Electron-optical column for a 4 MeV Ultrafast Electron Microscope

Many atomic processes occur on timescales that are as short as tens to hundreds 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 during the travel of the beam to the specimen. 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 into the deep femtosecond range without sacrificing total pulse charge and spatial resolution. High-energy, multi-MeV electron sources not only alleviate the impact of Coulomb interactions, they provide a path to single-shot real-space imaging and diffraction with spatio-temporal resolution in the range of Å - µs to ~ 10 nm - 10 ps. Electron Optica proposes to develop a novel electron beam column suitable for 4 MeV UEM and UED operating in single shot mode, with 10 million or more electrons per shot. The electron beam column is based on an electron source with a relative energy spread of 10-5 and utilizes state-of-the-art magnetic electron lenses and correction elements optimized for MeV operation. A detailed electron-optical analysis of the key column components, the objective lens, the illumination and projection optics, will be performed using state-of-the-art simulation software. Particular attention will be paid to the design of the objective lens, which defines the ultimate electron-optical performance of the column. The computation of the primary and higher order optical aberrations of the lens will be carried out in two steps: first, the magnetic flux distribution experienced by the beam is calculated with a proven high-accuracy, second-order finite element method (SOFEM); second, the ray-optical simulator calculates the aberration coefficients associated with the flux distribution along the column. For precision, the solver calculates the flux distribution in the magnetic circuit and coil windings, taking into account the effects of magnetic saturation in state-of-the-art magnetic materials. Furthermore, the requisite coil current for focusing the beam accounts for the relativistic mass increase of the electron, which is substantial at the beam energies of interest. The goal of the phase I research is to provide a detailed electron-optical design of a 4 MeV UEM/UED column that can be prototyped in phase II.