Electron-optical column for a 4 MeV Ultrafast Electron Microscope

Many atomic processes occur on timescales that are as short as tens of femtoseconds. While pulsed lasers have the temporal resolution to investigate these processes, they cannot provide the requisite spatial resolution. Ultrafast electron diffraction (UED) and Dynamic transmission electron microscopy (DTEM) are pulsed electron techniques that have been recently developed to examine the dynamics of these processes with adequate spatial resolution. Unfortunately, Coulomb interactions among the electrons broadens the temporal and spatial extent of the pulse. The Coulomb interactions mainly increase the beam energy spread (Boersch effect) from a fraction of an electron-Volt to hundreds or even thousands of electron-Volts. The Boersch effect has a two-fold impact on the electron optics: it spreads the arrival time window of the pulse from tens of femtoseconds to picoseconds and beyond; and it increases the objective lens chromatic aberration, which reduces the spatial resolution. Consequently, there remains a strong demand for improving the temporal resolution of the probing pulse without sacrificing total pulse charge and spatial resolution. In this SBIR project, EOI is developing a UEM column with atomic resolution, on the order of 3 Å, and capable of reducing the electron pulse length into the sub-nanosecond range for single shot applications with up to 10 million electrons/pulse and into the deep femtosecond range for stroboscopic applications with thousands of electrons/pulse. The key feature of this approach is the combination of a high energy electron gun, such as a superconducting radio-frequency (SRF) photocathode gun, and a novel electron column operating at up to 4 MeV. This approach has two major advantages compared to existing UEM designs: first, the acceleration to relativistic energies dramatically lowers the Coulomb effects due to electron-electron interactions, which allows the use of significantly more electrons/pulse; and second, it pro- vides high temporal resolution that is accompanied by atomic scale resolution owing to a lens system with low aberrations. The column comprises three distinct components: the illumination arm, the objective lens, and the projection arm. In the initial Phase II, EOI produced a detailed mechano-optical design of the illumination arm and the objective lens and is currently assembling the prototype. EOI plans to complete the prototype by the end of the initial phase II and ready it for integration with a high energy RF gun. In the sequential Phase IIA, EOI aims to complete the UEM column by designing, building and integrating the projection optics. EOI plans to assemble and test the UEM column, integrate it with the electron gun at a DOE facility and verify experimentally the performance of the instrument in diffraction and imaging modes.EOI’s novel UEM column provides a path to a product targeted for emerging ultrafast applications, thus making it suitable for enhancing the study of the structure, composition, and bonding states of new materials at ultrafast time scales to advance material science research in the field of nanotechnology and in particular in biomedical research.