Brightness optimization in a 200 keV DTEM source by geometry-driven aberration suppression.

High temporal resolution dynamic transmission electron microscopy (DTEM) requires substantially higher peak currents than conventional TEM sources can sustain without severe brightness loss, motivating a redesign of the electron gun. This work targets improving the brightness in a 200 keV electrostatic electron gun for DTEM by mitigating aberrations through optimizing conductor geometry. The cathode-anode assembly is reconfigured to act simultaneously as an accelerator and condenser pre-lens, confining the beam size in regions of strong field curvature. A third-order off-axis transfer map with Green's-function evaluation of image errors, validated against Superfish and GPT, quantifies how space-charge-driven divergence growth couples into spherical aberration and guides voltage/geometry choices. Along a minimum-spot operating contour, the spherical aberration coefficient is reduced from >150mm to ∼5-10 mm at 200 keV while preserving a ≤10μm exit-aperture spot for MTE = 0.1-1 eV and ∼1 mA peak current. The results establish geometry-driven gun redesign as a practical route to higher-brightness, low-aberration electron sources for next-generation time-resolved microscopy.