Generation of frequency-chirped density modulation electron beam for producing ultrashort THz radiation pulse

A new scheme is proposed to generate electron beams with a frequency-chirped density modulation for producing ultrashort THz radiation pulse in an optimally tapered undulator. In this scheme, an energy chirped electron beam is sequentially modulated in two modulators by interacting with two laser pulses with the same central frequency but different chirp rates, practically one laser pulse is linearly chirped and the other is chirpfree. Two dispersion sections are applied to transfer the energy modulation formed in the modulators to the desired density modulation. The central frequency of the density modulation of the electron beam can be continuously tuned by changing the compression coefficients of the dispersion sections, and the chirp rate of the final modulation frequency can also be tuned by either varying the compression coefficients or the linear chirp rates of the modulation laser pulses. The generation of ultrashort and broadband THz radiation pulses is also briefly discussed.