Comparison of TEM and positron annihilation lifetime spectroscopy on tungsten exposed to mixed D plasmas

Abstract

Tungsten is currently a main candidate as a divertor plasma facing material in fusion devices. Even tungsten, however, will not be able to withstand excessive heat fluxes during off normal events such as ELM's without significant erosion or other damages (e.g. melting or cracking) in the future fusion devices. One of the options to mitigate excessive power loads is by divertor impurity plasma seeding to dissipate the energy. Gases like Ar, Ne or N 2 can be used for that purpose. Moreover, the presence of He as a reaction product from DT fusion reaction is also unavoidable. All these impurities will have an effect on tungsten behavior under plasma exposure. The expected impact of impurities on the surface morphology will change the tungsten erosion behavior. The presence of different subsurface defects (dislocation loops, voids) will influence the retention of hydrogen isotopes, which is of prime importance for the operation of a fusion device. The main focus of this work is to compare TEM analysis with positron annihilation lifetime spectroscopy to investigate the near‐surface defects created by the impact of different plasmas. Tungsten samples were mechanically polished to obtain mirror like surface and recrystallized at 1800°C for 1 h. Such prepared specimens were exposed in the linear plasma generator PSI‐2 with an incident ion flux of about 10 22 m ‐2 s ‐1 and at an incident ion fluence of 5*10 25 m ‐2 , at a sample temperature of 500 K. Samples were biased to a potential of ‐ 100 V resulting in incident ion energy of 70 eV. Pure D plasma (reference sample) and D plasma with additional impurities of He (3%), Ar (7%), Ne (10%) or N (~5%) were applied. The impurity concentration was controlled by spectroscopy, except for N for which it was estimated from the puffing rates. The analysis covers the detection of subsurface defects and their density. TEM observation combined with positron annihilation techniques are employed to determine the thickness of the damaged zone and the presence and density of defects such as voids, dislocation loops and vacancies clusters. After the plasma exposure, the surface morphology was investigated using scanning electron microscope (SEM) combined with a focused ion beam (FIB) utilized for cross‐sectioning and thin lamella preparation for the transmission electron microscope (TEM) analysis. The reference sample exposed to a pure D plasma reveals at the surface the presence of two groups of blisters with a size of few mm and a few 100 nm. The presence of blisters is strongly correlated with the tungsten grain orientation. The addition of Ar and Ne results in surface erosion with different yields depending on grain orientation, confirmed also by electron backscattered diffraction (EBSD). Large blisters are present but show signatures of erosion. Less pronounced erosion is visible when adding N 2 . The presence of N in the plasma causes also blisters with cone‐like shapes. The addition of He leads to the formation of flatter blisters and very fine nano‐porosity on the surface.