As deuterium retention in plasma-exposed beryllium targets saturates after a given ion fluence (see Section 4.19.3.2.1), it is apparent that retention in codeposits will eventually be the dominant accumulation mech­anism with respect to beryllium PFCs. This is pri­marily due to the fact that the thickness of a codeposit will continue to grow linearly with time. It is, therefore, critical to understand both the

retention amounts and the release behavior of hydro­gen isotopes from beryllium codeposits. In this sec­tion, a ‘codeposit’ includes both the codeposition (where a BeD or BeD2 molecule is deposited on a surface) and co-implantation (where deposited layers of beryllium are bombarded with energetic hydrogen isotopes) processes.

Initial interpretation of studies of beryllium code­posits were made difficult by relatively high oxygen impurity content within the codepositing surface.102,103 Subsequent measurements104 with lower oxygen con­tent seemed to indicate that the oxygen level within the codeposit was correlated to the level of hydrogen isotope retention in the codeposit. The other variable that was identified to impact the retention level in these studies was the temperature ofthe codepositing surface.

Measurements seriously questioning the impor­tance of oxygen on the retention level in beryllium codeposits were made by Baldwin eta/.105 In this data set, the oxygen content throughout the codeposit was measured by depth profiled X-ray photoelectron spectroscopy and the oxygen content did not corre­late with the deuterium retention level (Figure 8), although the temperature of the codepositing surface was still a dominating term in determining the deu­terium retention level. Later, more detailed measure­ments confirmed that the presence of a beryllium
oxide surface layer was not correlated with an increase in retention in beryllium.106

A systematic study of beryllium codeposition fol­lowed,107 identifying three experimental parameters that seemed to impact the retention level in a code­posit. Along with the surface temperature, the inci­dent deuterium energy and the beryllium deposition rate were determined to be influential scaling para­meters. The previously reported data in the literature was also evaluated using the derived scaling and found to agree with the predictions of the retention levels measured under the various experimental conditions present in the different machines. Later the derived scaling was revised108 to use the ratio of the fluxes of the codepositing species, rather than the deposition rate to permit more accurate extrapo­lation to conditions expected in the edge of confine­ment devices.

The ability to predict the level oftritium retention in beryllium codeposits is an important aspect of a safety program; however, developing techniques to remove the trapped tritium from codeposits is a more important issue. The deuterium release behavior during thermal heating of beryllium code­posits has been investigated.1 9 The results show that the maximum temperature achieved during a bake — out is the figure of merit for determining the amount of deuterium release from beryllium. Increasing

the time spent at lower baking temperatures did not increase the amount of deuterium released from the beryllium codeposits. These results, along with the retention level predictions, should make it possi­ble to design baking systems for different areas of a confinement device to control the accumulation rate of tritium to a desired level.