Patent Number: 046577238
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a cooling arrangement for magnetic coils. This invention further relates to a cooling arrangement for a toroidal magnetic field coil assembly. More specifically, the present invention relates to a cooling arrangement for a toroidal field coil assembly for a tokamak fusion reactor. The invention further relates to a method of cooling an assembly of magnetic coils. 2. Background of the Invention Positioned within the plasma fusion region of a tokamak reactor is a plasma which is composed of a mixture of positively charged nuclei and free electrons. To maintain purity and to prevent instability, the plasma cannot be allowed to contact any other matter. Thus, the core of the tokamak fusion reactor (TFR) is in a hermetically sealed vacuum chamber. In addition, means must be provided to prevent the fusible nuclei from touching any structural members or walls before they have had sufficient opportunity to collide and fuse. However, at temperatures in the range where fusion reactions will occur, the nuclei are moving so rapidly that they would traverse the distance between structural members or walls in the plasma fusion region in less than a microsecond. Thus, a non-material means must be found to contain the plasma long enough for the nuclei to collide and fuse. One approach is to employ a magnetic field to confine the hot plasma. In a tokamak device, a circular ring of plasma is generated and maintained within a toroid-shaped region by the action of intense magnetic fields which themselves are shaped to form the toroid. The magnetic field acts as a non-material container liner that insulates the hot plasma from any walls or structural components of the tokamak. The magnetic field exerts an effective pressure on the contained plasma that is proportional to the square of the magnetic field strength. By maintaining this magnetic pressure at a greater value than the internal pressure of the plasma, containment is possible. In order to develop a magnetic field of sufficient density and intensity to contain hot plasma, large amounts of power are used to energize the magnetic coils that comprise the toroidal field coil assembly. In spite of the fact that the coils are typically made of a high conductivity material, the large currents involved create very considerable amounts of Joule heat. In addition, in a compact TFR, such as that disclosed in U.S. Pat. Nos. 4,367,193 and 4,363,775,; where the toroidal field coils are positioned adjacent to and surrounding the plasma fusion region, they experience considerable nuclear heating from the neutrons generated in the plasma fusion region. Thus, it becomes necessary to provide a means to cool the toroidal field coil turns to prevent their destruction. Furthermore, in a compact TFR, the toroidal field (TF) coils are of a compact size, and in order to generate the required toroidal field must carry a very high current density. Furthermore, the compact size puts additional constraints on designing effective cooling arrangements for the TF coils. It will be apparent that flowing coolant must be distributed in the toroidal field coil assembly of the compact TFR to remove the Joule and nuclear heat generated therein. The coolant must be distributed to the various cooling channels in the TF turns in such a way that the coolant distribution through the coil turns can be controlled to enhance the cooling characteristics. In U.S. Pat. No. 4,116,264 to Farfaletti-Casali et al, a modular toroidal assembly for forming a blanket structure for a fusion reactor is disclosed. The coolant header structure for the blanket illustrated appears to consist of several large tubes branching off of a ring-shaped manifold and entering a partial ring structure containing many small cooling tubes. The header tubes of U.S. Pat. No. 4,116,264 do not communicate with any toroidal field coils along the sides of a coil turn nor do the cooling channels bend out of the plane of the coil turns to join the header tubes thereby allowing for applicant's thermal symmetry and reduced bulk material temperature variation. In fact, no toroidal field coil structure is even disclosed in U.S. Pat. No. 4,116,264. Therefore, the cooling channels of U.S. Pat. No. 4,116,264 cannot be an integral part of any toroidal field coil as is a feature of the present invention. In U.S. Pat. No. 4,268,353 to Powell et al, there is disclosed a superconducting toroidal field coil assembly which contains cooling channels. However, those coils being both superconducting and located outside the region of the blanket and shield means are not subject to nuclear heating nor to intense Joule heat as are Applicants' TF coils. Moreover, the enormous size of those coils in comparison to the compact coils of the present invention renders the space required for cooling passages within them less crucial as a design consideration. Because of these looser design constraints, the coolant inlet passages of Powell et al are permitted to traverse the TF coil in a direction parallel to the flat face of the coil. Such a design would be unacceptable in a compact device since it would entail the removal of relatively large amounts of coil material in a local region. This in turn would result in