When the eV or slightly more energetic ions of hydrogen atoms impact the graphite plasma-facing surface, they will chemically react with the surface. The form this reaction takes is quite dependent on the temperature of the graphite, and as the majority of the ions are reacting over nanometer lengths, the result is the production of an amorphous film of hydrogenated carbon suit at the near-surface layer. An analytical expression for this thermally activated erosion process has been put forward by Roth and Garcia-Rosales,45 with further development by Mech46 and Roth.47 It is noted that in the ion energies between 10 and 20 eV, there is still an active debate regarding the synergy ofthe ion impact and the chemical erosion.

Temperature (°C)

The combination of energetic damage plus chemical reaction, which is sometimes referred to as ‘chemi­cal sputtering,’ is discussed by Jacob and Roth48 and others.

For low(RT) and intermediate temperatures, from 400 to 1000 °C (Figure 21), the volatilization of car­bon atoms by energetic plasma ions becomes impor­tant. As seen in the upper curve of Figure 21, helium does not have a chemical erosion component of its sputter yield. In the currently operating machines, the two major contributors to chemical erosion are the ions of hydrogen and oxygen. The typical chemi­cal species that evolve from the surface as measured by residual gas analysis49 and optical emission50 are hydrocarbons, carbon monoxide, and carbon dioxide.

The interaction of hydrogen with graphite appears to be highly dependent on the ion species, on mate­rial temperature, and on the perfection and type of the graphite. This is illustrated in Figure 22, which shows typical bell-shaped thermally acti­vated erosion yield curves for hydrogen and deute­rium ions on graphite. The shape of the yield curve is influenced by the competition for hydrogenation from the sp2 and sp3 hybridization states.51,52 Hydrogen ions incident to the surface are slowed down and pref­erentially attach to sp2 carbon atoms (such as graph­ite edge plane atoms) forming sp3 CH3 complexes. Above approximately 400 K, these CH3 complexes can be released, thus returning the structure to the sp2 state. It is important to note that this phenomenon will only happen in the presence of simultaneous ion damage. It will not occur simply due to a thermal process. This step leads to chemical erosion products (a host of erosion species are possible). The ability of hydrogen to continue to be bonded to carbon drops as the temperature goes up. If there are no CH or CH2 precursors on the surface, then no volatile CH3 or CH4 complexes can be formed, and thus there is no chemical erosion. This balance yields a maximum erosion rate, which for undamaged pyrolytic graphite resides at ^280-600 °C.53 It is noted that more recent work by Balden54 has determined the maximum to be in the range of 872-1222 °C. This mechanism was first elucidated by Horn55 and Wittmann.56 The rate of formation of CH2, CH3, and complex hydrocar­bons from atomic hydrogen in well-graphitized mate­rial is fairly low unless the material is altered (damaged) in the near-surface layer. For preirra­diated pyrolytic graphite (i. e., damaged graphite, meaning that a carbon atom has been removed from its lattice position, thus increasing the available sp2 sites) preirradiated by high-energy D+ of H+ ions, the total erosion yield following exposure to low-energy hydrogen increases dramatically. This is illustrated in the upper curves of Figure 22 that show more than an order of magnitude increase in erosion yield over
the undamaged case. This increased carbon loss has been attributed to the creation of active sites for Ho attachment.57’58 This structurally dependent mech­anism is supported by the data of Phillips et al.,59 which shows a factor of two difference in erosion yield between high — and low-quality pyrolytic graphite.