In modern high-gain free-electron lasers, ultra-fast photon pulses designed for studying chemical, atomic and biological systems are generated from a serial of behaviors of high-brightness electron beam at the time-scale ranging from several hundred femtoseconds to sub-femtosecond. Currently, radiofrequency transverse deflectors are widely used to provide reliable, single-shot electron beam phase space diagnostics, with a temporal resolution of femtosecond. Here, we show that the time resolution limitations caused by the intrinsic beam size in transverse deflectors, can be compensated with specific transverse-to-longitudinal coupling elements. For the purpose, an undulator with transverse gradient field is introduced before the transverse deflector. With this technique, a resolution of less than 1fs root mean square has been theoretically demonstrated for measuring the longitudinal profile and/or the micro-bunching of the electron bunch.
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