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We propose and demonstrate a novel scheme to produce ultrashort andultrastable MeV electron beam. In this scheme, the electron beam produced in aphotocathode radio-frequency (rf) gun first expands under its own Coulomb forcewith which a positive energy chirp is imprinted in the beam longitudinal phasespace. The beam is then sent through a double bend achromat with positivelongitudinal dispersion where electrons at the bunch tail with lower energiesfollow shorter paths and thus catch up with the bunch head, leading tolongitudinal bunch compression. We show that with optimized parameter sets, thewhole beam path from the electron source to the compression point can be madeisochronous such that the time of flight for the electron beam is immune to thefluctuations of rf amplitude. With a laser-driven THz deflector, the bunchlength and arrival time jitter for a 20 fC beam after bunch compression aremeasured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such anultrashort and ultrastable electron beam allows us to achieve 50 femtosecond(FWHM) resolution in MeV ultrafast electron diffraction where latticeoscillation at 2.6 THz corresponding to Bismuth A1g mode is clearly observedwithout correcting both the short-term timing jitter and long-term timingdrift. Furthermore, oscillating weak diffuse scattering signal related tophonon coupling and decay is also clearly resolved thanks to the improvedtemporal resolution and increased electron flux. We expect that this techniquewill have a strong impact in emerging ultrashort electron beam based facilitiesand applications.

Achieving 50 femtosecond resolution in MeV ultrafast electron diffraction with a double bend achromat compressor