Optimum shell separation for closed axial cylindrical magnetic shields

The effect of shell separation on the axial shielding with closed double-shell cylindrical shields is investigated numerically. It is found that the optimum shell separation for practical, equal-thickness shields of the above type is considerably smaller than that for transverse spherical and infinitely long cylindrical shields: in most cases, air gaps equal 5%-10% of the inner shell diameter are wide enough to bring the shielding to 90% of its maximum. This indicates that closely spaced axial shields can be used without much sacrifice in performance. Taking into account the computed optimum shell separation for double-shell shields, one can easily optimize and design a compact and effective multishell shield. Based on the numerical study, an analytical approximation is suggested for the axial shielding with narrowly spaced double-shell cylindrical shields.