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The spectra of sub-mm wavelength coherent transition radiation (TR) and diffraction radiation (DR) have previously been used to measure the bunch length of picosecond electron beam pulses. However, both the spectral and angular distributions of the radiation from a finite target or aperture with size r, are strong functions of the wavelength, when λ ≈ 2πr/γ where γ is the relativistic factor of the beam. This dependence must be taken into account in the determination of the bunch form factor and bunch shape. Also the spectral density of the bunch is a strong function of wavelength when λ ≈ d, the characteristic length of the bunch. When both the above conditions are fulfilled, i.e. λ ≈ 2πr/γ ≈ d, the spectral and angular distribution (AD) of the radiation are very sensitive to the longitudinal distribution of the bunch. We are investigating the use of the AD of TR or DR, to diagnose the bunch length and shape. Here we present a comparison of measured and calculated angular distributions from two targets: a solid disk and a rectangular slit, which we have used to determine the waveform of the beam bunch produced at PSI’s SLS pre-injector LINAC.
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