3.1.2. High-Z material sputtering by light incident particles

High-Z material sputtering will be discussed for the cases of tin and tungsten sputtering. High-Z material sputtering for refractory materials such as tungsten is attractive due to its relatively low-sputtering yield and high-sputtering threshold. However, due to the plasma low tolerance for high-Z impurities due to radiation losses, impurity levels must remain low, <10<sup>4</sup> (ratio of densities) in fusion plasmas [11]. Other high-Z materials such as tin are attractive from the standpoint of low-sputtering yield, relatively high-sputtering threshold, high thermal conductivity, and the potential for tin to be used as a liquid plasma-facing material due to its low melting point and low vapor pressure. For experimental data at normal incidence, the empirical formula (Eq. (1)) by Yamamura et al. was used as in the case for beryllium.

adjusted to 45 incidence for comparison [11]. TRIM-SP simulations were done for tungsten with a surface binding energy equal to its heat of sublimation of 8.68 eV. A mass density of

incidence data are adjusted to 45 using Yamamura's formula (Eq. (1)) for oblique particle incidence.

Figure 4. (a) Experimental and VFTRIM-3D simulation of tin sputtering by D, he, and Sn incident particles at 45 incidence. Normal incidence data are adjusted to 45 using Yamamura's formula (Eq. (1)) for oblique particle incidence. (b) Experimental and VFTRIM-3D simulation of tungsten sputtering by D, he, and self-ions at 45 incidence. Normal

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The VFTRIM-3D simulation predicts the experimental data reasonably well within the error bars. Helium bombardment shows a lower-sputtering threshold and larger yields compared to deuterium bombardment as expected from a lower mass ratio and a better energy transfer. Although the data are not shown for larger energies than 3 keV, the maximum tungstensputtering yield is expected at a lower incident energy for deuterium bombardment than for helium bombardment. This is due to the longer range of deuterium atoms in tungsten compared to that of incident helium, depositing less energy near the surface and thus turning the

The sputtering yield of lithium and beryllium decreases with deuterium saturation of the surface. This is due to preferential sputtering of deuterium atoms over lithium or beryllium atoms when bombarded by incident energetic particles. In the case of deuterium treatment for beryllium target, an extensive review has been presented in previous work and is only referenced here [10, 24, 27]. The net effect of embedded deuterium atoms is the effective reduction of the beryllium and lithium-sputtering yield as demonstrated by VFTRIM-3D simulations, shown in Figure 5a for deuterium bombardment. The simulations maintained the surface binding energy fixed at 3.38 eV. The level of deuterium saturation is that described earlier with a D/Be ratio of 0.33. For lithium sputtering, deuterium saturation is modeled with a D/Li ratio of 0.5 as discussed earlier. The lithium surface binding energy is kept fixed at

19.3 g/cm3 was used in the simulation as well.

1.68 eV.

sputtering yield curve at a lower incident energy than helium.

3.1.3. Effect of deuterium saturation on lithium and beryllium sputtering

Figure 4a shows the results for TRIM-SP simulation of tin sputtering. The VFTRIM-3D simulations are done for a surface binding energy equal to the heat of sublimation of tin, 3.12 eV. The data presented are as expected with helium bombardment, leading to a larger tin sputtering than deuterium bombardment due to an effective energy transfer. A maximum for deuterium bombardment is reached at a slightly lower incident energy than for helium bombardment. The argument for when this maximum yield is reached is the same as for tungsten, noting that in addition for heavy materials, the penetration depths of deuterium and helium at low energies will be quite similar and thus their maxima remain close. Tin shows promise, in that its sputtering yield at energies ranging from 100 to 400 eV is about a factor of five less than beryllium sputtering. However, one would have to contend with radiation losses from tin's high Z equal to 50.

Figure 4b shows the experimental and VFTRIM-3D simulation results for tungsten selfsputtering as well as tungsten sputtering from deuterium and helium bombardment. The experimental data are for normal incidence taken by Eckstein et al. at low energy and has been

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deuterium at room temperature, a surface composed of a D/Be ratio of 0.33 was used based on saturation experiments [33]. VFTRIM-3D simulations use a vectorized version of TRIM-SP known as TRVMC, which uses a binding energy of 1 eV for hydrogen isotopes [32] and beryllium's heat of sublimation. This binding energy was also utilized by TRIM-SP for

