Progress Update on the Development of a User Adjustable Pole-piece

The transmission electron microscope (TEM) is a powerful characterization instrument used in both physical and life sciences, with modern flagship instruments potentially representing an infrastructure investment of several million dollars. Pole-piece gap choice dictates much of a TEM’s performance which must balance resolving capability (smaller gaps) and sample accessibility/tilting (larger gaps) [1]. We have previously proposed an alternative approach, a User Adjustable Pole-piece (UAP) with a pole gap that can be adjusted by the microscopist to suit a variety of experiments, reducing unneeded duplication of resources [2]. To be of practical use, the proposed implementation must be; operable while maintaining high vacuum (to avoid delays and contamination), must be adjustable without the need to disassemble/remove adjacent lenses/detectors/goniometers, must be mechanically stable with ultra-precise alignment about the optic axis, and must be re-alignable to a good tuning without a specialist engineer present. Our present implementation is based around the JEOL 200/300kV geometry. The UAP is shown in Fig. 1, both integrated into the existing infrastructure, and separately on its own. In the smallest gap, we naturally expect the lowest aberrations, while in the largest gap we expect full 180-degree rotation for tomography, presenting a solution to the missing wedge problem [3]. In this presentation, we will elaborate on our progress in the design and manufacture of the UAP. Space in the objective lens is tight, and mechanical access to the chamber is achieved through sharing the anti-contamination device (ACD) port. Consequently, the cryogenic system has been redesigned to fit the UAP and its required mechanical feedthrough while preserving the vacuum in the chamber. We will show our results from both computational and physical testing of this revised cryogenic system. Heat flow calculations are used to predict the usable boil-off time of the ACD dewar. The mechanical challenges and constraints of manufacturing the UAP will be discussed. The formation of a high-quality magnetic field within the UAP requires extreme precision within the manufacturing process, so the tolerances of all UAP parts are carefully verified using a Coordinate-Measuring Machine (CMM). Furthermore, full part traceability is being realized for the UAP; each part can be tracked throughout the manufacturing process back to the serialized raw metal bar it was machined from. We will also show our progress in the manufacturing and validation process, including discussing aberration estimates derived from our dimensional measurements and 3D magnetic field simulations using COMSOL Multiphysics. These are both shown in Fig. 2. The UAP development has now reached the point where full system integration is needed (achieving the necessary manufacturing precision, integration with existing systems, PC control etc.). We are moving forwards with production, verification, and final testing stages of the UAP, and hope to present the first images taken with a UAP-equipped instrument this year [4]. (Left) System integration testing of the UAP in a JEOL GRAND ARM objective lens unit. The design includes an ACD extension flange, redesigned ACD system, and custom drive feedthrough system. (Right) Close up of the UAP, shown positioned at an intermediate pole gap of 4.0 mm, with the redesigned cryogenic block for the ACD. (Left) Verification of the pole face dimensions via Coordinate-Measuring Machine (CMM). (Right) Thermal simulations using COMSOL of our revised ACD system in the extended flange.