A recent review of mixed-material effects in ITER62 provides background information on mixed-material formation mechanisms and plasma-surface interac­tion effects. Here, the focus is on beryllium — containing mixed-material surfaces (i. e., Be/C and Be/W) and the conditions when one might expect these surfaces to dominate the observed plasma — surface interactions. In addition to plasma interac­tions with mixed-material surfaces, which will be discussed here, other aspects such as changes to ther­mal conductivity, material strength, and ductility, the impact of impurities on material joints, etc., must also be carefully evaluated.

4.19.3.3.1 Be-C phenomena

Beryllium and carbon have been observed to begin thermally interdiffusing at a temperature of around 500 °C,56 resulting in the formation of a beryllium carbide layer. However, beryllium carbide has also been observed to form during energetic carbon ion bombardment of beryllium surfaces at room temper­ature.110 As mentioned in Section 4.19.3.1.2, the change in the binding energy ofthe carbide molecule affects the sputtering yield of both the beryllium and carbon atoms. In addition, the formation of beryllium carbide also has a dramatic effect on the chemical erosion properties of a carbon surface bombarded with energetic beryllium ions.67,68,111

The presence ofberyllium carbide on the surface of a carbon sample exposed to deuterium plasma has been shown to correlate with the reduction of chemical erosion of the carbon surface.70 The speculation for the cause of this effect is that the carbide enhances the recombination of deuterium in the surface, thereby lessening the amount of deuterium available to interact with carbon atoms on the surface. This is similar to the impact of small amounts of boron carbide in a graphite surface affecting chemical erosion.112 However, the difference here is that instead of obtaining the carbide through an expensive production technique, the car­bide forms naturally as beryllium ions in the plasma interact with the carbon surface.

A systematic study of the time necessary to sup­press chemical erosion of a graphite surface due to the interaction with beryllium-containing plasma has been carried out.69 Increasing the surface tempera­ture of the graphite was seen to have the biggest impact on reducing the suppression time. Increasing the beryllium content of the plasma also reduced the suppression time in a nonlinear fashion. An increase of the incident particle energy was observed to increase the time necessary to suppress the chemical erosion of the surface, presumably due to an increase in the removal of the carbide-containing surface layer. A subsequent study showed that applying heat pulses to a graphite surface interacting with beryllium-containing plasma, to simulate surface heating due to intermittent events, acted to reduce the time necessary for the carbide surface to form and suppress the chemical erosion of the surface.1