CHEMICAL EROSION In contrast to physical sputtering and RES, the

chemical erosion of carbon depends strongly on the system of the implanted particle and the target material. Chemical sputtering has been observed only in systems where volatile molecules such as CO, C0² or C^xH^y are formed owing to interactions of projectile ions and carbon atoms. Desorption of molecules with thermal energies was recently con-firmed by Vietzke et al. [35] for the case of CD³ molecule formation under simultaneous D° and Ar +

impact.

However, for the mechanism of molecule formation, no comprehensive model exists which can describe all experimental data. For the interaction of hydrogen isotopes with graphite, a model was proposed [36]

which explains the occurrence of a peak in the erosion yield as a function of temperature. The release of hydrogen under steady state conditions competes with surface reactions for the formation of CH4 molecules and H2 recombination. This model predicts a flux dependence of the yield as 1/r at high incident fluxes.

It has been modified on several occasions to include the strong effect on the incident ion energy [37] or the simultaneous erosion due to energetic hydrogen ions and thermal hydrogen atoms [8, 38]. The semi-empirical model for synergistic methane formation [8]

leads to a reasonably good prediction of the temperature dependence and the flux dependence (including a shift in Tm for different fluxes) for the impact of H° only and H+ only, for fluxes of < 3 x 10'5 cm"2-s"'.

A reasonably good prediction of the synergistic factor (see below) was also obtained for the case of combined H°-H+ impact. However, none of the models takes into account that it is not only CH4 which is formed

10

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-1

-2

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~

-i

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'-^/"7V x .

^7 /1' ^

'100,'/ ' / \ : / / 3 0 0 / / \ \

1 \ *50 / / 20 1000 /

1000

1 1 , . 1

10 '

-300 600 900

TEMPERATURE (K)

FIG. 9. Yields of CH4 and CD4 as a function of the surface temperature for different ion energies. The data for D (solid lines) are from Ref [42] and those for H (dashed lines) are from Refs [47, 49].

but that also heavier hydrocarbon molecules are formed, which even dominate at ion energies below 300 eV [39-41]. No model describes the broadening of the temperature dependence at low ion energies, which leads to chemical erosion already at room temperature for energies below 100 eV [42]. It has been shown that H² recombination [43, 44], CH⁴ formation [44, 45] and even formation of heavier hydrocarbons [46] occur at the end of the ion range rather than at the surface.

The low energy yield at room temperature points to a possible diffusive release of volatile molecules.

Further, the chemical reaction yield is not proportional to 1/r, and no model describes the observed maximum yield at fluxes around 1 x 10'⁵ atoms-cm"²-s"¹ and the slow decrease towards higher fluxes [8, 47, 48]

(see Fig. 7). Thus, no model is available at present for reliable extrapolation of laboratory data to fusion reac-tor relevant fluxes higher than 10^l8 atoms • cm"²-s"'.

However, an extensive database exists for most of these reactions in certain parameter ranges. The main problem in the past was to extrapolate these data to the plasma edge conditions of a fusion device, i.e. the erosion yield of carbon under high hydrogen fluxes around 10'⁸ H atoms • cm"²-s"¹, at impact energies below 100 eV. Furthermore, the erosion due to simultaneous impact of carbon, oxygen and hydrogen may lead to combined effects.

For the most part, these questions have been answered. Here, we discuss only those results which are important for application in the field of plasma-wall interactions and which have not been included in the earlier compendium [1].

4.1. Carbon erosion by hydrogen

A typical set of graphs of the temperature depen-dence of methane formation on graphite at medium fluxes (10l6 H + -cm2 • s"1) is shown in Fig. 9 [42,47,49].

The temperature dependence of the formation of other hydrocarbons is in most cases very similar to that of methane formation (see below). The methane yield has a maximum around 800 K. For 300 eV ions, it reaches a value of nearly 0.1 CH4/H and 0.15 CD4/D. With decreasing energy, the maximum yields decrease and the temperature dependence becomes broader. A similar broadening of the temperature dependence at low ener-gies ( < 100 eV) and high fluxes (1018 D+-cm"2-s"') was also observed in plasma experiments in TEXTOR [50] and in PISCES [51]. While there is no clear threshold for chemical sputtering as a function of H +

ion energy, it appears that there are changes in the dominating reaction mechanisms for energies near and below 100 eV/H+ [47]. From the model proposing that methane formation occurs at the end of the ion

implan-10°

10 ^-3

T 1 I I I l l l | 1 1 I I 1 I I I | 1 1 I I I 114;

820 K

300K

I i i l l m l i i t i t l 11 -I I I I I I I I

10 ' 10 10u

En. ENERGY (keV)

10'

FIG. 10. Energy dependence of the total sputtering yield of graphite. The data are from mass loss measurements at room temperature and the maximum temperature for chemical sputtering for H (M, D) and D ( • , o) ions [52, 83]. The curves show the analytic fit obtained with Eqs (7) to (9).

tation zone [43-45, 50] we conclude that for the high energy cases the rate at which CH4 leaves the surface may be controlled by molecular diffusion.

