Long‐Term Hydrogen Outgassing from Plasma Facing Components

Abstract

The time evolution of hydrogen outgassing fluxes from plasma facing components(PFCs) in tokamaks at room temperature is investigated using a diffusion-reaction model with a single trap in the bulk of PFCs material. The release of hydrogen is surface-limited or diffusion-limited depending on properties of material and surface processes. In both surface and diffusion-limited regimes, the long-term decrease of the hydrogen outgassing flux follows a time power-law ∼ t −κ , for which κ depends on the concentration of hydrogen in the material and on surface desorption processes. In surface-limited regime, reaction-diffusion models of hydrogen outgassing from wall material(with hydrogen diffusion in the material bulk) are shown to be equivalent to models of outgassing from wall surfaces. Experimental results showing power-law decays of hydrogen outgassing fluxes from PFCs during long off-plasma events (κ = 0.7 for graphite and κ = 0.8 for ITER-like JET configuration) can be then reproduced using a diffusion-reaction model in surface-limited regime.

Short-term retention in metallic PFCs: modelling in view of mass spectrometry and LIBS

“…In PSI-2 a decay with a 1 @was measured for a bulk W target [18]. Several attempts of mathematical and computational analysis of postdischarge outgassing have been made to explain the observed power law decay of the outgassing flux [13,[19][20][21][22][23][24]. In the following we shall stick to power law as a good approximation for post-discharge outgassing and assume it to be an objective consequence of local physical processes.…”