As was pointed out in the previous section, it is impor­tant to have accurate knowledge of a target’s surface composition to predict its erosion rate. A small impu­rity concentration contained within the incident plasma can drastically alter the surface composition of a target subjected to bombardment by the impure plasma. Oxygen impurities in the plasma, either from ionization of the residual gas, or due to erosion from some other surface, will readily lead to the formation of beryllium oxide on the surface of a beryllium target. Depending on the arrival rate of oxygen to the surface compared to the erosion rate of oxygen off the surface, one can end up measuring the sputtering rate of a clean beryllium surface or a beryllium oxide surface. Careful control of the residual gas pressure in ion beam sput­tering experiments55 has documented this effect. Unfortunately, it is not always so easy to control the impurity content of an incident plasma.

In the case of a magnetic confinement device composed of groups of different plasma-facing mate­rial surfaces, erosion from a surface in one location of the device can result in the transport of impurities to other surfaces throughout the device. Mixed- material surfaces are the result. To first order, a mixed-material surface will affect the sputtering of the original surface material in two ways. The first is rather straightforward, and is true even for materials which do not form chemical bonds, in that the surface concentration of the original material is reduced thereby reducing its sputtering rate. The second effect changing the sputtering from the surface results from changes in the chemical bonding on the surface, which can either increase, or decrease the binding energy of the original material. If the chemical bonds increase the binding energy, the sputtering rate will decrease. If the bonding acts to reduce the surface binding energy, the sputtering rate will increase (assuming the change in surface concentration does not dominate this effect). A recent review of mixed materials62 provides some background information on the fundamental aspects of general mixed-material behavior.

If a plasma incident on a beryllium target contains sufficient condensable, nonrecycling impurities (such as carbon), it will affect the sputtering rate of the beryllium. This effect was first referred to as ‘carbon poisoning.’5,9,63 A simple particle balance model has been used to adequately explain the results for formation of mixed carbon-containing layers on beryllium at low surface temperature.64 However, as the target temperature increases, additional chemical effects, such as carbide formation, have to be included in the model.

An interesting change occurs when the bombard­ing species is a mixture of carbon and oxygen.

Measurements of the chemical composition of a beryllium surface bombarded with a CO+ ion beam showed almost exclusive bonding of the oxygen to the beryllium in the implantation zone.65,66 The formation of BeO on the surface left the carbon atoms easily chemically eroded. The amount of oxy­gen present in the incident particle flux plays a strong role in the final chemical state of the surface atoms and their erosion behavior.

The inverse experiment, beryllium-containing plasma incident on a carbon surface, has also been investigated.67-69 In the case of beryllium impurities in the plasma, a much more accurate measurement of the impurity concentration was possible. Contrary to the carbon in beryllium experiments, a simple particle balance model could not account for the amount of beryllium remaining on the surface after the plasma exposure. Clearly, the inclusion of chemi­cal effects on the surface needs to be taken into account to interpret the results.

Beryllium carbide (Be2C) was observed to form on the surface of carbon samples exposed to beryllium — containing deuterium plasma even during bombard­ment at low surface temperature. Carbide formation will also act to increase the binding of beryllium atoms to the surface and decrease the binding of carbon atoms. This effect will result in an increase in the concentration of beryllium on the surface compared to a simple particle balance equation and must be included to understand the evolution of the surface. In addition, the formation of the carbide was correlated with the decrease of carbon chemical ero — sion70 (see Section 4.19.3.3.1 for more discussion of the chemical erosion of the beryllium-carbon system).