D and D/He plasma interactions with diamond: Surface modification and D retention
Introduction
The coupling between the plasma and the plasma facing materials (PFMs) in magnetically confined nuclear fusion reactors is complex [1], [2]. PFMs are subjected to bombardment from a high flux of energetic particles (1024 ions m− 2 s− 1) and high heat fluxes (10 MW/m2) from the plasma, which may modify the PFM and release impurities into the plasma [3]. Plasma contamination must be controlled to prevent radiative heat losses in the plasma inhibiting the D–T fusion reaction from proceeding. In addition to minimising impurities released into the plasma, PFM must also minimise toxic tritium retention. High Z materials such as tungsten [4] exhibit low sputtering yields and high melting points. However, radiative heat loss scales with Z, so any tungsten impurities released to the plasma result in large heat losses. Low Z PFMs such as carbon-based materials suffer from higher erosion rates, but radiative losses are not as significant [5]. One substantial drawback of graphite and carbon fibre composites (CFCs) as PFMs is redeposition of hydrogenated carbon, which results in a high level of tritium retention relative to that in tungsten [3], [5], [6].
CVD diamond has been suggested as a PFM due to its excellent thermal and mechanical properties and it's resistance to chemical attack [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Diamond has been shown to have an erosion rate ~ 50% less than graphite when exposed to low temperature, high flux hydrogen plasmas [8], [9]. Furthermore CVD diamond displays a superior resistance to thermal shock compared to tungsten, graphite and CFCs [7]. The etch rate of sp3 bonded carbon is known to be much slower than that of sp2 bonded carbon in a hydrogen environment [23], [24], [25]. This implies that the erosion performance of diamond will be superior to that of graphite, however, it has been reported that partial amorphisation and conversion to sp2 bonded carbon can occur on diamond surfaces after plasma exposure [8], [9], [11], [26], [27].
This work aims to explore fundamental plasma–material interaction mechanisms by providing characterisation of the material condition after exposure to D plasma for fluences between 1022 and 1024 ions m− 2. Our previous work investigated plasma surface interactions of diamond with H and D plasmas, focusing on the effects of plasma ion energy [26] and pre-irradiation with high energy C ions to simulate neutron damage [22]. This work is extended through the addition of He to the D plasma to simulate the He produced from a burning D–T plasma [28], [29], [30], [31], [32] and the effect of varying ion fluence. The addition of He is expected to exhibit different plasma–surface interaction characteristics due to the lack of chemical effects and a larger atomic mass. The resulting modifications to surface morphology, inter-atomic bonding and D retention are reported.
Section snippets
Experimental
Polycrystalline CVD diamond samples (10 mm × 10 mm × 0.25 mm) were sourced from Element Six Ltd. (thermal management 100 grade, polished by the supplier to a surface roughness of Ra < 50 nm) and exposed to plasma in the MAgnetized Plasma Interaction Experiment (MAGPIE) [33]. MAGPIE provides a controlled magnetically focused plasma environment, capable of creating key plasma conditions to study fusion-relevant materials. The linear plasma device utilises a helicon plasma discharge, which provides an
SEM
Fig. 1 shows SEM images of the diamond surface with (a) no plasma exposure, and following exposure to D plasma for fluences of (b) 1.5 × 1022 ions m− 2, (c) 5 × 1022 ions m− 2, (d) 1 × 1023 ions m− 2, (e) 5 × 1023 ions m− 2 and (f) 1 × 1024 ions m− 2. The unexposed sample in Fig. 1(a) is featureless, indicating an extremely flat sample. No surface modification is visible following 1.5 × 1022 ions m− 2 or 5 × 1022 ions m− 2 of D plasma exposure in Fig. 1(b) and (c). Close inspection of Fig. 1(d) (1 × 1023 ions m− 2 D plasma exposure)
Discussion
The surface morphology exhibits some noteworthy differences when comparing diamond exposure to the D plasma, the D/He mixed plasma and the He plasma. Cracks on the surface were observed for the D/He mixed plasma and the pure He plasma, but not for the D-only plasma-exposed samples. These cracks are suggestive of He trapping and internal stresses, ultimately resulting in ~ 1 μm sized pieces of material flaking off the surface. This is indicative that flaking will be a significant erosion mechanism
Conclusion
CVD diamond was exposed to D and mixed D/He plasma for five exposure times between 30 s and 2000 s (1.5 × 1022 ions m− 2 to 1 × 1024 ions m− 2) in the linear plasma device, MAGPIE. SEM results showed the formation of 10 nm–100 nm surface protrusions, with a significant surface coverage for plasma exposure times ≥ 1000 s (5 × 1023 ions m− 2) and which required D present in the plasma. The source of these surface features was concluded to be via a process of chemical erosion of the diamond surface and subsequent
Prime novelty statement
CVD diamond is of interest as a plasma facing material in magnetically confined nuclear fusion devices; in this work CVD diamond has been exposed to nuclear fusion-relevant D and mixed D/He plasmas in the linear plasma device, MAGPIE. A significant surface coverage of 10 nm–100 nm sized hemispherical and conical surface features resulted from chemical erosion and subsequent redeposition. Near edge X-ray absorption fine structure (NEXAFS) spectra showed strong diamond features following plasma
Acknowledgements
The NEXAFS research in this work was undertaken on the soft X-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia. The authors gratefully acknowledge Joel Davis from the Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, for the assistance with collecting SEM images. CS Corr acknowledges the support from the Australian Research Council through the Future Fellowship scheme (FT100100825).
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