Ultrafast Electron Diffraction with Stray Sextupole Field Correction

Ultrafast Electron Diffraction (UED) is a technique which allows for the study of atomic and molecular scale dynamics at the picosecond timescale and below. In order to use UED on a material, it is necessary to create a periodic structure large enough that the probing electron beam can create a coherent diffraction image. For some materials, it is difficult to do this at length scales larger than a few microns. These simultaneous constraints on both the physical size and the momentum spread require an electron beam with an emittance near the single nanometer scale. To achieve this, we use a photocathode with the capability of producing beams with a mean transverse energy on the scale of tens of meV. A series of focusing magnets are then used to bring the beam to a focus just before the sample location. These focusing elements also create stray fields, which increase the emittance of the beam. We use a series of corrector magnets, which correct stray magnetic fields up to and including the sextupole field, which allow us to preserve the beam quality throughout the beamline.