Bunch length characterization at the TTF VUV-FEL

The characterization of the longitudinal density profile or bunch shape of picosecond and sub-picosecond relativistic particle bunches is a fundamental requirement in many particle accelerator facilities, since knowledge of the characteristics of the accelerated beams is of utmost importance for the successful development of the next generation light sources and linear colliders. The development of suitable beam diagnostics, non-invasive and non-intercepting, is thus necessary to produce and control such beams. First experimental evidences of the non-intercepting and non-destructive nature of DR diagnostics are presented. The precise longitudinal bunch distributions downstream of the second bunch compressor (DBC3) of the DESY TTF VUV-FEL have been reconstructed using a frequency-domain technique based on the autocorrelation of coherent diffraction radiation (CDR). A Kramers-Kronig analysis has then been applied to derive the minimal phase. The autocorrelation of diffraction radiation coming from a thin conducting foil placed in the vacuum chamber has been obtained using a MartinPuplett interferometer and sub-millimeter radiation. The measurement and the reconstruction of the longitudinal bunch distribution are discussed in detail showing also the strong dependence of the bunch length on the accelerating module phase. Due to the low and high frequency suppression, introduced by the experimental apparatus, only a portion of the CDR spectrum participates to the reconstruction of the longitudinal bunch profile. The knowledge of the system frequency response, in particular in the millimeter and sub-millimeter range is then crucial in order to correct the results and extrapolate a bunch shape as close as possible to the real one. At this purpose, a precise characterization over three different frequency regions of the Golay cell detector used for the measurement is also discussed, thus extending the cell calibration into the long wavelengths region.