Speaker
Description
In the realization of future fusion reactors, the strict management of tritium inventory and the minimization of tritium permeation from plasma-facing components (PFCs) into the coolant systems are critical issues. These challenges directly impact both the radiological safety of the power plant and the fuel efficiency of the fusion cycle. Tungsten (W) is currently the leading candidate material for PFCs due to its robust thermal and physical properties. However, establishing effective permeation barriers against the high flux of energetic particles anticipated in the reactor environment remains a high-priority technological objective. Unlike gas-driven permeation, plasma-driven permeation (PDP) is governed by complex surface dynamics, predominantly determined by the re-emission rate of hydrogen back to the plasma side.
This study aims to develop W coating materials capable of significantly suppressing PDP and to clarify the relationship between fabrication process parameters and hydrogen transport properties. W coatings were deposited on both nickel and W substrates using radio-frequency argon plasma sputtering. The argon background pressure during the deposition process was selected as the primary control parameter, varied systematically (e.g., from 30 Pa to 100 Pa) to modulate the kinetic energy of sputtered particles and, consequently, the microstructural growth of the films. The hydrogen permeation performance was evaluated by exposing the samples to an Inductively Coupled Plasma hydrogen source.
The experimental results demonstrated a strong dependence of the hydrogen permeation flux on the sputtering gas pressure. It was observed that W films deposited under higher argon pressure conditions (100 Pa) exhibited significantly superior barrier performance compared to those fabricated at lower pressures (30 Pa). Specifically, the permeation flux, normalized to account for differences in deposited mass, was reduced by approximately an order of magnitude in the high-pressure samples.
We interpret this behavior as likely being associated with changes in the film microstructure. High-pressure sputtering conditions are generally known to promote the formation of a porous, columnar structure, corresponding to Zone 1 in Thornton's structure zone model. It is inferred that such a structure, if present, would increase the effective surface area compared to denser films. In the context of plasma-driven permeation, a larger surface area is expected to facilitate the surface recombination of hydrogen atoms into molecules. This enhanced recombination would promote the desorption of hydrogen back into the vacuum chamber, potentially reducing the net flux diffusing through the bulk material.