  Ultralow emittance ($\leq20$ nm, normalized) electron beams with $10^5$
electrons per bunch are obtained by tightly focusing an ultrafast ($\sim$ 100
fs) laser pulse on the cathode of a 1.6 cell radiofrequency 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.

Accelerator Physics