The existence of tungsten beryllide alloys (i. e., Be2W, Be12W, and Be22W) is an excellent example of the importance of mixed-material surface formation in plasma-facing components.114 Figure 9 shows the tungsten-beryllium phase diagram. Each of the ber — yllides shown in the figure exhibits a lower melting temperature than one would expect from a tungsten plasma-facing surface. If plasma containing beryllium impurities interacts with a tungsten surface, there is a possibility of these lower melting temperature beryllide alloys being formed.

In thermodynamic equilibrium, various beryllide alloys of tungsten have been observed to form,115 and their reaction rates have been measured,116 at tempera­tures in excess of 800 °C. However, as was seen with beryllium carbide forming during plasma bombard­ment at lower temperature than expected thermody­namically, the concern exists that tungsten beryllide could form at temperatures below 800 °C as well.

Well controlled laboratory measurements in vac — uum117 and in plasma simulators118 have shown that although thin, nanometer scale, Be2W layers form at the interface between beryllium and tungsten surfaces, their growth below 800 °C is negligible. In addition, above 800 °C, rapid beryllium sublimation from surfaces can act to limit the amount of beryl­lium available for reacting with tungsten and thereby also limit the growth rate of the alloys. In the present low wall temperature confinement devices, modeling shows that the divertor strike point locations are the only areas where significant beryllide growth might be expected and in these regions there does not

appear to be enough beryllium deposition to raise significant concerns.11 One caveat to these predic­tions would be the existence of intermittent events that raise the temperature of surfaces where signifi­cant beryllium deposits are located, thereby possibly allowing the optimized beryllide growth conditions.

Another concern with regard to thin Be2W surface layers on plasma-exposed tungsten is the impact of these layers on tritium retention. While a thin Be2W surface layer is not likely to retain much tri­tium itself, the thin beryllide surface layer could alter the recombination characteristics ofthe bulk material and change the accumulation rate of tritium within the device. To date, there is little or no data available to address this issue.

While it appears likely that the most serious issues of tungsten beryllide formation may be avoided in present confinement devices, the issues associated with these alloys highlight the uncertainties and impor­tance of understanding and predicting mixed-material
formation in plasma environments. Mixed materials often interact with plasma in much different ways than their elemental components. In the case of the beryllium-carbon system (Section 4.19.3.3.1), the mixed material appears to offer the potential for bene­ficial effects, whereas in the case of the beryllium- tungsten system, the mixed material appears likely to be detrimental to the operation of the device. Each mixed-material system must, therefore, be individually evaluated to determine its potential impact on all aspects of operating surfaces in contact with plasma.