Sub-picosecond electron bunch length measurement by terahertz radiation S.Zhang S.Benson D.Douglas M.Shinn and G.Williams

This paper presents the measurement of sub-picosecond relativistic electron beam bunch length by analyzing the spectra of the coherent terahertz pulses through Kramers-Kronig transformation. The results are compared with autocorrelation from a scanning polarization autocorrelator that measures the coherent optical transition radiation. The limitations of the different methods to such a characterization are discussed. 1. Introduction Very high-power THz radiation has been successfully generated at Jefferson Lab[1]. The powerful short pulse THz source provides unprecedented opportunity for applications such as THz imaging and material studies. Since these THz pulses come directly from relativistic electron bunches, they also carry important characteristic information for the electron bunches. Measurement of the electron bunch length or the bunch longitudinal distribution is very important to FEL and accelerator physics studies. Coherent radiation from relativistic bunched electrons provides an effective way for obtaining bunch information, and has been studied by different research groups [2-4]. Coherent Transitional Radiation (CTR) produced by the electron bunch interacting with a thin aluminum foil has provided important clue from which the bunch form factor can be retrieved. The Coherent Synchrotron Radiation (CSR), on the other hand, is generated by the electrons through bending magnets and therefore requires no foils. There is no interruption to the running beam while the measurement is going on. We will use the CSR that is conveniently available form our FEL facility for the beam bunch measurement. The radiation spectral intensity produced by a bunch of N electrons can be expressed by