Patent Number: 046631089
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, a vacuum liner for a high energy plasma device 20 (FIG. 1), is best shown in FIG. 2 at reference numeral 22. While the liner 22 is exemplarily shown as a torus, it will be appreciated that it can be formed into any desired shape. The device includes a frame 24 for supporting the liner and a magnet system 26 which could be made up of toroidal, poloidal, helical and other coils or windings which may be either of superconductive or normal material. The purposes of the magnetic system are to produce and confine the plasma inside the vacuum liner 22. The liner 22 includes a vacuum tight liner wall 28 made up of a number of bellows sections 30 having corrugations forming interior ridges 32 and grooves 34 (see FIG. 3), and a number of port or gore sections 36 having generally smooth interiors. Each section has a closed peripheral wall defining an interior 38 with open ends. Adjacent interiors of adjacent sections form a plasma path 40. The corrugations of the bellows sections extend transversely to the axis of the plasma path. The bellows sections are preferably formed of stainless steel and have a relatively thin wall thickness, e.g., 20 mils. They have sufficient loop resistance that the penetration times for the magnetic fields provided by the magnet system, are acceptably short. The port sections, also made of stainless steel, are preferably somewhat thicker and have ports 42 and associated piping 44 for passage of the constituents of the plasma and for applying a vacuum. The various sections carry mating collars 46 at their ends so that the sections can be welded together to complete the vacuum tight wall 28. Theoretically, a properly designed magnet system would provide sufficiently homogeneous magnetic fields that the plasma is contained in the liner out of contact with the liner inner surface. However, available magnet systems do not provide such ideal fields and the plasma contacts the liner inner surface. When the plasma contacts the inner surface of a bellows, energetic particles from the plasma impinge on the bellows wall resulting in localized heating and causing melting and loss of vacuum integrity. Additionally metal ions from the sections enter and contaminate the plasma. These metal ions might have a charge of 10, whereas the electrons and hydrogen ions typically found in the plasma have a charge of 1. The introduction of the metal ions into the plasma causes increased radiation resulting in power loss in the plasma. In order to protect the bellows sections from the plasma, the vacuum liner of the present invention includes means for keeping the plasma from contacting the liner wall 28. This limiting means comprises a ring 48 made up of beads 50 nested in the interior groove 34 formed by each corrugation, as best shown in FIG. 3. The beads 50, which could be shaped as small right circular cylinders, each have an aperture 52 therethrough. A fastening means in the form of a wire 54 made of a material having resistance to high temperature and appropriate spring characteristics, such as a nickel-chromium-iron alloy, is laced through the beads, as shown in FIG. 4. The ends 56 of the wire are overlapped inside beads of the ring. The beads are formed of a material having a higher melting temperature than the material from which the liner wall 28 is formed. The beads are preferably formed of silicon carbide coated carbon, ceramic material or a nickel-chromium-iron alloy, with high density carbon being most preferable. The liner wall 28, with all its bellows sections 30, can include over 400 corrugations with each ring including 100 or more beads. Thus the installation of the rings must be simple to keep the cost low. The as-formed length of each ring is preferably slightly greater than the circumference of the groove portion to be filled. The ring can be fitted into the groove with an inwardly protruding remaining bulge of a few beads. By pressing the bulge outwardly into the groove, all the beads in the ring slightly compress and the bellows slightly stretches, allowing the bulged portion to snap into the groove. By the "keystone effect" the ring is firmly held in its groove. Each bead is in compression and the material forming the groove prevents expansion of the loop while the flanking ridges preclude lateral movement of the ring. Thus the ring is installed simply and quickly with strong mechanical retention, all without any need to weld or otherwise fasten the beads to the bellows section. As shown in FIG. 3, the beads 50 extend from their respective grooves 34 past the apices of the interior ridges toward the plasma path. Although the rings of beads do not overlie the inner ridges of the bellows sections, under many operating conditions they substantially protect the ridges from impingement by high energy charged particles which leave the plasma and give up their energy to the first surface they strike. This is because these particles predominantly follow the direction of the composite magnetic field. Typically the radial component of the composite magnetic field is much smaller than the toroidal component. Thus the paths taken by most of the escaping charged particles intersect the bellows section wall at shallow angles. These angles are so shallow that most charged particles strike the portions of the beads disposed above the level of the ridges and not the ridges. Put another way, although the ridges are visible in plan, they are in the shadow of the beads in view of the direction of the plasma. Besides the right circular cylindrical bead 50 of FIG. 6A, the beads could be made into other shapes. A T-shaped bead 50A includes a stem 58 for reception in the groove and oppositely extending arms 60 which at least partially overlie the ridges 32 flanking the groove 34 in which is disposed the stem 58. A bead 50B which is elliptical in cross section is shown in FIG. 6C. Because this bead configuration extends further toward the plasma path, it casts a longer shadow with respect to the escape trajectories of the charged particles. Thus, rings formed using the beads 50B could, under certain circumstances, be placed in every other groove with the remaining grooves left empty. The elliptical beads would then cast a shadow over all the ridges. Referring to FIG. 5 a bead configuration is shown which protects portions of the wire extending between the beads. Each bead 50C has a tail portion 62 with a central recess 64, and a pointed nose portion 66 sized for reception in the recess 64 of the next bead. Accordingly wire portions between the beads are shadowed by components of the tail portions 62. Referring to FIG. 7, bead configurations are shown which substantially fully cover the ridges with respect to the plasma. A ring formed of a first type of bead 50D is disposed in every other groove. The beads 50D are in the form of split or one half cylinders and are disposed so that the flat side of the bead faces the plasma path. The remaining grooves are fitted with rings formed of modified T-shaped beads 50E, each having a stem 58E and oppositely extending arms 60E of sufficient length and elevation to overlie the ridges flanking their groove and to overlap the beads 50D in adjacent grooves. Although melting and consequential vacuum loss is usually not a problem with the thicker port sections 36, introduction of metal ions therefrom into the plasma can be a problem. Thus it is preferable to cover also the interiors of the port sections with a limiter ring 66 shown in FIG. 8. The limiter ring for the port sections includes a plurality of T-shaped tiles 68 with each tile having a pair of spaced apertures 70, one adjacent each side, for receiving wires to form the tiles 68 into the ring 66. Spaced annular end stops 72 are welded to the interior of the port sections to locate the ring 66. The limiter ring is installed in a manner similar to the installation of the ring 48 described above. Note that two wires 74 are used to hold the limiter ring. These wires are positioned to flank any ports 42 formed in the port section. Thus the wires are positioned to hold remaining tile portions even through a central portion of a tile is removed to accommodate the port. FIGS. 9A-9D illustrate tile configurations 68A, 68B, 68C and 68D, respectively, for formation into rings to be fitted between various sections. These tiles include various arms and removed portions for overhanging the end ridge of a bellows section or for providing space to accommodate a welding bead. Through the use of bellows section rings 48, limiter rings 66 and other rings formed by tiles 68A-68D, substantially the entire inner surface of the liner wall 28 can be covered with a plasma limiter. The carbon limiter material is in intimate contact with the liner wall to preclude arcing. However the carbon limiter does not appreciably reduce the loop resistance of the vacuum liner 22. Even if adjacent rings touch, the carbon joint formed is of high resistance. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.