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-raypulses with unmatched brilliance at megahertz repetition rates based on supercon-ducting 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 adistance of 1.7 km to energies as high as 17.5 GeV. The bunches are distributed in-dividually at a maximum of 4.5 MHz to three FEL beamlines, where they generateultrashort radiation pulses in the soft and hard X-ray regime. The electron bunchcurrent profile is of fundamental importance for the generation of these XFEL pulsesand is required to have kilo-Ampere peak currents. However, diagnosis is challeng-ing because bunch durations as short as a few femtoseconds have to be measuredbunch-resolved at MHz repetition rates. In the scope of this thesis, the single-shotspectrometer CRISP has been installed and commissioned at European XFEL. Mon-itoring of coherent diffraction radiation (CDR) enables for the first time noninvasiveand 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 detectorelectronics and signal processing of the CRISP spectrometer have been modified forMHz repetition rates and exhibit in combination sufficient sensitivity for the rangeof bunch charges at European XFEL from 50 pC to 1 nC. The current profiles arereconstructed from the measured form factors with a further developed iterativeprocedure. Using a transverse deflecting structure and a bunch compression moni-tor, the current profiles have been confirmed by established comparative diagnosticmeasurements. The measurements presented in this thesis cover rms bunch dura-tions from 6 fs to 100 fs and peak currents up to 10 kA. Variations of bunch durationand peak current within the bursts of electron bunches have been identified with arelative rms bunch duration resolution of less than 1 fs