  Ultrafast electron experiments usually work with low-emittance few-electron
pulsed beams. The structures are usually much larger than the (average)
electron pulse size posing challenges to the resolution of simulations. We
present the tracking code FemtoTrack, which allows tackling these multi-scale
challenges in a simple manner. The computationally most heavy interpolation of
the fields is treated either by a moving window or by a supplemental grid,
leading to a significant speedup. Space charge is treated by direct
particle-particle interaction within each bunch, where however many bunches can
be simulated in the same window simultaneously. This allows to obtain
statistics similar to what is obtained on the screen in an experiment.
FemtoTrack is applied to two examples: An ultrafast nanotip electron source and
a length scalable laser-driven electron accelerator on a microchip. In these
setups, previous results have been reproduced with tremendous speedup, allowing
for parameter scans similar to the tuning of an experiment.
