With electron beam durations down to femtoseconds and sub-femtoseconds achievable in current state-of-the-art accelerators, longitudinal bunch length diagnostics with resolution at the attosecond level are required. In this paper, we present such a novel measurement device which combines a high power laser modulator with an RF deflecting cavity in the orthogonal direction. While the laser applies a strong correlated angular modulation to a beam, the RF deflector ensures the full resolution of this streaking effect across the bunch hence recovering the temporal beam profile with sub-femtosecond resolution. Preliminary measurements to test the key components of this concept were carried out at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory recently, the results of which are presented and discussed here. Moreover, a possible application of the technique for novel accelerator schemes is examined based on simulations with the particle-tracking code elegant and our beam profile reconstruction tool.
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