Initial nonintercepting measurements of subpicosecond bunches using coherent diffraction radiation

We report initial measurements of the subpicosecond (sub-ps) bunch structure in a 40 MeV linac beam using the autocorrelation of far infrared (FIR) coherent diffraction radiation (CDR). Since the radiation was generated as the charged particles passed through a slit or aperture in a metal screen oriented at 45 degrees to the beam direction, this is a nonintercepting technique. A compact Michelson interferometer was used to obtain the autocorrelation of the backward CDR with a Golay cell used as the FIR-mm wave detector. The longitudinal shape was calculated from the autocorrelation using the minimal phase approximation. A model was developed and used to calculate the CDR spectral emissions. The experimental results, analysis, and model are presented. Complementary measurements were also made with coherent transition radiation (CTR)