Patent Number: 046631089
Section: summary

The present invention relates to a vacuum liner for a plasma device and, more specifically, to such a liner which includes limiters for limiting the plasma from contacting a vacuum tight liner wall formed, in part, by bellows sections. BACKGROUND OF THE INVENTION Vacuum liners for defining a plasma path have been constructed using thin wall bellows sections made of stainless steel. These bellows sections offer high strength while forming a closed loop having sufficiently high electrical resistance that penetration times are acceptably low for magnetic fields generated by poloidal, toroidal or other associated magnetic systems for containing and energizing the plasma. Unfortunately, the thin bellows sections, when used in a high energy plasma environment may experience local surface melting due to plasma contact. The melting results in structural weakness and vacuum leaks in the liner. In one attempt to prevent melting of the bellows sections, short pieces of stainless steel rod limiters were installed on the inside surface of the sections. By absorbing the plasma energy, the rods protect the bellows. However, contact with the plasma causes erosion of the metal rods, resulting in contamination of the plasma due to the introduction of metal impurities which radiate and cause loss of power in the plasma. For further information regarding the structure and operation of such limiters, reference may be made to "Experimental and Computational Studies of Reversed-Field Pinch on TPE-IR(M)", by Ogawa et al. in "Proceedings of the 9th International Conference on Plasma Physics and Controlled Nuclear Fusion Research", Baltimore, 1982 (IAEA, Vienna, 1983), Vol. I, p. 575. Metallic limiters are also shown in U.S. Pat. No. 4,073,680. In order to avoid the introduction of metal impurities into the plasma, carbon tiles have been used instead of the metal rods. However, the discrete tiles are not readily reliably fastened to the interior of the bellows. In another attempt to protect the bellows and avoid contamination of the plasma, rings of carbon tiles were placed at spaced locations in the vacuum chamber in the effort to make the plasma only contact the tiles. However, the expected reduction of the heat level on the bellows was not found, and arcing occurred between the liner and the tiles which resulted in damage to the liner. For further information regarding such limiters, reference may be-made to "Mushroom Limiter Studies in ZT-40M", Downing et al., Bull. Am. Phy. Soc., 27, 1108 (1982); and "Temperature and Resistivity of the ZT-40M RFP with Poloidal Limiters", Haberstich et al., Bull. Am. Phy., 28, 1097. SUMMARY OF THE INVENTION Among the several aspects of the present invention may be noted the provision of an improved vacuum liner for a high energy plasma device. The liner is formed by a wall made up of a number of bellows sections, and limiters are provided to protect these sections from contact with the plasma. Limiter rings made of carbon beads, which may be right circular cylinders, are positioned in each interior groove of the sections, and the beads in each ring push against one another so that the entire ring is in compression. Because the flanking internal ridges formed by the corrugations of the bellows sections prevent lateral movement of the beads and the material defining the groove precludes substantial expansion of the loop formed by the ring, the ring is firmly retained in its groove. Certain beads may have an insert portion for reception in the groove and an arm extending over a ridge and overlapping a neighboring bead to further protect the ridge. Additionally, the beads, which have apertures, can be formed into the rings by lacing a wire through the apertures with the end portions of the wire being overlapped inside the beads, thereby simplifying installation of the rings. Other objects and features of the present invention will be, in part, apparent and, in part, pointed out hereinafter in the following specification and in the accompanying claims and drawings. Briefly, the vacuum liner of the present invention includes a vacuum tight liner wall formed by a series of sections with each section having an open ended wall closed peripherally to define an interior. Adjacent interiors of adjacent sections form a plasma path. Some of the sections are bellows-shaped, having a plurality of corrugations extending transversely to the axis of the plasma path. The liner also includes limiter rings formed by beads nested in the interior groove of each of the corrugations, with at least some of the beads extending from their grooves past the interior ridges flanking them. The material from which the beads are formed has a higher melting temperature than that of the material from which the sections are formed.