Roth and Bohdansky [42, 52] have investigated the total erosion process down to hydrogen energies of 20 eV, in an ion beam experiment at medium fluxes, as shown in Fig. 10. The energy dependence at the maximum of chemical sputtering is very similar for H and D ions and indicates a dependence on the energy which the projectile ions deposit near the surface in the form of broken C-C bonds [6, 37]. This might also explain the difference in the absolute yields due to H and D ions.

Because of the increased broadening of the tempera-ture dependence of carbon erosion at low ion energies, an almost energy independent erosion yield dominated by hydrocarbon formation is obtained at room tempera-ture and for ion energies in the range below 80 eV [42]. Therefore, a decrease of the ion impact energy in the range below 100 eV has only a minor effect on the total erosion yield. At high ion fluxes the broadening of the temperature dependence is enhanced and an energy independent yield is obtained below 200 eV [48]. In addition, a remarkable isotope effect is observed in the chemical yields for hydrogen and deuterium impact; this effect is not yet understood.

The flux dependence of the chemical sputtering yield, collected from various ion beam and plasma experiments, is given in Fig. 7, which also includes the RES data discussed in Section 3. An extrapolation of the data for fluxes of > 10^l6 H-cm~²-s~' indicates a gradual decrease for higher ion fluxes. It has been suggested [36] that the chemical erosion of graphite is drastically suppressed at high ion fluxes. This is not observed in the PISCES facility [30, 48, 51], where the erosion of graphite by hydrogen plasma was investigated at low energies and high fluxes (100 eV,

10'8 H-cm~2-s~'). The temperature dependence of the total yield is similar to that for lower fluxes as obtained with ion beams. The total yield is reduced only by a factor of two to three at high fluxes (10'⁸ H-cm"²-s"') compared to medium fluxes (10¹⁶ H-cnr²-s-') (see Fig. 7).

Similar results were also obtained by in situ measurements at high fluxes (10i8 cm~2-s~') in DITE [53] and TEXTOR [50]. The yields were also reduced by a factor of about two to three relative to the yields at medium flux densities. In the TEXTOR sniffer probe experiments, at room temperature the erosion yield of hydrogen plasma on graphite decreased with increasing ion energy (75-275 eV), whereas at the peak temperature the yield increased with increasing

energy. An isotope effect similar to that shown in Fig. 10 was also observed in these experiments.

The dependence of chemical erosion on ion energy, surface temperature and ion flux is tentatively

described [54] by the following equations:

*chem (EQ, 1) ~ 6 x l 0^{1 9}e x p (-1 eV/kT) l x l O1 5 + 3 x l 02 7 exp (-2 eV/kT)

x (200 QSⁿ + 1000 Y(EQ))

10¹ (8)

where Q, Sn and Y(E0) are taken from physical sput-tering (Eq. (1)) and T is given in ions • cm"2-s"'. To limit Ychem to values below 0.25, which cannot be exceeded if methane formation dominates, the addi-tional rule

Y' =

(1 + Y^chem/0.25) (9)

applies. The broadening of the temperature dependence at low energies has been taken into account rather crudely by the additional condition for T < 900 K and E0 < 100 eV:

Ychem — m a x {Ychem' Yl o w} (10) The values of Yi^ow for H and D as well as the

estimated value for T are given in Table I.

The predicted results from this formula are introduced in Figs 7 and 10 for the flux and energy dependences of graphite erosion, respectively.

The chemical erosion yield is only partly due to the formation of methane molecules. Systematic studies of heavier hydrocarbons due to H+ impact [39-41] and H° impact [41, 55] on carbon have been published recently. Figure 11 shows the energy dependence of the contribution of CHX, E C2Hi and E C3Hi to the total chemical erosion as a function of the impinging hydrogen ion energy at the peak temperature. The energy dependence of the product formation exhibits a similar tendency; with decreasing energy, the ratios E C²Hi/CH⁴ and E CjHj/CR, increase. The absolute yields obtained by different authors agree reasonably well, with the exception of the E C2Hi formation, especially the C2H2 component; this is the main reason for the indicated uncertainty in the total chemical erosion yield. The reason for the discre-pancy is unknown, since all data were obtained for

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3£C3Hn

-_J 'I I I i i m i l I i l l m i l | I 1 I i i n n 10"J 10~Z 10"'

