Noninvasive measurements of electron bunch current profiles with few-femtosecond resolution at MHz repetition rates

The upcoming generation of hard X-ray free-electron lasers (XFEL) delivers X-ray pulses with unmatched brilliance at megahertz repetition rates based on superconducting linear accelerators. The European XFEL, the first XFEL of this generation, accelerates bursts of up to 2700 electron bunches every tenth of a second over a distance of 1.7 km to energies as high as 17.5 GeV. The bunches are distributed individually at a maximum of 4.5 MHz to three FEL beamlines, where they generate ultrashort radiation pulses in the soft and hard X-ray regime. The electron bunch current profile is of fundamental importance for the generation of these XFEL pulses and is required to have kilo-Ampere peak currents. However, diagnosis is challenging because bunch durations as short as a few femtoseconds have to be measured bunch-resolved at MHz repetition rates. In the scope of this thesis, the single-shot spectrometer CRISP has been installed and commissioned at European XFEL. Monitoring of coherent diffraction radiation (CDR) enables for the first time noninvasive and bunch-resolved form factor measurements in two frequency bands: 0.7 THz – 6.6 THz (438 μm - 45 μm) and 6.9 THz - 58.4 THz (44 μm - 5.1 μm). The detector electronics and signal processing of the CRISP spectrometer have been modified for MHz repetition rates and exhibit in combination sufficient sensitivity for the range of bunch charges at European XFEL from 50 pC to 1 nC. The current profiles are reconstructed from the measured form factors with a further developed iterative procedure. Using a transverse deflecting structure and a bunch compression monitor, the current profiles have been confirmed by established comparative diagnostic measurements. The measurements presented in this thesis cover rms bunch durations from 6 fs to 100 fs and peak currents up to 10 kA. Variations of bunch duration and peak current within the bursts of electron bunches have been identified with a relative rms bunch duration resolution of less than 1 fs.