Superconducting lenses 5-9 are considered to be interesting devices for electron microscopy as it is then possible to increase H0 and dH/dz more than with the conventional lenses made of ferromagnetic materials. 4-6 Therefore it is possible to reduce the spherical aberration, which is essential for increasing the resolving power. Also, extremely high field stability can be obtained by operating the coil in the persistent current mode. For one measured field distribution H(z) (Figure lb) with Ho = 32-6 kOe and dH/dz = 140 kOe/cm, we calculated the electron optical data for a single-field-condensor-objective 'ยฐ using the bell-shaped field approximation: it Voltage of the electron microscope: 260 kV. Focal length: 2.5 mm. Spherical aberration coefficient: 0.7 mm. Resolving power: 1-3 A. 11
It is only possible to achieve the calculated resolving power if the axes of coil, cylinders, and electron beam are sufficiently adjustable and if one is able to compensate the astigmatism.
We are indebted to Dr. R. G. Maier, Siemens AG, Erlangen, for preparing the niobium stannide, to Dr. E. Fuchs for fruitful discussions, and we wish to ackknowledge the assistance of Mr. G. Lefranc and Mr. M. Exner with the experimental work.