Patent Number: 044141769
Section: summary

BACKGROUND OF THE INVENTION This invention relates to improved metal surfaces for the first wall and limiter in a plasma device and more particularly to metal surfaces with reduced loss of metal by erosion and particularly by sputtering during exposure to the plasma. In a second aspect of the invention, it relates to a metallic substrate containing an amount of the surface metal and providing a self-sustaining source of the surface metal. In a plasma device, plasma at an elevated temperature is magnetically confined within the first wall whose purpose is to limit the travel of particles escaping from the plasma. The limiter also serves to locate the plasma. During exposure to the hot plasma including bombardment by particles escaping from the plasma, surfaces of the first wall and limiter are subject to loss of material by erosion and particularly by sputtering. In the past, these surfaces have been constructed of metals such as stainless steel. Refractory materials such as ceramic and graphite coatings could also be useful. With ceramic materials, the extreme temperature changes may cause cracks to form and spallation to occur and therefore limit the effectiveness of the surface. Also, ceramic materials have limited value for removal of heat from the first wall and limiter and must therefore be applied as thin coatings over a metallic substrate. The failure of such coatings and clad layers could represent a serious materials problem for the development of plasma devices. With metal surfaces, the metal may be selected for its structural strength, resistance to corrosion, and ease of fabrication. However, metallic surfaces limit the effectiveness of the plasma device because they generate substantial amounts of sputtered neutral atoms which are not retained on the surface and contaminate the plasma. In addition, some metals selected for stuctural properties have a high atomic number and the presence of their atoms in the plasma reduces the energy available for the thermonuclear reactions. It has been suggested that the alkali and alkaline earth metals could be useful as components of the first wall and limiter. Since these metals are electropositive, they are potentially capable of providing a high secondary ion yield. However, these metals in the pure state do not possess the desired structural, fabricating, physical and chemical properties. In addition, a significant portion of the metal spattered from pure alkali metal surfaces in practice escapes as neutral atoms and not as secondary ions. As metal is lost from the surface, the useful life for operating of the plasma device is reduced because a new first wall or limiter or new surfaces for these metallic members must be installed. Accordingly, improvements in the first wall and limiter of the plasma devices are desired. One object of the invention is a first wall or limiter for a plasma device with a surface exhibiting reduced loss of metal by erosion and particularly by sputtering. A second object of the invention is a first wall or limiter for a plasma device with a surface exhibiting an improved secondary ion/neutral ratio. Another object of the invention is a first wall or limiter with means for replenishing metal on the surface exposed to plasma in a self-sustaining manner. Yet another object is a first wall or limiter with a surface coating of a low atomic number. An additional object of the invention is a first wall or limiter with reduced loss of structural metal. These and other objects will become apparent from the detailed description below. SUMMARY By the invention, a metallic member exposed to plasma in a plasma device is provided with a surface having reduced loss of metal by erosion and particularly by sputtering. Further, the member is constructed primarily of a metal having desired structural and fabrication features together with a surface having an improved secondary ion/neutral ratio. In addition, a substrate is provided with desired structural and fabrication properties together with properties which provide a source of the surface metal as it is depleted or lost from the surface. In the plasma device, the metallic member comprises a bulk portion or a metallic substrate composed of a first metal, and a thin surface layer composed of at least a major amount and advantageously a predominant amount in the order of at least 90 at.% of a second metal selected from the group consisting of alkali and alkaline earth metals and exhibiting a vapor pressure below the vapor pressure of the alkali or alkaline earth metal in its bulk form. Preferably, the surface layer consists essentially of the second metal with the surface layer being a monolayer and the atoms of the second metal in the surface being primarily bonded to atoms of the first metal. It is also preferred that the substrate include a portion of the second metal to serve as a source for replenishing metal on the surface lost during operation of the plasma device. The invention further includes a substrate containing a mixture of first and second metals, including an alloy or crystalline compound, with further characteristics which enhance its effectiveness as a source of the second metal. In one process or method whereby the substrate serves as a self-sustaining source of surface metal for maintaining the surface, the substrate is composed of first and second metals in an alloy with the metals being selected to satisfy the equation EQU H.sub.1,2 =.OMEGA.+1/2(H.sub.1,1 +H.sub.2,2) where .OMEGA..ltoreq.0 and H represents the enthalpy of sublimation for the alloy and the pure first and second metals. With the alloy having these characteristics, a structural member, such as the first wall, composed of the alloy is heated to an elevated temperature or subjected to another energy source such as particle radiation sufficient to cause atoms of the second metal to become segregated at the surface and to form a high concentration of the second metal in a monolayer at the surface with a sharp decrease in concentration in the next layer below the surface. Beyond the second layer, the concentration returns to the bulk concentration. As the surface metal is lost during operation of the plasma device, the segregation effect transfers atoms of the second metal to the surface layer to maintain the surface. When the composition of metals in the mixture forms an intermetallic compound, segregation of the second metal becomes limited. However, a second process or method becomes effective to maintain the surface during operation of the plasma device. Initially, the surface of the structural member will have atoms of both the first and second metals. During bombardment by particles from the plasma, atoms of both metals escape from the surface, with those of the first metal being primarily neutrals. Since those escaping atoms of the second metal which have ionic bonds leave as secondary ions, they are returned to the surface by the electrical and magnetic fields of the plasma device. Further, since the surface binding energy between atoms of the first and second metals is higher than the energy for like atoms of the second metal, returning atoms of the second metal will seek vacant sites on the surface and form a monolayer which in some instances need not be atomically smooth.