Using stretching-modulation-compression effect to generate isolated few-femtosecond MeV electron bunches

Femtosecond electron beams serve as effective tools for investigating ultrafast structural and dynamic processes in matter, providing complementary capabilities to femtosecond laser beams. In this paper we propose and demonstrate the feasibility of a scheme combining an undulator with THz modulation to generate isolated few-femtosecond electron bunches. We have developed a theoretical method that incorporates the transport dynamics of low-energy relativistic electrons interacting with the THz modulation field in the undulator and the space charge effects within the bunch itself. The results indicate that, under currently achievable conditions in infrared laser technology and undulator engineering, the proposed scheme can generate single isolated ultrafast electron bunches with kinetic energy 3 MeV, bunch length about 9 fs (rms) with core charge about 8 fC. The normalized slice emittance of the core is about 0.4μm and the rms beam size remains within 1 mm. We have also evaluated the potential influence of several relevant physical quantities on the final bunch length and arrival time. We believe that the proposed scheme and the developed theoretical model presented in this paper can provide useful insights for generating few-femtosecond electron bunches or even shorter attosecond electron bunches in accelerator-based ultrafast electron facilities.