Biasing effects on hydrogen isotope transport and permeation in the gaps of monoblock plasma-facing components

• Biasing increases ion energy but not net permeation in monoblock gaps. • Sheath electric field geometry, not total particle flux, governs hydrogen transport in castellated PFCs. • A transition from multi-reflection deposition (floating) to direct-acceleration reflection (biased) is revealed. Hydrogen isotope (HI) permeation through millimeter-scale gaps in tungsten monoblocks poses a critical challenge to tritium safety in fusion reactors. Although substrate biasing is widely employed to simulate the fusion boundary environment, its specific impact on hydrogen transport and permeation within these gaps remains inadequately understood. By integrating experimental measurements from the PREFACE facility with 2D3V particle-in-cell (PIC) simulations, this study clarifies how a negative bias of –100 V affects HI permeation behavior. Contrary to initial expectations, biasing enhances ion flux but does not lead to a net increase in permeation flux at the gap bottom. Simulations reveal that this results from a key decoupling between ion deposition and net retention: the applied bias directs ions toward the gap bottom with elevated impact energies (∼100 eV), which promotes sputtering and reflection, thereby reducing effective retention. In contrast, under floating conditions, funnel-like sheath fields guide ions through multiple sidewall reflections, gradually channeling low-energy particles to the bottom region, where permeation is more efficient. This work clarifies the biasing effect on permeation by establishing electric-field-controlled ion energy and trajectory as the governing mechanism, offering valuable insights for the design of plasma-facing components to mitigate tritium leakage.