Surface treatment of PFMs, while extremely effec­tive for the current day short-pulse tokamaks (pulses typically less than a few seconds), are of limited value for the next-generation (quasi-steady state) machines because of the significant surface erosion expected. However, forming graphite or CFC homogenously with various erosion-mitigating elements is possible. The mechanisms for this mitigation are twofold: (1) geometric shielding by low erosion yield par­ticulate, and (2) changes in local chemical reactivity due to the presence of doping atoms. Promising results have been obtained by doping of graphite with boron, which resides substitutionally in the graphite lattice, trapping migrating interstitials and altering the electronic structure of the material. Boron doping has been shown60 to both reduce the erosion due to oxygen, and to significantly reduce the sputtering yield due to methane formation. How­ever, other factors, such as the drastic reduction in thermal conductivity that is unavoidable in boro — nized graphite, need to be factored into the overall picture. Boron is discussed in some detail here, mostly because it had received great early attention. However, newer dopant combinations61 have served to suppress erosion, and they have also not had such a negative impact on thermal conductivity, and are therefore considered superior for PFC application.

As discussed by Garcia-Rosales,62 until the mid-1990s, boron was the primary ‘dopant’ of interest in fusion PFM graphites for chemical erosion mitiga­tion. As seen in Figure 23,63 the inclusion of up to 15% boron in graphite can result in significant (an order ofmagnitude in the peak regions) reduction in the erosion yield. Studies to date indicate that for effective suppression a minimum of 3% boron in graphite is required. The mechanism behind this suppression may include the reduced chemical activ­ity of the boronized material, as demonstrated by the increased oxidation resistance64 or the suppressed diffusion caused by the interstitial trapping at the boron sites. From the mid-1990s onward, many other metallic element additions to graphite were studied for

their possible beneficial effect on erosion mitigation. Specifically, elements such as silicon, titanium, tung­sten, and vanadium have been studied with varying levels of success.62 These elements are somewhat less effective in erosion mitigation than boron, though a factor of two reduction is to be expected.65-67 In a recent review by Balden,61 considerably higher reduc­tions in erosion are noted. More recently, emphasis has been on the use of multielement doping strategies.61

Because the removal of graphite is significant both in terms of gross material loss (possible consumption of the entire wall for power devices) and enhanced tritium retention for the resulting carbon dust, the effect of any additive to graphite in terms of physical properties or impact on plasma performance when eroded (Section 4.18.2.1) needs to be considered. With the exception of boron, additive elements dis­cussed in the previous paragraph will all have a negative impact on plasma performance in compari­son with carbon atoms, and therefore the balance of reduced mass loss compared to enhanced parasitic radiative plasma loss (eqn [2]) must be considered. As for physical properties, at levels well below the threshold at which they are effective for erosion suppression (~3%), they are direct substitutional elements in the graphite lattice, effecting significant reduction in thermal conductivity (due to their mass-defect phonon scattering.) In contrast, titanium doping, as evidenced in materials such as the Russian RGTi material, serves to enhance the graphitization

process, resulting in very well-crystallized materials of high (though somewhat anisotropic) thermal con­ductivity. A comparison of several element additions to graphite and their effects on the properties of graphite has been carried out by Paz68 and discussed by others.47,69 It is seen (Figure 2468) that all the metallic inclusions studied, with the exception of silicon,70 had, at these graphitization temperatures, the effect of enhancing the effective length (perfec­tion) of the basal plane of the graphite crystals, which is directly linked to enhanced thermal conduc­tivity. In comparison the basal crystal lengths of the Poco nuclear graphite see Table 2 and the Russian RGTi,71 which is processed at a similar graphitiza — tion temperature but with an applied electric current are shown in Table 2. The right hand side of Figure 24 shows the effect of varying the amount of titanium on the crystallite size, indicating that there is an increase in crystallite size with an increase of up to a few atomic percent of titanium.

In addition to the thermal process for chemical erosion described in the previous section, a second route to erosion, which is limited to the regions very near the surface, is also of importance.62 Specifically, for ions of <100eV, the formation of sp3 complexes occur at the surface of the graphite with very low binding energy (<2 eV compared to the 7.4 eV bind­ing energy for carbon in bulk graphite). Because of this very low binding energy the complex can be easily physically sputtered from the surface. Doping of graphites is also somewhat effective in reducing this surface erosion, attributed to the build-up of higher mass ‘dopant’ elements as the carbon atoms
are preferentially sputtered from the surface, effec­tively armoring the surface.62

For machines that will run in steady state such as ITER, moisture and oxygen evolving from the sur­face may not be a significant issue. However, oxygen is the most damaging impurity to current tokamaks through its presence in the molecular form, or as water vapor, and its tendency to be strongly adsorbed by carbon PFMs. Consequently, this impurity has a large impact on the plasma performance and erosion. The release of oxygen from irradiated carbonaceous films has been reviewed by Haasz72,73 and others. It has been clearly demonstrated that the carbon flux away from the first wall is directly related to the evolving oxygen. Typically, the oxygen enters the plasma from the PFMs in the form of CO or CO2. Figure 25 shows the strong temperature depen­dence of the erosion yield of a variety of graphites and codeposited (near amorphous redeposited car­bon) materials.74 It is noted that the data of Figure 25 are measurements of erosion yield by thermal oxida­tion in an O2 environment. Without special PFM surface treatment, such as plasma glow discharge and bake-out of the surface material, these fluxes dominate the surface erosion. For this reason, exten­sive research has been conducted into modifica­tion of graphite surfaces with impressive success in enhanced plasma performance.75 These improve­ments are due not so much to suppressed carbon erosion as to the decrease in the amount of oxygen released from the graphite. Toward this end, doped graphites have been modified to incorporate thermally and physically sputter-resistant carbides by doping

75 76 77 78 79

with titanium,‘ boron,‘ ’ 7 beryllium,‘ and silicon.’ Comprehensive reviews can be found for the chemi­cal erosion of graphite46,47,55,56,80 doped graphite by hydrogen,62 and also an article on the surface treat­ment of a graphite wall by Winter.75