Of the flux of particles that will impact the PFMs, the highest particle flux will be the ionized fuel itself. The energy of the impacting fuel ions on the various plasma-facing areas depends on many variables. For the divertor, where most of the interactions will occur, the majority of particles will have energies in the eV range. On the first wall where the interaction intensity is less, the charge exchange particles will mostly be in the keV range. For larger fusion devices of the future where dense plasmas and higher mag­netic fields result in the thermalization of the ener­getic helium (eqn [1]), those helium ions will have energies similar to the fuel ions. Electrons, which are in number density equilibrium with the plasma ions, also travel along the plasma field lines, albeit in the opposite direction. The high-energy neutrons pres­ent in the D+T reaction (14.1 MeV), or those for the D+D reaction (2.4 MeV) have mean free paths of several centimeters in graphite and so will not interact strongly with the first wall. However, these neutrons will be scattered and slowed down within and behind the first wall, resulting in a nearly isotro­pic flux of high-energy neutrons throughout the fusion device. The reaction of the plasma neutrons, ions, and electrons with graphite PFMs, which is discussed in some detail in the following sections, can have a wide range of effects. These effects include physical and chemical erosion of the first wall and thermomechanical property degradation of the bulk and surface material.

The discussion thus far has been limited to the operation of tokamaks in the quasi-steady state (long pulse). All present-day large tokamaks are pulsed machines with pulse lengths of seconds, where the plasma discharge consists of a rapid heating phase, a steady state, and a cool down phase. In this case, the heat flux is approximately uniform around the cir­cumference of the machine and scales with the machine power. However, a significant number of these plasma shots end in an abrupt and somewhat violent fashion referred to as disruption. When this occurs, the plasma rapidly becomes unstable and instantaneously ‘dumps’ its energy onto the PFC. This causes significantly larger heat loads than dur­ing normal operation, and in many cases, defines the design limits for these components.