Single-shot Longitudinal Bunch Profile Measurements at Flash Using Electro-optic Detection: Experiment, Simulation, and Validation

At the superconducting linac of FLASH at DESY, we have installed an electro-optic (EO) experiment for singleshot, non-destructive measurements of the longitudinal electric charge distribution of individual electron bunches. The time profile of the electric bunch field is electrooptically encoded onto a chirped titanium-sapphire laser pulse. In the decoding step, the profile is retrieved either from a cross-correlation of the encoded pulse with a 30 fs laser pulse, obtained from the same laser (electrooptic temporal decoding, EOTD), or from the spectral intensity of the transmitted probe pulse (electro-optic spectral decoding, EOSD). At FLASH, the longitudinally compressed electron bunches have been measured during FEL operation with a resolution of better than 50 fs. The electrooptic process in gallium phosphide was numerically simulated using as input data the bunch shapes determined with a transverse-deflecting RF structure. In this contribution, we present electro-optically measured bunch profiles and compare them with the simulation. SINGLE-SHOT ELECTRO-OPTIC DETECTION Precise measurements of the temporal profile of extremely short electron bunches are essential for a detailed understanding of the bunch compression and lasing mechanisms in a SASE FEL. Single-shot electro-optic (EO) detection techniques are ideally suited for this purpose since they are non-destructive and permit correlation studies between the time profile of electron bunches and the properties of FEL pulses produced by the same bunches. The transverse electric field of a relativistic electron bunch passing close to an electro-optic crystal corresponds to a THz pulse traveling through the crystal. This THz pulse induces a transient birefringence in the EO crystal which can be sampled by a linearly polarized optical laser pulse. In this experiment we use gallium-phosphide (GaP) instead of the more common zinc telluride (ZnTe) as it offers a factor of two better time resolution. The electro-optic temporal decoding (EOTD) technique was applied [1, 2] ∗ email: [email protected] fs laser CCD