2015_Конференция "Взаимодействие плазмы с поверхностью (XVIII; 5-6 февраля 2015 г. ; Москва)

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Просмотр 2015_Конференция "Взаимодействие плазмы с поверхностью (XVIII; 5-6 февраля 2015 г. ; Москва) по Автор "KRASHENINNIKOV, S. I."

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    He CLUSTER DYNAMICS IN TUNGSTEN IN THE PRESENCE OF CLUSTER INDUCED FORMATION OF He TRAPS
    (НИЯУ МИФИ, 2015) KRASHENINNIKOV, S. I.; SMIRNOV, R. D.
    Experiments with the irradiation of Tungsten by He ions with the energies below the sputtering threshold reveal both strong modification of surface morphology (e.g. fuzz growth, at the temperature of the sample T above ~1000 K) and formation of a layer of He nano-bubbles in the near-surface region (at T below 1000 K) [1-3]. In Ref. 4 the dynamics of the formation of the layer of He nano-bubbles in the near-surface region was considered both analytically and numerically. Two different mechanisms of bubble formation were analyzed: i) bubble nucleation caused by the self-trapping of He atoms accompanied by the formation of immobile He clusters and following trap mutation and bubble growth, and ii) absorption of He atoms by existing immobile traps (e.g. associated with impurities) and following trap mutation and bubble growth. It was shown that both mechanisms of bubble nucleation result in the formation of a plug of large, immobile helium clusters. These clusters serve as a plug, which is the dominant reaction sink for He atoms that prevents further penetration of the helium and decreases its concentration. With time, the front boundary of the plug effectively moves toward the surface leading to the formation of the layer of nano-bubbles. This physical picture is, in a ballpark, consistent with available experimental data (e.g. see Ref. 3). However, for the case of the nucleation of bubbles via He self-trapping, theoretical estimates of the thickness of the layer of nano-bubbles for experimental relevant He flux, appears to be somewhat larger than that seen in experiments. For the case of bubble nucleation through existing traps, the width of the layer should depend of the concentration of existing traps, while experimental data seem to suggest that the purity of the sample (which, probably, alter the concentration of existing traps) does not change much the thickness of the nano-bubble layer. As we see, it seems that some important ingredient, affecting the width of the layer of nano-bubbles, is missed on our model, which, otherwise, gives quite consistent physical picture of the formation of nano-bubble layer.