On the origin of ‘fuzz’ formation in plasma-facing materials

Abstract

Tungsten, the material used in the plasma-facing components (PFCs) of nuclear fusion reactors, develops a fuzz-like surface morphology under typical reactor operating conditions. This fragile ‘fuzz’ surface nanostructure adversely affects reactor performance and operation. Developing predictive models, capable of simulating the spatiotemporal scales relevant to the fuzz formation process is essential for understanding the growth of such extremely complex surface features and improving PFC and reactor performance. Here, we report the development of an atomistically-informed, continuous-domain model for the onset of fuzz formation in helium plasma-irradiated tungsten and validate the model by comparing its predictions with measurements from carefully designed experiments. Our study demonstrates that fuzz forms in response to stress induced in the near-surface region of PFCs as a result of plasma exposure and helium gas implantation. Our model sets the stage for detailed descriptions of this complex fuzz formation phenomenon and similar phenomena observed in other materials.

Theoretical Model of Helium Bubble Growth and Density in Plasma-Facing Metals

“…This correlation, much like other studies of helium bubble energetics and bubble growth in tungsten [21][22][23][24][25][26][27][28][29][30] , has been useful in developing coarse-grained models of helium transport and surface morphological evolution in tungsten [31][32][33][34][35][36] . However, the correlation in Eq.…”

Plasma Facing Material by Self-Interstitial Solid Solution Strengthening – Problem, Proposition, and a Solution

“…Historically, it has been described in chemical engineering sense as most of activities of fusion research started on east cost in US in Princeton Plasma Physics Laboratory where physicists and chemical engineers were handling experiments. In physical chemistry sense, He atoms have strong repulsion to W atoms [80,92]. This ultra-low solubility forces He atoms to self-precipitate into small He bubbles [83] that become nucleation sites [90] for further void growth [93] under radiation induced vacancy supersaturations [94], resulting in material swelling [69,86,95] and high temperature He embrittlement [71,96,97], as well as surface blistering [75][76][77][78] under low energy and high flux He bombardment [54,98] at elevated temperatures [99].…”

“…Other structures such as Helium cavities are also observed which may be suppressed by defect sinks, nano clusters, or interfaces. In physical metallurgy terms, it is a point defect [100,101] in a crystal lattice -a precursor of fuzz [56,80,102] or under dense nanostructure [40] formation. It is produced as a result of self-ejection of W from interstitial positions in its own lattice by He atom or cluster of atoms (usually 9 atoms at 700 K [40]) (Cluster: 7 -8 atoms (bulk), < 7 -8 atoms (surface) and its bonding with vacancy [88,[103][104][105][106] forming a Frenkel pair [107][108][109][110][111].…”

“…Selection of suitable crystal lattice which inhibit Frenkel pair [111] generation and promotes host atom self-interstitial solid solution strengthening contributes effectively towards making plasma facing material -subject area of present study. More detailed mechanisms involves loop punching after spontaneous organization of W self-interstitials (SIA) into a prismatic <111> -dislocation loop, ligament thinning [80], crater and nano tendril formation and trap mutation reactions [80] HenVm + He ------------------> Hen+1Vm+1 + I where V = W vacancy, I = self-interstitial atom. He3V1 is a stable cluster.…”

“…To this end, various materials (such as W [47][48][49][50], addition of Rh in W [51], use of bcc Fe [52,53], Ta [54], W-Ta [55], Ta/Fe [56], Pd [57], nanocrystalline Cu [58], SiOC/Crystalline Fe nanocomposite [59], W-K [60], reduced activation steel [61], ferritic [62], ferritic/martensitic steels [63], Be pebbles [64][65][66][67], Be and beryllides [68], graphite, carbon fiber composite [69]) and high Z atoms (Zr, No, Mo, Hf, Ta) [70] have been tested but none proved satisfactory [71][72][73][74]. All show rapid surface degradation exhibiting surface blisters [75][76][77][78] and formation of fuzz [51,[79][80][81][82] or under dense nanostructure [40] after bubble. These rapidly degrade their physical (e.g.…”