Patent Number: 046541829
Section: summary

The present invention relates to plasma devices and, more specifically, to a rotatable limiter for protecting the confinement vessel of the device from damage by the plasma. BACKGROUND OF THE INVENTION Vacuum liners for defining a plasma in a high energy plasma device have been constructed using relatively thin wall sections made of stainless steel or Inconel. These 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 sections, when used in a high energy plasma environment, may experience local surface melting upon plasma contact. The melting results in structural weakness and vacuum leaks in the liner. In one attempt to prevent melting of the sections, short pieces of stainless steel rod limiters were installed on the inside surface of the sections. By absorbing the plasma energy, the rods protected the sections. However, contact with the plasma caused the plasma to deteriorate upon 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 Fashion 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 sections. In another attempt to protect the sections 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 contact only the tiles. However, the expected reduction of the heat level on the sections 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. Soc., 28, 1097 (1983). Mirror plasma apparatus has been proposed which utilizes shielding by arc discharge to form a blanket plasma and lithium walls to reduce neutron damage to a solid material wall which rotates to keep a liquid lithium layer against it. For further information regarding the structure and operation of this apparatus, reference may be made to U.S Pat. No. 4,260,455. SUMMARY OF THE INVENTION Among the several aspects and features of the present invention may be noted the provision of improved apparatus for containing plasma in a high energy plasma device. Several armature rings are positioned at spaced locations inside a vacuum liner wall, and each ring has conductors which carry current which interact with the magnetic field inside the liner wall, resulting in rotation of the armature rings. Each ring carries armor tiles facing the plasma and serving as plasma limiters. The tiles transfer heat through the ring and the vacuum liner wall. Rotation of the rings results in more even heating of the tiles to prevent damage to the liner wall due to localized heat concentrations. The coefficient of expansion of the material of the armature rings is greater than the coefficient of expansion of the liner wall material so that with heating, the rings expand into firm engagement with the liner wall. This more efficiently permits heat transfer from the armor tiles to a cooling medium disposed outside the liner wall. Continued expansion occurs after the plasma is turned off due to thermal inertia. After cooling, the rings contract into position for rotation in response to the next plasma pulse. Other aspects and features of the present invention will be, in part, apparent and, in part, pointed out specifically in the following specification and accompanying claims and drawings. Briefly, the apparatus of the present invention includes a vacuum tight liner wall which may be made up of a series of sections each of which has a closed peripheral wall defining an interior with open ends. Adjacent interiors of adjacent sections form a plasma path with each section having an inside surface and an outside surface with the interior being generally circular in cross section. A magnet system has helical conductors disposed outside the liner wall for generating a magnetic field extending inside the liner wall. Armature rings are positioned inside the liner wall with each ring carrying rollers for entering into engagement with the inside surface of the wall. The rings also have current carrying armature conductors extending at an angle to lines of force of the magnetic field. The rings carry armor tiles facing the plasma path which function as plasma limiters (a first wall). The interaction of the magnetic field and the current in the armature conductors results in rotation of the armature rings to prevent damage to the liner wall by localized heat concentration.