Thermal release of D2 from new Be-D co-deposits on previously baked co-deposits

Past experiments and modeling with the TMAP code in [1, 2] indicated that Be- D co- deposited layers are less (time- wise) efficiently desorbed of retained D in a fixed low- temperature bake, as the layer grows in thickness. In ITER, beryllium rich co- deposited layers will grow in thickness over the life of the machine. Although, compared with the analyses in [1, 2], ITER presents a slightly different bake efficiency problem because of instances of prior tritium recover/control baking. More relevant to ITER, is the thermal release from a new and saturated co- deposit layer in contact with a thickness of previously- baked, less- saturated, co- deposit. Experiments that examine the desorption of saturated co- deposited over- layers in contact with previously baked under- layers are reported and comparison is made to layers of the same combined thickness. Deposition temperatures of similar to 323 K and similar to 373 K are explored. It is found that an instance of prior bake leads to a subtle effect on the under- layer. The effect causes the thermal desorption of the new saturated over- layer to deviate from the prediction of the validated TMAP model in [2]. Instead of the D thermal release reflecting the combined thickness and levels of D saturation in the over and under layer, experiment differs in that, i) the desorption is a fractional superposition of desorption from the saturated over- layer, with ii) that of the combined over and under - layer thickness. The result is not easily modeled by TMAP without the incorporation of a thin BeO inter- layer which is confirmed experimentally on baked Be- D co- deposits using X- ray micro- analysis. (C) 2015 Elsevier B.V. All rights reserved.