Patent Number: 045308128
Section: summary

FIELD OF THE INVENTION The present invention relates generally to composite magnetic coil windings and more particularly to composite magnetic coil windings for use in a tokamak reactor. The present invention also relates generally to toroidal reactors for producing fusion reactions and more particularly to toroidal field coil windings for toroidal reactors where the coil windings are composite members consisting of two or more segments of dissimilar materials. More particularly still the invention relates to segmented toroidal field coil windings for tokamak reactors where the coil windings are made of a copper or copper alloy section and an aluminum or aluminum alloy section. BACKGROUND OF THE INVENTION Prior art tokamak fusion reactor (TFR) concepts were directed to large machines with blanket and shield elements positioned in between the plasma fusion region of the TFR and the large superconducting toroidal field (TF) coils. In U.S. Pat. Nos. 4,367,193and 4,363,775, there is disclosed a small machine with the blanket means positioned external to the normally conducting TF coil assembly. It is known in the art that such blankets can advantageously use the neutrons generated in the fusion plasma to breed new fuel, to produce thermal energy and to create additional energetic reactions. This invention is directed to those TFRs utilizing external blankets (XBTFR) such as those disclosed in the commonly assigned U.S. Patent applications referred to above. In the case where a TFR uses the deuterium-tritium (d,t) reaction, approximately 80% of the energy output is in the form of the kinetic energy of fast neutrons. In the small machine referred to in the above-referenced U.S. patents, the TF coil is exposed to the flux. The neutron radiation damage and heat loads preclude the use of superconducting materials for the TF coils in this small machine design. Applicants have found that the materials used in the TF coils must have both high electrical conductivity to carry the high currents necessary to generate the TF and also high tensile strength to withstand the forces accompanying the strong magnetic fields. Applicants have found that TF coils of high electrical conductivity can be made from high strength copper alloys. However inasmuch as in the small TFR design, the TF coil surrounds the plasma region, the neutrons created as a result of the fusion reactions must pass through it. In this regard, it has been found by Applicants that copper and copper alloy coils will absorb a considerable fraction of the neutrons and that those that do emerge without being absorbed in the TF coils will have lost much of their kinetic energy in the copper or copper alloy. While it is a feature of the TFR design disclosed in the above-referenced U.S. patents to remove the energy deposited in the TF coils and recover it as useful heat, energetic neutrons are far too valuable for breeding fuel for fusion and fission reactors and for generating high temperature heat in the blanket to be used merely as a source of low temperature heat in the TF coils. Applicants have also found that one of the consequences of the TFR geometry is that the current density and mechanical stresses imposed on the TF coils are much greater in the region of the inner part of the TF coil, the region nearest the center or the main axis of the machine. Another consequence of the XBTFR geometry is that most of the neutrons generated in the fusion plasma exit through the outer part of the TF coil or the region farthest from the central or main axis of the TFR. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved magnetic coil structure. It is also an object of this invention to provide a magnetic coil structure that has high electrical conductivity and high strength. It is a further object of this invention to provide an improved TF coil winding for TFRs. It is also an object of this invention to provide improved TF coil windings for XBTFRs that have high strength and high electrical conductivity. It is a still further object of this invention to provide improved TF coil windings for XBTFRs that have high strength and high electrical conductivity and which absorb relatively few energetic neutrons passing through the TF coils. It is a still further object of this invention to provide improved TF coil design for XBTFRs that utilize aluminum or aluminum alloys wherever possible to provide improved neutron economy, reduced TF coil nuclear heating and heating gradients and reduced activation. It is a still further object of the instant invention to provide an improved TF coil design for XBTFRs that shift the nuclear heating load from the magnets, coils and supporting structures to the surrounding media (e.g., blankets). To achieve the foregoing and other objects in accordance with the principles of the present invention, there is provided a coil structure comprising at least two conductor segments, one segment made of copper or copper alloys and the other of aluminum or aluminum alloys. Preferably the conductor segments are joined at a high strength, high electrical conductivity joint. It is also preferred that the joint be constructed so as not to interfere with heat removal from the coil. In a further aspect of the present invention, there is provided a coil structure for a TFR wherein the coil structure consists of a copper or copper alloy segment and an aluminum or aluminum alloy segment wherein the segments are joined at a high conductivity, high strength joint. In a further aspect of the present invention, a TFR is provided for producing fusion reactions in a region generally adjacent to the TF coils which are exposed to the neutron flux. The TF coils, in accordance with the principles of the present invention, consist of a copper or copper alloy segment and an aluminum or aluminum alloy segment joined together at high conductivity, high strength joints. Preferably, the side of the TF coil nearest the center of the TFR is made of copper or copper alloy. This has been found to be the portion of the TF coil where the current density and the mechanical stresses imposed are the greatest. It is also preferred that the opposite side of the coils be made of aluminum or aluminum alloy. This has been found to be the region of the TF coils where the much larger fraction of the neutrons produced pass through the TF coils to the outside. It is also preferred that the joint be constructed to provide a generally smooth and continuous surface with the coil segments and be constructed so as not to interfere with heat removal from the TF coils. It has been found by Applicants that aluminum has the advantage that energetic neutrons passing through it suffer very much less absorption and very much less energy loss than when they pass through copper. Another advantage of aluminum is that it is not activated by exposure to neutrons as much as copper and what radioactive elements are formed are shorter lived. The attendant waste disposal problem of an all copper TF coil is thereby eased. While the mechanical strength of aluminum and aluminum alloys may not provide an adequate strength margin for the inner portion of a TF coil, it is quite adequate for the outer part. The much larger fraction of neutrons passing through the aluminum-copper composite coil of the present invention allow for more efficient use of the neutrons generated for breeding tritium fuel for fusion reactors, for breeding fissile fuel for fission reactors, for transmuting and fissioning fission reactor radioactive waste products and generally for all other purposes for which persons skilled in the art utilize neutrons. The neutrons passing through the aluminum or aluminum alloy segment of the TF coil lose less of their kinetic energy making them more effective for the purposes disclosed hereinabove. In addition, inasmuch as the neutrons passing through the aluminum or aluminum alloy sections of the TF coils result in less energy deposition in the TF coil than for the case of copper, the cooling system burden is reduced. It is preferred that alloys of Cu and Al be utilized, rather than the pure metals in order to achieve necessary strength and other mechanical properties. In particular, copper beryllium, copper beryllium-nickel and MZC (Mg-Zr-Cr) alloys are preferred. The most important consideration for the selection of the particular Al alloy is its mechanical strength. Preferably alloys in the 2000, 6000 and 7000 series are utilized due to their strength. Aluminum reinforced with graphite or carbon may also be used. It should be understood that lower strength Al alloys can be utilized in some of the lower stressed areas of the coils. It should be further understood that some alloying elements of aluminum (e.g., Fe and Ni) will result in increased activation compared to pure Al. However, the Al alloys still retain the property of being more transparent to neutrons than copper alloys, thus enhancing neutron economy by about the same magnitude as if pure Al were used. Additional objects, advantages and novel features of the invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.