Intrinsic limit of field homogeneity of helical dipoles

The intrinsic limit of field homogeneity and the relation between the field homogeneity and multipoles of helical dipoles are analytically and geometrically investigated. The field homogeneity of helical dipoles, deformed by the twisting of the 2D dipole is intrinsically limited by the degree of twist, differently from that of the 2D dipole. It results that the highest homogeneity of the dipole field for the circular homogeneous cross-sectional region is accomplished with the non-zero helical sextupole. (author)