  We propose and demonstrate a novel scheme to produce ultrashort and
ultrastable MeV electron beam. In this scheme, the electron beam produced in a
photocathode radio-frequency (rf) gun first expands under its own Coulomb force
with which a positive energy chirp is imprinted in the beam longitudinal phase
space. The beam is then sent through a double bend achromat with positive
longitudinal dispersion where electrons at the bunch tail with lower energies
follow shorter paths and thus catch up with the bunch head, leading to
longitudinal bunch compression. We show that with optimized parameter sets, the
whole beam path from the electron source to the compression point can be made
isochronous such that the time of flight for the electron beam is immune to the
fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch
length and arrival time jitter for a 20 fC beam after bunch compression are
measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an
ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond
(FWHM) resolution in MeV ultrafast electron diffraction where lattice
oscillation at 2.6 THz corresponding to Bismuth A1g mode is clearly observed
without correcting both the short-term timing jitter and long-term timing
drift. Furthermore, oscillating weak diffuse scattering signal related to
phonon coupling and decay is also clearly resolved thanks to the improved
temporal resolution and increased electron flux. We expect that this technique
will have a strong impact in emerging ultrashort electron beam based facilities
and applications.
