Deuterium and helium retention and corresponding modifications of W under heat loads relevant to ITER transient plasma events: Part I. The power load below the tungsten melting temperature

Abstract In order to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts, tungsten samples were exposed to pulsed heat loads using pure deuterium (D) and with 10% helium (He) seeding plasmas in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms that is relevant to ELMs and number of pulses was varied from one to thirty. The power load was 0.7 MJ/m2 that is below the tungsten melting temperature (Tm). Two tungsten samples were used, namely, polycrystalline tungsten without (W) and with pre-existing He-induced W ‘fuzz’ (Wf). Similar to the steady state plasma, the presence of He in tungsten leads to a reduction of the D retention during transient events at temperature below Tm. We explained it as interruption of the D diffusion towards to the bulk of tungsten by (i) the strain field induced by He bubbles and (ii) the formation of interconnected He bubbles at high temperature which leads to an open porosity for accelerated D desorption, thus, decreasing the D influx into the tungsten bulk. But as the He retention in Wf decreases below 1019 He/m2, the effect of He on the D retention after the plasma gun irradiation disappears: the D retention in W and Wf is the same after 30 pulses of the exposure to pure D plasma. In both cases of pure D and He seeded D plasma gun exposures, the D retention is higher compared to the steady state plasma exposure at sample temperature above 600 K.