Stress-driven surface swell and exfoliation of copper as the plasma-facing materials in NBI ICP source

Abstract

Neutral beam injection (NBI) heating is a significant auxiliary heating method used in Tokamak fusion devices. The material of faraday shield (FS) and accelerator grids in the NBI inductively coupled plasma (ICP) source can be damaged during operation by the high-density hydrogen plasma irradiation, and thus affecting the stability of the NBI system. In this paper, a series of hydrogen plasma exposure experiments are performed on oxygen-free copper (OFC) specimens at 400 K–850 K with ion energy of 20–200 eV and irradiation fluence up to 1.0 × 1025 m−2. Meanwhile, the rate equation model is adopted for numerical simulation of the bubble growth and hydrogen retention. The influence of OFC surface temperature, hydrogen ion energy and fluence on OFC damage are experimentally and numerically investigated. Surface observations show that swell and exfoliation are formed on the OFC samples at 400 K and 600 K by scanning electron microscopy. The hydrogen ion energy varying from 20 to 200 eV at 400 K is observed to have little effect on OFC surface microstructure. The simulative results show that there exist different critical temperatures when the initial bubble radius changes. The bubble surface density rises and the bubble size decreases with increasing temperature (below the critical temperature). In addition, adjacent bubbles get closer to each other with the growth of hydrogen bubbles, and the strong tensile stress is produced inside the surrounding material of hydrogen bubbles. Some cracks caused by hydrogen bubbles appear on the surface of the OFC to relax the pressure-induced stress, ultimately leading to OFC FS/grids material damage. This investigation helps to understand hydrogen retention and failure mechanisms of OFC materials under extreme operation conditions in the NBI devices.

Temperature-dependent bubble growth under synergistic interactions of hydrogen and helium in tungsten

“…Our research group has previously developed theoretical models to study the H/He retention and bubble growth behaviour in W under either pure H or pure He plasma irradiation [29][30][31][32]. These models consider three forms of H/He in W: solute H/He atoms, trapped atoms, and H/He bubbles.…”

“…Nonetheless, when both H and He atoms coexist in W, the dynamics of defect capture and bubble growth behaviours become more complex, requiring consideration of their interactions. The rate equation employed in the model to predict the nanobubble growth in W under individual H or He ion irradiation [30,32] is adapted to account for these interactions in this study. It is noteworthy that, considering the complexity of the model and computational feasibility, the current work is limited to the effects of intrinsic defects.…”

“…At 473 K, the contribution of H to bubble growth predominates, whereas at 1273 K, the role of He becomes more pronounced. This variation can be attributed to the enhanced retention of H in W at lower temperatures and the increased mobility of He at higher temperatures [29,32]. The growth of H-He bubbles is primarily driven by the higher internal pressure resulting from the increased number of H and He atoms within bubbles.…”

Initial growth of tungsten fuzz induced by bubble-driven surface stress layer under helium irradiation

“…It is obvious that the stresses at the bubble surface could combine to produce tensile stress in a direction perpendicular to the plane containing the bubbles (see figure 4), considering a set of identical coplanar bubbles with excess internal pressure. Therefore, it tends to split this plane [33]. Furthermore, if it is assumed that the bubbles are in a simple cubic array with distance d = C −1/3 b between nearest neighbor centers, then the average total tensile stress (δ 0 ) across the plane can be given by,…”

Automatic recognition of defects in plasma-facing material using image processing technology

“…This exposure can induce the formation of a fragile nanofiberlike crystalline structure known as 'fuzz' on the surface of W at temperatures of 1000-1900 K. The formation of this nanostructure contains many helium bubbles, which play a crucial role in inducing the surface modification of PFMs [2][3][4]. Rate theory (RT) and molecular dynamics (MD) simulations are useful for gaining insight into these modifications [5][6][7][8]. However, modeling nanoscale modifications of PFMs necessitates a significant amount of experimental data.…”