Direct Measurement of Sub-10 fs Relativistic Electron Beams with Ultralow Emittance.

Ultralow emittance (≤20  nm, normalized) electron beams with 10^{5} electrons per bunch are obtained by tightly focusing an ultrafast (∼100  fs) laser pulse on the cathode of a 1.6 cell radio frequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to <10  fs rms bunch length. The measurement is performed using a thick high-voltage deflecting cavity which is shown to be well suited to measure ultrashort durations of bunching beams, provided that the beam reaches a ballistic longitudinal focus at the cavity center.