Superconducting objective lens utilizing field trapping in Nb3Sn disks

摘要

A high‐quality superconducting objective lens has been designed, and the optical characteristics have been tested in a conventional electron microscope. The lens uses Nb3Sn disks held in the gap of a ferromagnetic container. The magnetic field is trapped in the disks using an excitation coil that is then deenergized leaving a persistent field that has a Gaussian distribution. Using a stack of 20 disks in a gap of 2.5 mm with a bore diameter of 3 mm, a peak field of 22 kG and a half‐width of 1.76 mm is produced. The spherical and chromatic aberration coefficients given by this field have been calculated. The lens was tested in an experimental cryostat mounted in the column of an Hitachi HU‐11A. The images obtained showed reasonable quality with the resolution limit of 10 Å set by mechanical instabilities.