Time resolved phase space tomography at flash using a transverse deflecting RF-structure

To initiate self-amplification of spontaneous emission (SASE) in single-pass free electron lasers (EEL), electron bunches with high peak current and small slice emittance are necessary. At FLASH at DESY, this is accomplished by longitudinal shortening in two magnetic bunch compressors. The compression process may be accompanied by distortions from coherent synchrotron radiation (CSR) and space charge (SC) forces. Their effect on transverse phase space can be studied with a vertically deflecting RF-structure, which allows to measure the horizontal slice emittance and, in combination with tomographic methods, the horizontal phase space distribution within ´time slices´. In this paper, experimental results for SASE operation at 27 nm are presented. Bunch fractions with a normalized emittance of 3 mum and a current of 1 kA have been detected. CSR effects have been found to be the dominating source for slice emittance growth.