LAUSR

The measurement of electron bunch temporal profile is one of the key diagnostics in accelerators, especially for ultrashort bunches. The electro-optic (EO) technique enables the precise longitudinal characterization of bunch electric field in a single-shot and nondestructive way, which can simultaneously obtain and analyze the time jitter between the electron bunch and the synchronized laser. An EO monitor based on spatial decoding for temporal profile measurement and timing jitter recoding has recently been demonstrated and analyzed in depth for low-energy bunches at the Tsinghua Thomson scattering X-ray source. A detailed description of the experimental setup and measurement results are presented in this paper. An EO signal as short as 82 fs (rms) is observed with $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ gallium phosphide for a 40 MeV electron bunch, and the corresponding length is 106 fs (rms) with $300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ zinc telluride. Owing to the field-opening angle, we propose a method to eliminate the influence of energy factor for bunches with low energy, resulting in a bunch length of $\ensuremath{\sim}60\text{ }\text{ }\mathrm{fs}$ (rms). The monitor is also successfully applied to measure time jitter with approximately 10 fs accuracy. The experiment environment is proved to be the main source of the slow drift, which is removed using feedback control. Consequently, the rms time jitter decreases from 430 fs to 320 fs.