A high brightness source of femtosecond polarized electrons for electron microscopy

Electron scattering probes like electron microscopes conventionally probe the electrostatic potential of the sample, but techniques such as Lorentz microscopy, which utilizes high energy unpolarized electrons, and spin-polarized low energy electron microscopy (SPLEEM), are sensitive to the spin/magnetic texture of the sample. As polarized beams are typically generated via photoemission, femtosecond pulses sources of polarized electrons are very feasible, but to date no commercial polarized electron gun delivers femtosecond polarized beams with electron- microscope scale beam quality. If such a source existed, we would open the door to time-resolved dynamics of out-of-equilibrium magnetic phases, which to date have mostly been investigated with X-ray facilities, but now in table-top devices with superior surface sensitivity. Further, the spin-polarized electron beam itself is of fundamental interest to materials science--understanding the interplay of coulomb repulsion and degeneracy pressure in the model system of a beam may help understand this interplay in quantum magnetic materials. Xelera Research, in close partnership with Cornell University, has multiple decades of combined expertise in the generation of polarized electron sources for particle accelerators and electron scattering instruments. We aim to provide a complete electron source system, (including cathode activation, drive laser delivery, beam generation, acceleration, steering, and one focusing stage) and for the generation of beams in SPLEEM and SEM columns. In this STTR project we aim to develop an ultrafast pulsed, spin-polarized photocathode source with output kinetic energy ranging from 30 eV to 30 keV, to span the range of SPLEEM and SEM applications. Pulse lengths as low as 100 fs, energy spreads as low as 200 meV, and with beam quality/coherence equivalent to that found in Schottky-gun SEM/TEM applications. A critical advancement of our design is a lithographically-defined polarized (up to 40%) GaAs photocathode with an array of nano- to micro-scale source diameters, so users can choose the appropriate balance of delivered average current, beam quality, time-resolution, and energy spread appropriate for their application. These novel photocathode sources will be based on recent advancements in lithographically-defined photocathodes from Cornell University's photocathode lab led by Jared Maxson. The team at Xelera Research LLC is uniquely experienced in the design and construction of novel optics, detectors, particle beam sources, injectors, beam transport systems, and beam dumps, and has pioneered work with using modern optimization techniques for applications of interest to the Department of Energy.