  Accelerator-based MeV ultrafast electron microscope (MUEM) has been proposed
as a promising tool to study structural dynamics at the nanometer spatial scale
and picosecond temporal scale. Here we report experimental tests of a prototype
MUEM where high quality images with nanoscale fine structures were recorded
with a pulsed 3 MeV picosecond electron beam. The temporal and spatial
resolution of the MUEM operating in single-shot mode is about 4 ps (FWHM) and
100 nm (FWHM), corresponding to a temporal-spatial resolution of 4e-19 s*m,
about 2 orders of magnitude higher than that achieved with state-of-the-art
single-shot keV UEM. Using this instrument we offer the demonstration of
visualizing the nanoscale periodic spatial modulation of an electron beam,
which may be converted into longitudinal density modulation through emittance
exchange to enable production of high-power coherent radiation at short
wavelengths. Our results mark a great step towards single-shot
nanometer-resolution MUEMs and compact intense x-ray sources that may have wide
applications in many areas of science.
