To enable more groundbreaking scientific discoveries, imaging, diffraction, and spectroscopy techniques must evolve beyond traditional static or low-speed imaging by dramatically improving temporal resolution. Being able to resolve processes at the ps and fs time scales would enable visualization of the dynamics of atomic motion at its fundamental timescales but requires the development of new electron sources and advanced optics. Among the techniques under active research, ultrafast electron diffraction (UED) is the most advanced, while ultrafast transmission electron microscopy (UTEM) and ultrafast electron energy loss spectroscopy (EELS) are still in earlier research and development stages. Concept designs for these instruments leverage high extraction field radiofrequency photoinjectors as these sources can produce dense electron bunches with the necessary brightness to enable high spatial and temporal resolution. This work presents several techniques developed in order to preserve the initial beam brightness from the cathode to the detector, thereby meeting the stringent requirements for capturing ultrafast dynamics and advancing ultrafast electron scattering instrumentation.
We focus on the opportunities offered by operating a photoinjector in the so-called cigar regime, where the beam’s elongated aspect ratio in its own reference frame enables the requisite brightness for high spatiotemporal resolution in UTEM, and investigate the longitudinal phase space manipulation of these beams using RF fields. Pairing the Pegasus RF photoinjector source with a newly installed 3rd harmonic RF cavity, we show the 6D phase space of the electron beam can be shaped to achieve optimal bunching conditions or minimal energy spread, making Pegasus well-suited for high-fidelity UED or UTEM. An envelope equation-based approach is employed to derive analytical scaling laws for RF-based pulse compression, revealing the dependencies on beam energy and charge. Our results indicate that relativistic energies are crucial for achieving sub-femtosecond pulse lengths with electron bunches containing $10^6$ electrons. We further demonstrate experimentally that the 3rd harmonic removes non-linear effects of RF curvature, hence, shrinking energy spread by nearly two orders of magnitude to 10 parts per million and paving the way for ultrashort beams in the sub-femtosecond regime.
Next, we theoretically address the effects of space charge fields on imaging performance in single-shot time-resolved TEM. By employing a Green's function perturbation method, we derive analytical estimates of space charge-induced aberration coefficients and validate them through particle tracking simulations. Our findings provide critical insights into how space charge nonlinearity affects image formation and offer fundamental scaling laws for balancing temporal and spatial resolution in time-resolved TEM. These results provide an important framework for improving the performance of ultrafast electron scattering instruments, particularly in high-charge, single-shot modes.
Finally, we propose advancing UED to higher beam energies, potentially exceeding 10 MeV. Higher beam energies flatten the Ewald sphere, bringing higher-order Bragg reflections into the field of view, while also reducing space charge effects, allowing more charge to be loaded into the bunch and enhancing the intensity of Bragg orders. Additionally, higher energies provide greater penetration depths and improved temporal resolution, though they introduce challenges related to beam rigidity and focusing. We address these challenges by utilizing post-sample strong focusing permanent magnet quadrupole (PMQ) optics, which provide angular magnification to overcome the point spread of the detector. Our method employs a triplet of compact, high field gradient ($>500$ T/m), small-gap (3.5 mm) Halbach PMQs. These PMQ lenses allow us to maintain high-quality diffraction patterns at higher energies. With this optical setup, we demonstrate a tunable camera length, achieving a $6\times$ improvement and reciprocal space resolution better than $0.1$ \AA$^{-1}$ with an $8.2$ MeV electron beam and a crystal Au sample. Future designs should consider larger aperture PMQs to capture more Bragg orders, as larger apertures also reduce the aberrations.
