Dynamic outgassing of deuterium, helium and nitrogen from plasma-facing materials under DEMO relevant conditions

Abstract

In confined plasma magnetic fusion devices significant amounts of the hydrogen isotopes used for the fusion reaction can be stored in the plasma-facing materials by implantation. The desorption of this retained hydrogen was seen to follow a tα law with α ≈ -0.7 in tokamaks. For a pulsed fusion reactor this outgassing can define the inter-pulse waiting time. This work presents new experimental data on the dynamic outgassing in ITER grade tungsten exposed under the well-defined conditions of PSI-2 to pure and mixed D2 plasmas. A peak ion flux of 1022 D+ m-2 s is applied for up to 6 h at sample temperatures of up to 900 K. Pure D2 and mixed D2 + He, D2 + N2 and D2 + He + N2 plasmas are applied to the sample at 68 V bias. The D2, He, N outgassing at 293 K and 580 k are observed via in-vacuo quadrupole mass spectrometry covering the range of 40 s 200 000 s after exposure. The outgassing decay follows a single power law with exponents α = -0.7 to -1.1 at 293 K, but at 580 K a drop from α = -0.25 to -2.35 is found. For DEMO a pump-down time to 0.5 mPa in the order of 1-5 h can be expected. The outgassing is in all cases dominated by D2.