Effect of high-flux H/He plasma exposure on tungsten damage due to transient heat loads
Introduction
Edge-Localized Modes (ELMs) are a key concern for the lifetime of the full tungsten (W) ITER divertor, now to be installed from the first day of operations [1]. Unmitigated ELMs might expel 6–9% of the total plasma stored energy (350 MJ for a Q = 10 discharge) leading to intolerable heat loads at the divertor targets [2]. Even in the case of mitigated ELMs, the large number of expected events might lead to material damage due to thermo-mechanical fatigue [3]. Extensive studies have been performed using, for example, electron beams [4] to understand the basic damage mechanisms, such as cracking and roughening, caused by ELM-like thermal shocking of materials. Those studies have mainly been performed on as-received materials. However, in a fusion device, the surface is exposed simultaneously to a high flux plasma and to the transient heat/particle load associated with an ELM i.e. ELMs interact with a surface dynamically loaded with the plasma species (D, T, He, etc.). It has been shown that plasma exposure can affect the thermal shock behaviour of W [5] and synergistic effects can occur under combined loading [6].
The first ITER full-W divertor will have to operate through the H, He, D and first DT phases and it is thus important to understand the impact of the various operation phases on the W thermo-mechanical properties. For instance, the formation of He bubbles due to plasma exposure during the ITER non-active He operation phase, has been found to decrease the surface thermal conductivity [7] and might affect the material’s resilience to ELM events. Nanostructure formation under high flux H exposure, caused by the development of nanovoids in the near-surface region, has also recently been reported [8]. This contribution reports on a series of laboratory studies which have been undertaken at DIFFER to address the effect of high flux plasma exposure on the thermal shock behaviour of W.
Section snippets
Experimental
Three types of investigations have been conducted.
- 1
Laser-induced and electron beam transient loads with a variety of energy densities and pulse numbers on W targets unexposed to plasma.
- 2
Pre-exposure of samples in the Pilot-PSI [6] and Magnum-PSI [9] linear magnetized plasma devices with different ion species (H, He), ion fluence (up to 5 × 1026 m−2) and energies (5–50 eV) and with different base surface temperatures, Tbase (room temperature (RT) – 600 °C), and then subsequently exposed to
Hydrogen plasma exposures
The first step was to characterize the material damage induced by the laser only for the W grade used in this study. This was achieved by exposing samples at RT to 100 laser pulses with FHF in the range 13–43 MJ m−2 s−1/2. In addition, the effect of pulse number (in the range 1–5000) was investigated, for a fixed FHF = 37.5 MJ m−2 s−1/2. During these experiments, the surface cracking often reported in other experiments [12], was not observed.
It has recently been demonstrated that the damage mechanism
Summary and conclusion
A detailed study of the effect of high-flux plasma exposure on the thermal shock behaviour of rolled W has been performed. In the case of H plasma loading, the behaviour of the pristine material has been compared with that of surfaces after H plasma pre-irradiation and with the case of combined laser plus plasma exposure. While cracking is absent for the non-exposed material, exposure to a fluence of 3 × 1026 m−2 at 200 °C leads to an embrittlement of the material and the appearance of a crack
Acknowledgements
This work is part of the research program of the FOM, which is financially supported by NWO. It is supported by the European Communities under the contract of Association between EURATOM and FOM. Partial support through Grant No. 14-12837S (Czech Science Foundation) is also acknowledged. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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