Self-compensation of the residual field gradient in double-shell open-ended cylindrical axial magnetic shields

An important and not-obvious demonstration' made in this work Is that replacing a closed double-shell axial cylindrical shield with a similar but open-ended shield can lead to an increasing in not only the axial shielding factor, but in the residual magnetic field uniformity as well. It is shown that a self-compensation of the residual field gradient and a significant improvement of the total residual field uniformity are potentially possible, by combining two open-ended cylindrical shells having different length-to-diameter ratios and providing opposite signs and appropriate magnitudes of the residual field gradients. A numerical example confirms the applicability of the method. It is shown that an -6-fold reduction of the residual field gradient along the axial direction and an ~2fold reduction of that along the radial direction over a relatively wide area inside a double-shell cylindrical axial shield are possible if its both ends are open.