Deuterium Retention in Mixed Layers with Application in Fusion Technology

Abstract

Formation of Be-W mixed layers in thermonuclear fusion reactors can potentially alter the retention of hydrogen isotopes and affect the retention and release properties of these isotopes. This paper reports on the retention and release characteristics of D from reference Be, W layers as well as three Be-W mixed layers with well-defined atomic concentrations (2:1, 1:1, 1:2). The layers resulted from the sputtering of Be and W materials in Ar:D (1:1) mixture at 2 Pa using DC magnetron sputtering. The mixed layers’ deposition parameters were varied to adjust accordingly the deposition rate for each material in order to obtain the desired concentrations. Scanning electron microscope images showed that morphology is independent of composition for samples deposited on silicon substrates. In contrast, layers deposited on tungsten revealed a textured surface and morphological changes with W concentration variation. X-ray diffraction patterns of mixed layers evidenced the presence of a polycrystalline tungsten phase. Additionally, the degree of crystallinity is highly influenced by the plasma parameters and enhanced amorphization is evidenced by a decrease of crystalline size by a factor of 10 for mixed layers compared to the W reference layer. The release behavior of D from the layers is affected by the trapping contribution of both Be and W. Compared with implanted layers, presented in literature studies, the co-deposited layers show a high D occupancy of low energy trapping states, the majority of the D retained in the samples being released at temperatures below 623 K. High energy trapping becomes more pronounced for layers with a high Be concentration. The oxygen contamination observed for Be layers points to a mitigation of D retention in low energy trapping states and shifts the desorption chart towards a higher temperature due to enhanced retention in BeO associated traps. The D retention presents a linear decrease of W concentration in the sample.