Optimization of Focusing Systems for Ultrafast Electron Microscopy

Ultrafast electron microscopy promises the ability for very high combined spatial and temporal resolution. Focusing elements based on arrays of permanent magnet quadrupoles, with very high gradients, offer the potential to achieve high image resolution, with aberration control. However, using permanent magnet quadrupoles presents difficulty in machining, assembly, and alignment, when used in multiple-lens arrays. Permanent magnet quadrupoles can achieve gradients in excess of 1 kT/m, and higher-order harmonic suppression is possible. When arranged in arrays, the focusing properties at high beam energies offer significant advantage for aberration correction. Multi-objective genetic algorithms offer optimization capabilities to solve these challenges and guide engineering and fabrication of high-precision permanent magnet quadrupole arrays. In Phase I, we studied permanent magnet quadrupole construction, and successfully developed a simulation tool to analyze individual magnets and multiple-lens arrays. We used a magnetostatic code with a multiple-objective genetic algorithm to optimize key parameters. Simulated field maps were used in a particle tracker optimization to determine multiple-lens configurations that minimize geometric and spherical aberrations while maintaining adequate magnification. The Phase II plans include upgrades to the algorithm for multiple-lens design. The design work will guide the engineering and fabrication of a permanent magnet quadrupole array for use as an objective lens. The lens will be installed on a beamline at Brookhaven National Laboratory with the goal of characterizing the focal strength, stigmatism and aberrations. Finally, a virtual diagnostic based on the machine parameters will be developed for online tuning capabilities. Commercial Applications and Other Benefits: The program results will guide commercial implementation of a complete, turn-key and user- friendly, tabletop ultrafast electron microscopy system. The online tuning capabilities will permit “plug-and-play” style capabilities for operators and scientists, and seed the deployment of these devices across the scientific landscape at universities and research institutions.