a plurality of conductor layers, and

a plurality of substrate layers,

wherein each respective substrate layer carries a respective corresponding conductor layer,

wherein each respective substrate layer and corresponding respective conductor layer together form a respective ring having a radial slot extending through the entire respective ring,

wherein the plurality of conductor layers and the plurality of substrate layers together form three or more rings in a spiral arrangement with an initial ring, an end ring, and one or more middle rings,

wherein the initial ring and the end ring, each having an end adjacent to a respective slot are, via a contact section, in current-conducting contact with at least one respective middle ring of the one or more middle rings at an end of the respective middle ring adjacent to the respective slot,

wherein each middle ring has two ends adjacent to a respective slot and is, via a contact section, in current-conducting contact with two other rings, and

wherein the three or more rings are arranged alternately such that a ring with a downward-facing conductor layer follows a ring with an upward-facing conductor layer.