Temperature and composition dependence of the high flux plasma sputtering yield of Cu-Li binary alloys

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Because of its very high thermal conductivity, actively cooled copper is an attractive plasma-interactive material for long pulse fusion devices such as ETR and devices with very high wall power loadings, such as reversed-field pinched (RFPs) and the proposed compact ignition torus (CIT). Pure copper however, has an unacceptably low threshold energy for runaway self-sputtering. Low Z materials such as graphite and beryllium are not subject to runaway self-sputtering, but suffer from high light ion erosion rates and very nonuniform redeposition. It has been suggested that strongly segregating alloys such as Cu-Li might be used to provide a low- Z self-sustaining coating while maintaining the desirable redeposition, thermal and mechanical properties of the majority alloy component. High flux deuterium plasma sputtering and ion beam experiments have been performed on Cu-Li alloys to determine if the reduction in copper erosion previously predicted and observed in low flux ion beam experiments occurs at particle fluxes representative of an RFP first wall or tokamak limiter. Partial sputtering yields of the copper and lithium components have been measured as a function of alloy composition and sample temperature using optical plasma emission spectroscopy, weight loss and catcher foil techniques. It is found that the lithium sputtering yield increases with increasing sample temperature while the copper yield decreases by as much as two orders of magnitude. The temperature required to obtain the reduction in copper erosion is found to be a function of bulk lithium concentration. Consequences of these experimental results for anticipated erosion/redeposition properties are calculated, and the Cu-Li alloy is found to compare favorably with conventional low- Z materials.