Self-Sustaining Coatings for Fusion Applications - Copper Lithium Alloys

Dilute binary alloys have been discussed previously as a means of producing self-sustaining coatings for fusion applications. The anticipated advantages of such coatings are described in a companion paper.¹¹ Issues addressed in this paper concern experimental observation of the formation of a low-Z coating by solute segregation in a Cu-Li alloy, maintenance of the coating in a sputtering environment, and a comparison of the calculated net erosion for W, Mo, and Cu-Li when used as either the divertor plate or the bottom limiter for INTOR. Auger electron spectroscopy has been used to monitor the surface composition of an alloy consisting of 3.0 at.% Li in Cu while sputtering with 1–3 keV Ar⁺ or He⁺ at a flux of 10¹² – 10¹⁴ cm⁻² sec⁻¹ (corresponding to a gross erosion rate of several mm/yr) at temperatures up to 430°C. It is found that the alloy is capable of reproducibly maintaining a complete lithium overlayer. The time-dependent thickness of the overlayer depends strongly on the mass and energy spectrum of the incident particle flux. It has been experimentally demonstrated that a significant fraction of the sputtered lithium is in the form Li⁺ and is returned to the surface by an electric field such as the sheath potential at the limiter, or a tangential magnetic field such as the toroidal field at the first wall; consequently, the overlayer lifetime is essentially unlimited. The TRIM computer code has been used to calculate the sputtering yield for pure metals and the partial sputtering yields of binary alloy components for various assumed solute concentration profiles. It is found that even with very low-Z coatings, the majority of the sputtered atoms originate in the uppermost atomic layer and that the partial sputtering yield of an alloy component is significantly reduced if that component is excluded from the uppermost atomic layer. It is predicted that the self-sputtering behavior of Cu-Li when used as a limiter or divertor plate will compare very favorably with that of tungsten. Calculations using the REDEP code bear out this expectation. At low plasma edge temperatures (< 50 eV), the net erosion (erosion minus redeposition) due to D,T,He and self-sputtering is nearly zero, while the gross erosion is less than that of Mo, For edge temperatures > 50 eV, W, and Mo are unusable due to self-sputtering. It is calculated that in the intermediate edge temperature regime (50–200 eV), a limiter made of copper with a lithium coating 1.5 monolayers thick would show net growth or erosion of < 3 mm per year. Consequently, Cu-Li alloy may be the only material suitable for use with intermediate plasma edge temperatures.