All fusion reactors, whether tandem mirror, tokamak, or alternate concept, will be surrounded by blankets, whose principal function is to extract useful energy from the reactor. The "first wall" is the most interior surface of the blanket, and is that part of the blanket actually facing the fusion plasma. The first wall degrades due to the hostile environment, hence the blanket requires frequent service. For the well known tandem mirror reactor, the main chamber ("central cell") for housing the fusion reaction is comprised of a series of independent annular blanket modules which form a cylinder. To maintain the vacuum inside the central cell, an effective seal is necessary for mating the blanket modules in the desired series array.
First wall and blanket structure service and replacement require the reactor to be shut down. In view of the huge capital costs, down time is exceedingly expensive. To facilitate service, the fusion blanket is divided into modular annular-shaped segments for easy handling. The division plane(s) between modules are potential leakage paths for gas to enter and quench the reacting plasma housed inside the blanket. The many proposed blanket concepts address this blanket seal problem in different ways.
In one proposal, designers prefer remotely welded joints which must then be cut for disassembly by remote operator tools. High radiation environment due to structure activation makes remote operations a necessity, even at the outside of the shield which protects the magnets. The "shield" is the outer most region of the blanket, and is placed before the superconducting solenoid magnets to minimize irradiation of the magnets.
A second proposal suggests bolted joints with metal seals. A suitable arrangement according to this approach would include annular flanges mounted on the outside of the ends of each individual cell. Holes would be drilled through the flanges in a direction parallel to the centerline of the cell. The holes would be positioned opposite one another, thus permitting the insertion of bolts, to which would be attached nuts on the other end of the bolt. Before inserting the bolts, a malleable annular metal seal would be placed between the cell-to-cell interface surface (i.e., the division plane between each cell), selected so the metal would deform into the cell face as the bolts were tightened, thereby forming a bolted joint providing the desired seal. The disadvantage with this approach is that a new mealleable metal seal would have to fabricated each time the seal was broken by removal of the bolts.
A third approach is to use metal seals of the knife-edge type or metal "o"-rings or other similar devices which are forced closed against each other. More than one seal can be employed with differential pumping between the seals. However, the most serious disadvantage of this design is the problem of remaking a seal after one use. Overlapping seal indentations are seldom successful in forming the vacuum seal desired. Therefore, with each servicing of a modular segment of the blanket, a new seal would have to be fabricated.
The pressure in pounds per square inch (psi) is often quite high at the contact inferface of the seating apparatus or pressure cushion surfaces joined by bolts, knife-edge seals or metal o-rings. In order to obtain the sealing quality required for the "hard" vacuum (on the order of 10.sup.-6 torr) of the tandem mirror magnetic fusion reactor, the compression pressure on the contact interface surfaces for a removable reusable seal will preferably not exceed the elastic deformation limit of the seal material, because plastic deformation might destroy the ability to use the seal again. Bolts, knife-edge seals and metal o-rings work on the principle that the surfaces actually forming the seal have a very high psi compression pressure at the contact interface.
For example, the knife-edge seal typically consists of a "soft" metal such as copper, against which is pressed a "hard" metal such as stainless steel formed to have a knife-edge shape at the point of contact between the soft and hard metal. The hard knife-edge cuts into the soft metal to form the seal. The seal is formed at the small cross-sectional area where the knife-edge and "soft" metal are in contact. The result is that molecules outside the vacuum have an undesirably short distance across which to travel to reach the vacuum (i.e., the seal width, which in the case of an annular seal would be the seal's outside diameter minus the inside diameter).
Problems and shortcomings exist with current technologies directed to forming seals, as mentioned above. The knife-edge type or metal "o"-ring type can be used only once before the seal has to be replaced. The large diameter bellows have the pressurization and squirming problems. Therefore, an improved apparatus and method must be devised in order to obtain the repeatability of use and high quality vacuum seal required in high vacuum environments such as that found in a fusion reactor.