  MeV ultrafast electron diffraction (UED) is a widely used technology for
ultrafast structural dynamic studies of matters in numerous areas. The
development of laser wakefield accelerator (LWFA) envisions great potential of
advanced all-optical electron source based on LWFA in UED applications. We
experimentally demonstrated that an LWFA-based device with a miniaturized
permanent magnet beamline can generate and manipulate electron beams suitable
for UED. In the beam transmission, the LWFA electron beams with intrinsic short
duration stretch due to energy spread and then are compressed by a following
double bend achromat. The optimized double bend achromat can make the beamline
isochronous such that the arrival time jitter induced by the shot-to-shot
energy fluctuation can be eliminated, and allow the advantage of the natural
laser-beam synchronization for LWFAs to emerge. With the energy filtering, the
beam energy spread can be reduced to 3% (FWHM) while a sufficient amount of
charge (11.9 fC) per bunch for diffraction is retained. Start-to-end
simulations showed that the bunch length reaches ~30 fs (rms) with the same
experimental configuration. Clear single-shot and multi-shot diffraction
patterns of single-crystalline gold samples are obtained and the derived
lattice constant agrees excellently with the real value. Our proof-of-principle
experiments open the door to the detection of ultrafast structural dynamics
using MeV LWFA beams, and pave the way for the UED applications with sub-10-fs
temporal resolution.
