Light-weight aluminum foil solenoids for traveling-wave tubes

Most traveling-wave tubes use solenoids to constrain their electron beams. Each tube requires a certain diameter, length, and strength of magnetic field, which are the basic specifications for the solenoid. In addition, for airborne applications it becomes important to minimize weight and power, and to provide suitable cooling. Methods have been developed for winding coils of aluminum foil, together with a suitable insulation. The ends of the coils can be machined, then bonded to a heat-conducting plate or cooled by an air stream. There is only one insulating boundary; thus heat is readily removed from the winding. This design permits operation at high current densities without exceeding safe hot-spot temperatures, so that aluminum coils can be made about the same size, for given characteristics, as copper wire coils. The saving in weight is thus proportional to the densities of the two materials. A further interesting feature of the aluminium foil solenoids is that the ends of the coils can be machined to almost any desired shape. Slots or holes can be cut in the coils to permit the introduction of power leads or mechanical devices.