hot spots and high stresses. Burgeson et al in U.S. Pat. No. 4,277,768 disclose coolant means for their TF coil, each channel of which completely traverses the TF coil from the inner contour to the outer contour thus requiring the creation of large voids for coolant flow in the TF coil which, as explained above, would be unacceptable in Applicants' compact device. Such a design is possible in the device of Burgeson et al because of the massive size of the coils, the location of the coils remote from the plasma region, and the use of superconducting TF coils. SUMMARY OF THE INVENTION It is, therefore, an object of the invention to provide a cooling arrangement for magnetic coils. It is an additional object of the invention to provide a cooling arrangement for cooling a magnetic coil assembly consisting of a plurality of magnetic field coils. It is a further object of the invention to provide an arrangement for cooling the toroidal field coil assembly of a tokamak fusion reactor. It is a still further object of the invention to provide a method for cooling magnetic coil turns. It is a still further object of the invention to provide a method of cooling toroidal field coil turns in the toroidal field coil assembly of a tokamak reactor. It is a still further object of the present invention to provide a method and apparatus for distributing coolant to a toroidal field coil that will result in near thermal symmetry and reduced bulk material temperature variation in the circumferential direction around individual coil turns. The above objects and others are accomplished in the present invention by means of the use of header structure positioned on the side face of flat-washer shaped magnetic coil turns. The header structure, positioned on the side face of the coil turn, utilizes the gap or space between adjacent turns and is located outside of the body of material that physically constitutes the coil turn. Preferably, the coolant channels located within the magnetic coil turn bend toward the flat face of the coil turn to intersect the header structure at coolant inlets and coolant outlets. The header structure may be of a semicircular cross-section for pressure containment enhancement and for transitioning into a round tube or manifold that may be used to supply and withdraw coolant from the main coolant source. The header structure may preferably be tapered or otherwise shaped to accommodate itself to space limitations and also to control the flow distribution into and out of the coolant channels. It is apparent that geometries other than semicircular could be used to construct the coolant channel headers. For instance, the channel headers could be rectangular, square or elliptical as well as other cross-sections that would suggest themselves to one of skill in the art. Preferably, the headers are welded or brazed to the coil turn to provide a strong and leak-tight joint. The method and apparatus may use coolant headers on both sides of a coil turn to serve parallel cooling channels that alternate with respect to coolant flow direction. In another aspect of the invention, a cooling arrangement as described above is provided to cool the TF coil turns of a tokamak reactor. Each TF coil turn is provided its own supply and return header and has within it cooling channels beginning at a coolant inlet opening and terminating at a coolant outlet which are respectively in fluid communication with the supply header and the return header. The coolant channels bend towards the flat surface of the TF coil turn to form the inlet and outlet openings thus minimizing the amount of void space due to coolant channels in the TF coil turn. Preferably, the supply header and the return header are configured such as to optimize the coolant flow distribution through the channels and the coolant inlet and outlet openings are positioned adjacent to the joint between consecutive TF coil turns. Also in accordance with the present invention, a method is disclosed of flowing coolant through supply headers into coolant inlet openings in a TF turn, through the TF turn coolant channels, out of the outlet terminals and into a return header. The method in accordance with the present invention positions the supply header and the return header along a flat face of the TF coil turn. In accordance with another aspect of the present invention, substantial variations in the bulk temperature of the coil turn are reduced by providing coolant headers on the sides of coil turns and parallel cooling channels within the coil turns that are served by the headers. The coolant flow in the parallel channels alternates with respect to coolant flow direction. Large temperature variations in toroidal field coils are undesirable inasmuch as those variations cause a greater coil maximum temperature than would a more uniform bulk temperature. In addition, a higher bulk temperature lowers the substantial structural strength of the coil. Moreover, temperature variations around the coil cause distortions and unequal thermally-induced growth in the coil. This growth can result in structural deflections which, if resisted, will cause stresses. Additional objects, advantages and novel features of the invention will be set forth in part 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.