A high power density generator contains an armature structure having a plurality of conductors and at least one heat transfer tube thermally coupled to the conductors. The tube is hollow so as to allow a heat transfer fluid to pass through it. The tube is made of a material having a thermal conductivity λ of at least 20 W/mK, an electrical breakdown strength of at least 60 V/mil, and a mechanical strength adequate for handling, manufacturing and operation. In an embodiment of the invention, the tube is made of a ceramic material.