Subpicosecond-resolved emittance measurements of high-brightness electron beams with space charge effects at PITZ

Abstract

In this thesis a scheme to measure the slice emittance of a high-brightness electron beam which undergoes space charge effects is developed and employed. For this, a transverse deflecting structure (TDS) is added to the single-slit scan technique, which is used at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) to measure projected emittance. The beam is focused with quadrupole magnets behind the slit mask to ensure sufficient signal-to-noise ratio for a reliable slice phase space reconstruction, and hence, slice emittance calculation. The quadrupole magnets also allow time resolution below one picosecond. The beam optics is probed before the slice emittance measurements to ensure precise knowledge of the transport which is needed for the phase space reconstruction when using focusing magnets. Numerical simulation of the slice emittance measurement was performed to estimate the systematic error arising from various factors, including space charge forces, drift distance, and focusing strength. Additionally intensity cuts were implemented to also consider errors arising from finite signal-to-noise ratio in experiment. For the parameters achieved at PITZ the net systematic error in the slice emittance reconstruction in the centre stays well below 10%. The slice emittance measurement scheme is also analysed in methodology studies observing the measured emittance values varying the quadrupole focusing and TDS shearing strength proving reliable slice emittance measurements. The slice emittance setup is used to characterise beam properties from electron bunches emitted with laser pulses of different shape. The main achievement is, that using a temporal and transverse flattop laser pulse shape reduces the slice emittance compared to a temporal Gaussian laser pulse with a transverse flattop distribution. The slice emittance is further improved when using a transversely-truncated Gaussian laser pulse with a temporal Gaussian profile, increasing the beam brightness additionally when employed e.g. in an free-electron laser (FEL) facility. The projected emittance is decomposed in measurement and simulation to identify slice emittance and mismatch emittance contributions to the projected emittance.