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In this work, we have found that the key to minimizing the effect of the end-cap openings in multishell shields lies with widening the axial air-gaps between the end-caps. We have revealed that the axial air-gaps strongly influence the shielding with openings, while practically not affecting-if the gaps are not too narrow, of course-the shielding with no openings. As a result, widening the axial air-gaps can bring the axial shielding with large openings very close to that with no openings at all. To investigate as general case as possible we have described with the help of special charts the five-shell shields with typical 1.25, 1.5, and 1.75 aspect ratios for the innermost shell. The charts showed that while the shielding with no openings increases monotonically with either shortening the innermost shell or increasing the radial air-gaps, the shielding with openings reaches an extreme, which depends on the normalized permeability. Another important finding is that in contrast to small openings, the effect of large openings depends strongly on the radial air-gaps. Using the charts developed, a typical five-shell shield can be easily optimized in order to compensate for the effect of the openings and match the performance of the corresponding closed shield.
Paperno et al. (Thu,) studied this question.