Perfect containment ofthe high-density plasma needed for power production, where perfection means no interaction of the high-energy plasma and its sur­roundings, is not a practical reality. Whether through normal operation, or in off-normal incidents such as plasma ‘disruptions,’ plasma-material interaction (PMI) will occur in fusion devices. Components

in line of sight with the plasma, and therefore impacted by the hot gasses and particles, are referred to as plasma-facing components (PFCs) or materials (PFMs). The reactions between the fusion plasma and the PFMs are quite severe and typically cause melting or sublimation, component mechanical fail­ure due to high thermal stress, and excessive surface erosion. The plasma ion flux and associated heat loading to the PFMs can be highly nonuniform and quite dependent on the tokamak design.

The hot plasma gasses are made up of unburned hydrogen fuel, fusion byproducts such as helium, plasma electrons, and impurities, which include ele­ments previously removed from PFCs. As can be seen in eqn [1], the types of particles that may strike the PFMs are dependent on the fusion fuel. For the D+T fuel system, the plasma will contain not only the

D+T fuel, but also high-energy alpha particles (3.5 MeV He) and neutrons (14.1 MeV). The parti­tioning of the reaction energy between helium and the neutron is both an advantage and a disadvantage for the D+T fuel system. Because the energetic helium nucleus quickly collides with the surrounding gasses, most of its energy remains in the plasma and helps to sustain the high plasma temperature. Con­versely, the neutron has very little chance of collision in the low-density plasma and loses its energy outside of the plasma, usually over meters of path length inside the structure of the reactor. Because less than 30% of the D+T reaction energy remains in the plasma, only this fraction is eventually dumped on the PFCs, thus reducing the heat load handling requirement and material erosion. However, as discussed in Section 4.18.3, the material damage associated with the 14.1 MeV neutron collisions is significant and perhaps offsets the advantages of reduced D+T heat loading.

A characteristic classifying the fusion device type is the manner in which the plasma edge is defined and the plasma power handled. The classic approach is to define the plasma edge by placing a sacrificial com­ponent in contact with the plasma. This component, which intercepts the plasma edge particle flux, is known as a bumper or bumper limiter, and extends circumferentially around the torus. A second approach to defining the plasma edge is magnetically capturing and diverting the edge plasma onto a divertor plate well removed from the central plasma. Once the plasma gasses strike the surfaces and are thus cooled, they are pumped away. Unless mitigated, the energy
deposited locally on the ‘divertor’ can be excessive. Many techniques, such as magnetic sweeping to spread the load and puffing of gas to ‘soften’ the ion impact, have been used to reduce the particle flux and energy. Regardless of whether the limiter or divertor design is employed, the majority of the par­ticle and heat flux is intercepted by these components (Table 1). However, a significant flux also impacts the balance of the torus lining, generally referred to as the first wall. A convenient comparison for the heat load­ings given in Table 1 is that the maximum output from a conventional propane torch is approximately 10MWm~ , or about the maximum seen in current fusion devices.