Deuterium plasma-driven permeation in chemical vapor deposition tungsten-copper composite

Abstract With the potential application of tungsten (W)-copper (Cu) bonding in the plasma-facing components (PFCs) in fusion devices, hydrogen isotope (HI) transport through the W/Cu interface has become a key concern for tritium self-sustainment and operational safety. To investigate HI permeation through the W/Cu interface, a series of low-energy deuterium (D) plasma-driven permeation experiments were performed on chemical vapor deposition tungsten (CVD-W)/Cu composite, bare CVD-W, and bare Cu, across a temperature range of ∼600 K–800 K. The effective D diffusion coefficient of CVD-W was found to be higher than that of rolled W, likely due to the grain boundaries serving as high-diffusivity pathways for D diffusion. Under the identical experimental conditions, an unexpected result was found that the steady state permeation flux in CVD-W/Cu was higher than that in bare Cu, with values of ∼3.1 × 10 18 m −2 s −1 in CVD-W/Cu and ∼4.1 × 10 17 m −2 s −1 in Cu at 741 K. And the time required for CVD-W/Cu to reach steady state permeation exceeded the sum of the times required for CVD-W and Cu individually. Rate equation simulations suggested that a high D concentration segment with a low HI solution energy of 0.6 eV was necessary to replicate the observed high permeation flux in CVD-W/Cu. The length of the segment was in line with the experimental observation in the Cu-entry region of CVD-W near the CVD-W/Cu interface. Density functional theory calculations confirmed that the presence of Cu in W could reduce the HI solution energy in W. It is therefore suggested that the reduction in HI solution energy in the Cu-entry region of CVD-W led to substantial D accumulation near the interface, enhancing the steady state permeation flux in CVD-W/Cu compared to bare Cu. Furthermore, an analytical solution for the steady state permeation flux in a generalized three-layer composite was derived using a modified analytical equation for the fast evaluation of the steady state permeation flux. This work provides valuable insights and foundational parameters for understanding and evaluating HI transport in PFCs using W and Cu in fusion devices.