Single shot longitudinal bunch profile measurements at FLASH using electro-optic techniques” EPAC

For the high-gain operation of a SASE FEL, extremely short electron bunches are essential to generate sufficiently high peak currents. At the superconducting linac of FLASH at DESY, we have installed an electro-optic ex-periment with temporal decoding and spectral decoding to probe the time structure of the electric field of single sub-200 fs electron bunches. In this technique, the field-induced birefringence in an electro-optic crystal is encoded on a chirped ps laser pulse. The longitudinal electric field profile of the electron bunch is then obtained from the encoded optical pulse by a single-shot cross correlation with a 30 fs laser pulse using a second-harmonic crystal (temporal decoding) or by a single-shot measurement of its spectrum (spectral decoding). In the temporal decod-ing measurements an electro-optic signal of 110 fs FWHM was observed, which is limited by the material properties of the particular electro-optic crystal used. Bunch profile and time jitter measurements were obtained simultaneously with SASE operation