When a relatively high-energy impacting particle transfers energy to a near-surface carbon atom in an amount sufficient to overcome the lattice bond energy or surface binding energy, some carbon atoms may be displaced and these may move in a direction defined by the angle between their path and the initial path of the impacting atom. Analogous to striking a billiard ball, this angle must be between 0° and 90°. The energy imparted to the displaced atom follows that given in eqn [6]. For a relatively high-energy atom striking a surface normally, the recoiling atom cannot be sputtered from the surface. However, in off-normal impact or displacement cas­cade events from fusion neutrons that occur near the surface, some fraction of atoms will be emitted (physically sputtered) from the graphite surface. The amount of material lost from the surface is defined by the sputtering yield (Y), which is the number of target PFM atoms emitted per plasma ion impacting the surface. From eqn [6], the energy transferred to a target PFM atom, which is directly
related to the erosion yield, is a strong function of the impacting particle mass and the mass of the material being sputtered. The impact angle also has a large effect on the number of atoms that receive adequate kinetic energy normal to the PFM surface to be physically sputtered. The plasma ions travel along the magnetic field lines that are at a shallow (grazing) angle with the PFM, typically 1-5°, though the ion impact angle will be modified by surface potentials and collisional processes.81

The quantitative effect of the mass, energy, and angle of impact on the sputter yield of impacting deuterium ions is shown in Figure 26(a) and 26(b). As the kinetic energy of deuterium increases, the total amount of energy transferred to the target atoms increases, and the average amount of energy per colli­sion results in greater erosion. From Figure 26(a), it may be seen that the physical sputtering yield of light target atoms is considerably greater than that for the heavy atoms, primarily due to the reduced impact energy required to overcome the displacement energy of the higher target atoms. For example, in purely kinetic terms, approximately 20eVis required to dis­place an atom of carbon from the surface, while 220 eV is required for an atom of tungsten. In the sub-keV energy range of plasma fuels, the high yield materials are therefore carbon and beryllium. As the impacting ion energy increases, the sputtering yield for all materials decreases as the depth of interaction of

the impacting ion becomes too great for displaced atoms to back scatter to the surface. In the case of graphite, the majority of the sputtered material comes from the top few atomic layers.82

With the correct combination of incident energy and target mass, it is possible for the sputtering yield to exceed unity, that is, more than one atom leaves the surface for every impacting particle. This quickly leads to what is called the catastrophic ‘carbon bloom,’ that is, the self-accelerating sputtering of carbon. As can be seen in Figure 26(b), this problem is the worst for carbon self-impacts at grazing angles to the surface.