Review of Wall Conditioning and Wall Materials for Fusion Research Devices
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10 20 30 40 50 Z, IMPURITY ATOMIC NUMBER
Figure 1. Maximum plasma concentration of impurities for which ignition can be achieved, based on an expérimental power reactor. '•"
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must be kept low because impurities resuit in radiative losses that at best decrease the efficiency of the plasma "burn" and at worst prevent ignition completely. Radiative losses scale with the effective charge, Z, of the plasma. Radiative losses due to bremsstrahlung are proportional to Z2; those due to recombination are proportional to Z4. Figure 1 shows the impurity concentration above which ignition cannot be achieved as a function of impurity atomic number for an expérimental power reactor.1 The advantage of using a low-Z material as the first wall of a fusion machine (i.e., as the plasma-facing material) is évident. The major plasma impurities observed in most experiments are oxygen, carbon, and first-wall material. Thèse impurities are introduced into the plasma by many mechanisms, including outgassing, desorption, chemical interaction, sputtering, and évaporation. Chemical and physical interactions between the first wall and the energetic hydrogenic ions produced in the plasma are important factors in impurity production. (Hère "hydrogen" refers to hydrogen and its isotopes, deuterium and tritium.) The energy contained in the plasma can be quite large (many megaj ouïes) and can interact with the first wall on a short time-scale (e.g., a millisecond). It is possible that no material will prove suitable for ail opérational scénarios. Therefore, the burden of reducing the plasma/first-wall interaction to an acceptable level rests on both plasma and materials scientists. Requirements for materials used for
vacuum Systems and first walls hâve changed significantly over the last décade. In the early 1980s, nonmagnetic stainless steel served as both the vacuumatmosphere interface and the first wall in most experiments. It was soon recognized that the requirements for the first wall were incompatible with those of the vacuum-atmosphere interface, so liners and coatings of various materials were implemented. This review describes techniques for cleaning and conditioning the materials used for liners, coatings, and the vacuum-atmosphere interface. The materials used for liners and coatings are discussed, and their performance as plasma-facing components is described. Finally, future directions are presented. Cleaning and Conditioning Techniques for cleaning and conditioning materials in fusion devices hâve evolved as the demands on materials hâve changed and as various techniques hâve become feasible. Most vacuum vessels for fusion devices hâve been constructed of nonmagnetic stainless steel and Inconel. Liners hâve been made of stainless steel, carbon, graphite, molybdenum, béryllium, and other materials, while most coatings hâve been made of carbon and carbon compounds, such as silicon carbide (SiC) and titanium carbide (TiC). Other materials are required by many of the plasma diagnostics (e.g., laser-based électron température and dens
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