Fig. 2
Design and characterization of the final focusing lens system. a Single quadrupole element. An aluminum disk of 15.875 mm diameter holds 4 permanent magnets placed in quadrupolar configuration. The outer square flexure is specifically designed to minimize the footprint, and provide at the same time high precision control on transverse alignment of the magnetic element. b Three-dimensional view of the sample area. The sample holder can be moved horizontally and vertically with 100 nm precision. The permanent magnetic quadrupole (PMQ) focusing system can be moved longitudinally and horizontally with better than 10 nm precision. Additionally, the longitudinal position of the last quadrupole element can be adjusted with sub-μm precision. c Transverse and longitudinal alignment of magnetic elements. The plot shows the magnetic field magnitude moving along the focusing system. With no offset (x = 0) the peak measured field was below 1.5 mT, corresponding to a transverse misalignment error of less than 14 μm. The inset shows the measured focusing strength of one of the elements. d Beam dynamics simulation of a collimated electron beam entering the focusing system using general particle tracer (GPT). Such simulations were performed using the actual measured gradient profiles, with the distances between the elements optimized to minimize the overall focal length of the system