The data shown in Figure 3b show a maximum between 300 and 500 eV, closely resembling BeO data taken by Roth et al. [20]. Beryllium has a high affinity for oxygen at room temperature, thus the surface binding energy is effectively increased, reducing the sputtering yield. In addition, deuterium-treated surfaces effectively decrease beryllium sputtering due to preferential sputtering of embedded deuterium atoms. As a consequence, the beryllium sputtering yield from deuterium-treated surfaces measured in IIAX is predicted well by VFTRIM-3D simulations. TRIM-SP simulations do not account for deuterium treatment, and thus their yields are higher than anticipated, coincidently matching VFTRIM-3D results. If deuterium saturation was used by TRIM-SP modeling, beryllium sputtering would be effectively decreased, thus not predicting the experimental data in IIAX. The ability for VFTRIM-3D to effectively model surface roughness also leads to the high predictability of experimental data

High-Z material sputtering will be discussed for the cases of tin and tungsten sputtering. High-Z material sputtering for refractory materials such as tungsten is attractive due to its relatively low-sputtering yield and high-sputtering threshold. However, due to the plasma low tolerance for high-Z impurities due to radiation losses, impurity levels must remain low, <10<sup>4</sup> (ratio of densities) in fusion plasmas [11]. Other high-Z materials such as tin are attractive from the standpoint of low-sputtering yield, relatively high-sputtering threshold, high thermal conductivity, and the potential for tin to be used as a liquid plasma-facing material due to its low melting point and low vapor pressure. For experimental data at normal incidence, the empir-

Figure 4a shows the results for TRIM-SP simulation of tin sputtering. The VFTRIM-3D simulations are done for a surface binding energy equal to the heat of sublimation of tin, 3.12 eV. The data presented are as expected with helium bombardment, leading to a larger tin sputtering than deuterium bombardment due to an effective energy transfer. A maximum for deuterium bombardment is reached at a slightly lower incident energy than for helium bombardment. The argument for when this maximum yield is reached is the same as for tungsten, noting that in addition for heavy materials, the penetration depths of deuterium and helium at low energies will be quite similar and thus their maxima remain close. Tin shows promise, in that its sputtering yield at energies ranging from 100 to 400 eV is about a factor of five less than beryllium sputtering. However, one would have to contend with radiation losses from tin's

Figure 4b shows the experimental and VFTRIM-3D simulation results for tungsten selfsputtering as well as tungsten sputtering from deuterium and helium bombardment. The experimental data are for normal incidence taken by Eckstein et al. at low energy and has been

ical formula (Eq. (1)) by Yamamura et al. was used as in the case for beryllium.

consistency.

both in IIAX and from Roth et al. [20].

high Z equal to 50.

3.1.2. High-Z material sputtering by light incident particles

56 Plasma Science and Technology - Basic Fundamentals and Modern Applications

Figure 4. (a) Experimental and VFTRIM-3D simulation of tin sputtering by D, he, and Sn incident particles at 45 incidence. Normal incidence data are adjusted to 45 using Yamamura's formula (Eq. (1)) for oblique particle incidence. (b) Experimental and VFTRIM-3D simulation of tungsten sputtering by D, he, and self-ions at 45 incidence. Normal incidence data are adjusted to 45 using Yamamura's formula (Eq. (1)) for oblique particle incidence.

adjusted to 45 incidence for comparison [11]. TRIM-SP simulations were done for tungsten with a surface binding energy equal to its heat of sublimation of 8.68 eV. A mass density of 19.3 g/cm3 was used in the simulation as well.

The VFTRIM-3D simulation predicts the experimental data reasonably well within the error bars. Helium bombardment shows a lower-sputtering threshold and larger yields compared to deuterium bombardment as expected from a lower mass ratio and a better energy transfer. Although the data are not shown for larger energies than 3 keV, the maximum tungstensputtering yield is expected at a lower incident energy for deuterium bombardment than for helium bombardment. This is due to the longer range of deuterium atoms in tungsten compared to that of incident helium, depositing less energy near the surface and thus turning the sputtering yield curve at a lower incident energy than helium.

