Fast retrieval of temporal characteristics of FEL pulses using streaking by THz field

A simple and very fast method for reconstruction of temporal structure of linearly polarized FEL pulses from the THz streaking spectra of photoelectrons is suggested. The method is based on a quantum mechanical approach within the strong-field approximation. The method is suitable for online retrieval of the temporal characteristics of the FEL pulses. It can be applied for any photon frequency in a broad range of FEL pulse duration with a proper selection of the streaking field. To enhance its accuracy, it is suggested to simultaneously analyze the streaking spectra for several emission angles.