MSE Research Spotlight: Plasma-Wall Interactions for Fusion Reactor Design

In one set of experiments performed with colleagues T. Schwarz-Selinger, K. Schmid, M. Schlüter and W. Jacob at Max-Planck-Institut für Plasmaphysik, Garching, Germany, the interaction of quantified deuterium (hydrogen) atom beams with hard a-C:H (a-C:D) films was studied at a substrate temperature of 320 K. The results of this work provide new insights on this aspect of the interaction of a deuterium plasma with plasma-facing componenents (PFC) based on carbon. Film modification and erosion were investigated in real-time by ellipsometry in an ultrahigh-vacuum particle-beam setup at Garching. The change of deuterium (D) areal density as a function of D (H) atom beam exposure time was measured ex situ by nuclear reaction analysis with 3He ions at 690 keV.

We were able to distinguish three sequential stages of D (H) interaction with hard a-C:H (a-C:D) films. The first stage is isotope exchange of the hydrogen isotopes in the film with those from the beam. The first stage is relatively fast and extends to a depth of ≈1.5 nm from the surface. It is completed after after a total hydrogen fluence of ≈2x1018 cm-2 and corresponds to an exchange of ≈5x1015 atoms cm-2. The cross-section of isotope exchange determined is equal to that of hydrogen abstraction from an a-C:H surface. This is followed by creation of additional C-D (C-H) bonds at an areal density of ≈3x1015 cm-2 in the near-surface region. A soft a-C:D layer with a thickness of ≈1.5 nm is formed, and its formation is complete after a total hydrogen fluence of about 2x1019 cm-2. During the third stage, steady-state erosion of the a-C:H film is seen. In this phase, the soft a-C:D layer with roughly constant thickness (≈1.5 nm) remains at the film surface and is dynamically reformed as the underlying hard a-C:H (a-C:D) becomes thinner. The current results on D (H) atom/a-C:H (a-C:D) film interactions provide new insights on the consequences of long-time plasma exposures of PFCs which are relevant to future plasma systems operated under steady-state conditions.