The bowtie steering slot: A novel electron optic enabling large, flat photocathodes in streak tubes

Streak tubes with large photocathodes typically steer electrons into the subsequent electron optics by using an inward-concave photocathode surface. The use of a curved photocathode surface introduces optical, mechanical, and manufacturing complications. A novel bowtie-profile steering slot anode has been developed for use in place of a curved cathode and conventional slot extractor. The grounded bowtie profile slot shapes the electric field of the photocathode to produce field gradients within the plane of the aperture, creating a spatially inward steering effect, which directs electrons into the downstream focusing electrodes. We present simulations describing the effects of the bowtie steering slot and experimental measurements of its implementation in the BHx streak tube [Raymond et al., Rev. Sci. Instrum. 96, 103503 (2025)]. Experimental work supports the spatial magnification and geometric distortion produced by the simulations. Implementation of the bowtie steering slot in the BHx results in 1.8× spatial magnification and 25 mm of usable photocathode height with a maximum of ≈2% geometric distortion at the detector. Here, we present design considerations for this electrode by exploring how its geometry can be optimized for various steering configurations. Simulations have demonstrated the potential for the bowtie to improve the usable cathode area of existing streak tubes.