Speaker
Description
Pulsed electron beams probe the dynamics of matter out of equilibrium with high spatial and temporal resolution. Ultrafast electron diffraction in particular is sensitive to sub-angstrom, sub-picosecond scale atomic motion. To collect all the structural information available in an electron diffraction pattern, the experimentalist must control the angular magnification onto the detector plane. We present a case study demonstrating the advantage of angular magnification: investigating periodic strain in moiré materials. Strain waves with 10 nm wavelength appear in diffraction as satellites closely clustered around brighter Bragg peaks. We describe a quadrupole lens triplet that varies the effective drift distance $M_{12}$ between sample and detector from 80 cm to 8 m for our 140 keV electron beam, allowing us to zoom in on these moiré satellites. Three independently powered quadrupoles make it possible to eliminate astigmatism from a point-like probe. With the field strength achievable using quadrupole magnets, this magnification technique is also suitable for MeV beam energies.
Funding Agency
This work was supported by the U.S Department of Energy, awards DE-SC0020144 and DE-SC0017631, and U.S. National Science Foundation Grant PHY-1549132, the Center for Bright Beams.
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