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New accelerator technologies such as laser wakefield accelerators (LWFA) or dielectric laser accelerators (DLA) have pushed the electron bunch length down to femtosecond or sub femtosecond regime. These ultrashort electron bunches find many applications, e.g. seeding for free-electron lasers (FEL), ultrafast electron diffraction (UED) and coherent Smith-Purcell radiation (cSPr) sources etc. The characterization of such ultrashort bunches is becoming a challenging task, especially at low energy regime due to the space charge effects. Usually, the streak cameras based on RF cavities are used to obtain accurate bunch length. However, the phase jitter between the incident beam and the electromagnetic field phase in the cavity set a resolution limit. A bunch length diagnostic based on a self-emission THz driven split-ring resonator (SRR) is proposed to reach the sub-picosecond (ps) or femtosecond (fs) resolution. Since the coherent SmithPurcell radiation from the incident electron beam produces the driving THz pulse, it can essentially eliminate the time jitter between the incident beam and the deflection THz field in the SRR gap. Besides, this THz pulse frequency can be tunable to easily match the SRR resonance frequency. In this paper, we describe the mechanism of the THz generation method and present the simulation results of the novel bunch length measurement based on a THz-driven SRR. The results show that this novel method can successfully measure the bunch length with the temporal resolution of 2-10 fs.
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