  We present a method for producing sub-100 fs electron bunches that are
suitable for single-shot ultrafast electron diffraction experiments in the 100
keV energy range. A combination of analytical results and state-of-the-art
numerical simulations show that it is possible to create 100 keV, 0.1 pC, 20 fs
electron bunches with a spotsize smaller than 500 micron and a transverse
coherence length of 3 nm, using established technologies in a table-top set-up.
The system operates in the space-charge dominated regime to produce
energy-correlated bunches that are recompressed by established radio-frequency
techniques. With this approach we overcome the Coulomb expansion of the bunch,
providing an entirely new ultrafast electron diffraction source concept.
