The beam for FLASH2 is accelerated using the same accelerating modules as for the initial FLASH beam line (FLASH1), but then deflected downstream of this shared linac (linear accelerator). The electron bunches generated at a photocathode have an rms (root mean square) length of 6.5 ps or 4.5 ps when the standard injector lasers are used. These bunches are then compressed in two bunch compressors in the linac to about 30 fs to 200 fs. To shorten the bunch lengths even further, a short-pulse injector laser was installed at FLASH, which generates short, low charge electron bunches directly at the cathode. These short electron bunches with a length of about 1 ps can be further compressed in the linac to produce ultra-short FEL (Free-Electron Laser) pulses down to a few femtoseconds in the FLASH2 undulators. Measurements of such pulses as well as tracking simulations of the FLASH2 beam line are presented in this thesis. Both studies demonstrate the feasibility of ultra-short FEL pulses down to single-spike lasing at FLASH2. Up until now, no hardware to directly measure the electron bunch length has been installed in the FLASH2 beam line. As exact knowledge of the pulse duration is essential for time-resolved user experiments, the beam line downstream of the FLASH2 undulators has been redesigned for the installation of a variable polarization Transverse Deflecting Structure (TDS). In combination with a dipole magnet it is possible to map the longitudinal phase space density of the electron bunches onto a beam screen. Additionally, the photon pulse duration as well as the slice emittance in both transverse planes can be measured using such a TDS. This thesis presents the final layout of the beam line, the accelerator optics, and also simulations for the aforementioned measurements. However, the temporal resolution of an X-band TDS is limited to the order of 1 fs rms. In most cases, this is not sufficient to resolve single self-amplified spontaneous emission (SASE) spikes within one photon pulse for hard X-ray radiation. By adding the intensity spectrum from a high-resolution spectrometer to the temporal profile from the TDS the overall resolution can be enhanced. This is done by applying an iterative reconstruction algorithm that is presented in this thesis. Additionally, the algorithm is tested using simulation data of the Linac Coherent Light Source (LCLS) and applied to data taken at LCLS.
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