A range of particle types, fluxes, and energies strike the PFMs and interact in the near-surface region of the PFM. The most common interactions are with hydrogen fuel ions, ranging in energy from a

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few eV to hundreds of eV. In addition to hydrogen ions, fuel by-product ions such as helium, and impu­rities from the first wall also impact the surface. Severe surface layer damage occurs because of such ion impacts, and significant erosion of surface material additionally occurs. Various mechanisms are responsi­ble for erosion, dependent on the surface temperature of the graphite. The mechanisms can generally be characterized in order of increasing temperature phe­nomenon as physical sputtering, chemical erosion, and radiation enhanced sublimation (Figure 21).43 Above 2000 °C, the vapor pressure of graphite dominates the erosion. In addition to the obvious issue of erosion-degraded component lifetime, removal and redistribution of carbon contributes to a high tritium inventory trapped in redeposited carbon.44