Ultracold electron sources, which are based on near-threshold photo- and fieldionization of a cloud of laser-cooled atoms, offer the unique combination of low emittance and extended size that is essential for achieving single-shot, ultrafast electron diffraction of macromolecules. Here we present measurements of the effective temperature of such a pulsed electron source employing rubidium atoms that are magneto-optically trapped at the center of an accelerator structure. Transverse source temperatures ranging from 200K down to 10K are demonstrated, controllable with the wavelength of the ionization laser. Together with the 50 μm source size, the achievable temperature enables a transverse coherence length of ≈ 20nm for a 100 μm sample size.
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