ION ENERGY (keV)

10 u 101

FIG. 11. Energy dependence of the contribution of different hydrocarbon molecules to the total erosion of graphite at the maximum temperature for chemical sputtering. Open points [41. 102], half filled points [39], solid points [40]. Data for thermal energies: Open points [41, 102], boxes [55].

pyrolytic graphite at similar ion fluxes of about 10^l5 to 10l6 atoms • cm-2-s"1. Recently, the studies on the distribution of molecular species have been extended over a wide range of H ⁺ fluences [56], indicating that the C²H² contribution increases with increasing ion fluence. This gives the first hint for the origin of the wide range of the C2H2 contribution reported. In all studies, the energy at which the maximum erosion yield occurs decreases with increasing carbon content of the hydrocarbon molecule.

Here again, we have some evidence of the existence of different controlling mechanisms for low and high energies. This observation appears to be consistent with the model discussed in Ref. [50]. If we assume that the implanted energetic hydrogen reacts only after thermalization, hydrocarbon formation should be independent of the H⁺ ion impact energy. Only the depth at which the reaction occurs is different. There-fore, for high energy impact, the larger hydrocarbons formed in deeper subsurface layers (compared with CH⁴) may be prevented from penetrating to the outer surface. For relatively low energy bombardment, on the other hand, the implantation zone is very near the outer surface and, thus, heavy hydrocarbons formed there are more readily released [50].

While molecular hydrogen does not react with graphite, the chemical reaction between atomic

hydrogen and graphite occurs also at thermal hydrogen energies in the absence of radiation induced displace-ments. The formation of hydrocarbons due to thermal

H° impact on different types of graphite has been studied extensively [8, 44, 57-59].

Thermal atomic hydrogen reacts with carbon materials, forming the radical CH3 and a wide spectrum of higher volatile hydrocarbons. At temperatures up to 500 K, the hydrocarbon spectrum (not the yield) depends only slightly on the temperature. Above 500 K, the lighter hydrocarbons dominate [41, 55];. Balooch and Olander [60] reported a special branch of C2H2 production above 1200 K. This result has not been reproduced by other investigators. The different results on heavier hydrocarbon formation due to the reaction of thermal atomic hydrogen are included in Fig. 11 [41, 55]; see also Ref. [102]. Since for carbon erosion the number of carbon atoms in each molecule has to be taken into account, the chemical erosion of carbon by H° is dominated by formation of C2HX and C3HX molecules. The total sputtering yield shown in Fig. 11, which is about two orders of magnitude smaller than that for energetic ions, is the yield for annealed graphite. The sputtering yield from a surface activated by prebombardment or from deposited a-C:H layers is increased and approaches the yield for energetic ions [61-63]. Using thermal tritium and protium atoms, no isotopic effects were observed in the temperature dependence of hydrocarbon forma-tion [64].

4.2. Carbon erosion by oxygen

Apart from its application in fusion, carbon is widely used in other high temperature applications, for example by the aerospace industry. Typically, the atmosphere of interest is that of air, and the primary reactions involve oxygen. Therefore, the literature gives extensive data on the erosion of graphite due to oxidation; results for the reaction of carbon with molecular and thermal atomic oxygen have been published in Ref. [1].

Relatively low reaction rates are observed for molecular oxygen at temperatures below 1000 K, and the measured sputtering yields depend on the type of graphite or on the surface orientation of pyrolytic graphite. The main reaction product is CO; the C02

yields are about two orders of magnitude lower. The reactivity of atomic oxygen with graphite is much higher, even at lower temperatures [33, 65].

Since the publication of the previous data compen-dium [1] in 1984, carbon erosion due to energetic oxygen ion impact has been studied in some detail [31, 33, 66-70]. In general, no pronounced tempera-ture dependence of the erosion yield is observed. The

_ 10'

ION ENERGY (keV)

FIG. 12. Energy dependence of the total sputtering yield of graphite at room temperature bombarded with oxygen ions and of the reaction yield for various molecular species. The data are taken from Refs [31, 33, 66-69].

energy dependence of the reaction yield and of the released molecular species is shown in Fig. 12 for measurements at room temperature. When energetic oxygen impinges on carbon, the implanted oxygen is retained in the carbon until the oxygen concentration saturates. The saturation concentration decreases with increasing carbon temperature and decreasing impact energy, similar to the results for hydrogen impact [33, 71]. When saturation is reached, all the implanted oxygen reacts to form CO or C0². The total yield is about unity, independent of the incident energy, as can be seen in Fig. 12. The energies of the released molecules can be assumed to be thermal, while the sputtered carbon atoms have energies in the 5 eV range [14].