Generation of Ellipsoidal Laser Pulses with Periodic Intensity Modulation for Photoinjectors

Abstract

Modern linacs are considered for generation of THz pulses with up to mJ energies at high repetition rates due to the capabilities of modern photo injectors. A Self-Amplified Spontaneous Emission (SASE) FEL is proposed for such a powerful THZ source [1]. In order to improve the Carrier-Envelope-Phase (CEP) stability of the THz SASE FEL a seeding option using temporally modulated electron bunches from the photo injector is under study. One of the methods is to generate electron beams with a sub-picosecond structure. Modern linac based FELs (like the European XFEL at DESY and LCLS at SLAC) start from photo injectors, where electron bunches are generated by laser pulses from a cathode surface. In this case, the phase space distribution of the electron bunches strongly depends on the 3D spatio-temporal intensity distribution of the photocathode laser pulses. So, it is necessary to modulate the temporal intensity distribution of the laser pulses to obtain rippled (or microbunched) electron beams. Spectral phase or spectral amplitude modulation of linearly chirped laser pulses can be used to obtain laser pulses with ∼1 ps modulation period. A spatial light modulator (SLM) placed inside the optical compressor with zero frequency dispersion is an attractive way to produce the modulation [3, 4].