Systems with Superconducting Lenses
Superconducting lenses have not been much used yet as condensers, intermediate lenses, and projectors in conjunction with superconducting objective lenses. Projectors are used, as in conventional electron microscopy, to magnify the intermediate image produced by the objective lens. Since the magnification should be a maximum for a given column length, the lens strength with the smallest asymptotic focal length should be used. This leads to k2 ≈ 1 in the bell-shaped field approximation. For lenses with a field distribution with steep slopes, the asymptotic focal length increases less as the excitation increases. An example is the shielding lens R (Table 4.5 for the case of a 1500-k V beam voltage) for which f1 ≈ f0 was found by numerical calculation, although the Glaser value of k2 in the bell-shaped field approximation corresponds to k2 = 2.1. But since the magnification should be controlled with the intermediate and projector lenses, the strong hysteresis in the shielding lens is very inconvenient and thus only iron-circuit lenses have been used up to now for further magnification.
KeywordsObjective Lens Liquid Helium Temperature Carbon Foil Beam Voltage Adjustment Screw
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