A STUDY OF SELF-SUSTAINING THIN-FILMS AS A MEANS OF FUSION PLASMA IMPURITY AND WALL EROSION CONTROL (PLASMA SHEATH, DEFECT MIGRATION, SPUTTERING, COLLISION CASCADE, COPPER-LITHIUM & ALLOYS)

A simulation of self-sustaining (low-Z) "thin-films" as a means of fusion plasma impurity and wall erosion control has been performed through the development and/or extension of potential sheath, sputtering mechanics, and metal-surface kinetic models. Angular impact behaviour determined from the potential sheath model as a function of plasma-edge conditions provides the parameterization necessary for calculating thin-film alloy sputtering yields. The sputtering yields and damage profile behaviour resulting from the sputtering mechanics model for heterogeneous alloys provide the athermal driving force characteristics necessary for modeling the metal-surface kinetics of thin-film systems. The coupling of athermal and thermal phenomena establishes the framework for investigating the ability of thin-film systems to sustain themselves in an irradiation environment.

The application of the sheath, sputtering, and metal-surface kinetic models assumes a "worst case" scenario of a potential field in the presence of a grazing magnetic angle with the fusion conditions of edge densities greater than O(10 m) and edge termperatures less than 0(10 eV). Potential sheath calculations predict that low-Z ions impact at angles closely coinciding with the magnetic angle, while high-Z ions may be assumed to impact normally for magnetic angles of 80 degrees or less. If the low-Z secondary-ion fraction exceeds 50%, low-Z self-sustaining thin-film systems are potentially advantageous in comparison to elemental surfaces due to reduced sputter erosion. For one such system, a Cu-Li alloy, the kinetic modeling is predictive of a self-sustaining Li thin-film, if the Li secondary-ion fraction approaches 90%. Comparison of the kinetic modeling to experiment for the Cu-Li alloy suggests that preferential sputtering is not the sole determinant of the equilbrium surface composition for mass-disparitive alloying elements.