Investigation of damages induced by ITER-relevant heat loads during massive gas injections on Beryllium

Abstract

a b s t r a c tMassive gas injections (MGIs) will be used in ITER to mitigate the strong damaging effect of full performance plasma disruptions on the plasma facing components. The MGI method transforms the stored plasma energy to radiation that is spread across the vacuum vessel with poloidal and toroidal asymmetries. This work investigated the impact of MGI like heat loading on the first wall armor material beryllium. ITER-relevant power densities of 90-260 MW m −2 in combination with pulse durations of 5-10 ms were exerted onto the S-65 grade beryllium specimens in the electron beam facility JUDITH 1. All tested loading conditions led to noticeable surface morphology changes and in the expected worst case scenario, a crater with thermally induced cracks with a depth of up to ∼340 μm formed in the loaded area. The level of destruction in the loaded area was strongly dependent on the pulse number but also on the formation of beryllium oxide. The cyclic melting of beryllium could lead to an armor thinning mechanism under the presence of melt motion driving forces such as surface tension, magnetic forces, and plasma pressure.

Oxide segregation and melting behavior of transient heat load exposed beryllium

“…This particularly weak region (black area below the molten material) had vertical thickness of up to 50 µm, as can be seen in figure 1( f ). The weakly bound grains acted not only as a thermal barrier, but also appeared to reduce the wettability of the bulk material, as it was observed in [11]. Moreover, the thermal barrier led to an overheating of the overlaying material during each transient heat pulse.…”

“…The transient beryllium surface temperature could not be accurately measured with a fast pyrometer due to the unknown emissivity at the varying surface conditions and elevated temperatures. To support the interpretation of the experimental results, all given maximum temperature (T max ) values were calculated using the FEM model in [11].…”

“…The results indicated that the transition region formation depended on the maximum temperature T max reached during the transient heat pulses with a threshold at T max = 1211 °C. T max was calculated via the FEM model in [11] which considers the electron penetration depth in JUDITH 1 experiments (~120 µm for 120 keV, determined with Monte Carlo simulations).…”

“…Therefore, it remains vital to investigate the performance of beryllium under the influence of transient power fluxes and especially in the case that melting occurs to predict the reliability of the FW armor. Laboratory experiments that apply power fluxes by quasi stationary plasma accelerators [4,6] or focused electron beam guns [7][8][9][10][11] are suitable to simulate the thermal loads during transient plasma events in a tokamak.…”

“…This work aimed at a more detailed understanding of the observed deterioration of the transient thermal load affected layer on beryllium [11,12]. Furthermore, the role of the intrinsic beryllium oxide content in the as-received ITER qualified beryllium grade S-65 under transient heat loading was in the focus of the investigation.…”