Pulse length measurements of ultracold and ultrafast electron bunches extracted from a laser cooled gas

We are developing an ultrafast and ultracold electron source, based on near-threshold, femtosecond photoionization of laser-cooled and trapped Rubidium gas. Recently, we demonstrated electron crystallography of graphite for the first time using the ul- tracold source. The ultimate goal is ultrafast, single-shot electron crystallography of macromolecules, which requires a high degree of control of the dense electron phase space distribution. The transverse phase space distribution was characterized using the waistscan method and yielded electron temperatures as low as 10 K. For char- acterizing the longitudinal phase space distribution we have developed a microwave cavity based diagnostic element to correlate electron bunch lengths to streak images. This allows us to measure the pulse length with sub picosecond temporal resolution. We present the first measurements of both ultracold and ultrafast electron pulses which have an rms pulse duration of 20 picoseconds containing at least 1000 elec- trons. These bunches are sufficiently short to be compressed to 100 fs bunch lengths using established RF compression techniques.