Cool beams for ultrafast electron imaging

By near threshold photoionization of a laser-cooled and trapped atomic gas we create dense, picosecond electron bunches at electron temperatures three orders of magnitude lower than in conventional field and photoemission sources. The superior coherence properties of this ultracold source will enable single-shot electron diffraction of macromolecules and ultrafast nanodiffraction. Recently we have recorded the first diffraction patterns of graphite using the ultracold source. To control and manipulate highly coherent, ultrashort pulsed beams we are developing compact 3 GHz microwave cavities as versatile time-dependent electron optical elements. We have demonstrated bunch compression – longitudinal focusing – to below 100 fs using a 3 GHz microwave cavity in TM010 mode. Alternatively, a cavity in TM010 mode may be used to lower the energy spread of an electron bunch by longitudinal defocusing. We use microwave cavities in TM110 mode for measuring bunch lengths, but also to chop the continuous beam of an electron microscope into a high repetition rate train of femtosecond single-electron pulses while conserving emittance