#### 3.1.3. Effect of deuterium saturation on lithium and beryllium sputtering

The sputtering yield of lithium and beryllium decreases with deuterium saturation of the surface. This is due to preferential sputtering of deuterium atoms over lithium or beryllium atoms when bombarded by incident energetic particles. In the case of deuterium treatment for beryllium target, an extensive review has been presented in previous work and is only referenced here [10, 24, 27]. The net effect of embedded deuterium atoms is the effective reduction of the beryllium and lithium-sputtering yield as demonstrated by VFTRIM-3D simulations, shown in Figure 5a for deuterium bombardment. The simulations maintained the surface binding energy fixed at 3.38 eV. The level of deuterium saturation is that described earlier with a D/Be ratio of 0.33. For lithium sputtering, deuterium saturation is modeled with a D/Li ratio of 0.5 as discussed earlier. The lithium surface binding energy is kept fixed at 1.68 eV.

The effect of deuterium saturation on beryllium sputtering is a bit stronger for helium bombardment (not shown here) than for deuterium bombardment. This is due to the effective energy transfer from the incoming deuterium to the embedded deuterium atoms in beryllium and lithium. This effect, however, is lessened with a lower amount of deuterium in the beryllium or lithium lattice. Therefore, the energy dependence for a given deuterium saturation level in a material cannot be simply determined from the energy dependence at 0% saturation by a constant multiplication. Similar results are found for lithium sputtering except for larger yields and a lower-sputtering threshold mostly due to a better energy transfer from deuterium and helium atoms to lithium target atoms.

the lightest component and for the least bound species. The deuterium is sputtered preferentially and the surface, in time, is enriched in lithium. However, at doses in IIAX and doses found in typical plasma-facing conditions in tokamaks, the one-to-one ratio of lithium matrix atoms and saturating-deuteride species are kept over at least the depth of origin of sputtered species as a constant flux of deuterium atoms impinges on the lithium sample and a source of

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Another factor is the competition between preferential sputtering on the one hand, and mixing or segregation on the other [37, 38]. These latter effects are less pronounced here since we have a surface that is "soaked" with deuterium atoms and not an alloy composed of deuterium and lithium constituents. Therefore, preferential sputtering mechanisms are justified as a viable interpretation. The binding of deuterium and lithium atoms is less likely than deuterium

Self-sputtering at the plasma boundary in fusion devices will occur for impurities, which have been injected into the plasma, are ionized, and return to strike solid surfaces at the wall or divertor regions. The momentum transfer between like masses is extremely effective due to maximum energy transfer, leading to increased sputtering and potentially diluting the plasma with impurities. There are two components in self-sputtering that must be considered. One is the erosion component due to physical sputtering and the other is the reflection of the incident particle into the plasma. Since both sources cannot be distinguished from each other, except their average particle energy, these sources must be added to obtain the total amount of particles injected into the plasma. Figure 6a shows the self-sputtering yield (sputtering +

Figure 6. (a) Self-sputtering experimental and simulation yields (total = reflection and sputtering for TRIM-SP simulations) of tungsten, tin, beryllium, and lithium at 45 incidence. Normal incidence data are adjusted to 45 using the Yamamura formula for oblique particle incidence. (b) VFTRIM-3D, TRIM-SP and experimental data of 2 keV D<sup>+</sup> bom-

bardment of graphite versus particle angle of incidence.

implanted deuterium atoms segregates to the surface over the time of dose [34–36].

atoms penetrating and sitting at interstitial sites in bcc lithium.

3.1.4. Self-sputtering

Figure 5b further shows the importance of deuterium treatment on the measured absolute sputtering yield of solid lithium (i.e., IIAX experiments). Figure 5b shows experimental and VFTRIM-3D simulation results for He<sup>+</sup> bombardment on D-treated and non-D-treated lithium at 45 incidence. The figure plots the energy dependence of the absolute sputtering yield of lithium in atoms per incident ion. The lithium-sputtering yield functional behavior of the non-D-treated lithium target is shifted toward a maximum at higher energies (~1000 eV) for one of the experimental cases. The VFTRIM-3D results begin to diverge the experimental data at energies above 500 eV. In addition, the computational model used for the non-D-treated data is based on a mechanism for channeling energy from subsurface layers to the top layer [23]. The simulation model used to predict the D-treated data does not utilize this mechanism. This result implies that the absence of deuterium atoms at interstitial sites of the lithium bcc lattice allows for atoms from deeper in the sputtering cascade to transfer their momentum up to surface layer atoms, thus contributing to more sputtering.

The D-treated lithium-sputtering yield is measured to be significantly lower than bombardment with no deuterium treatment. As explained earlier, preferential sputtering is expected for

Figure 5. (a) VFTRIM-3D simulation of deuterium sputtering of D-treated and non-D-treated lithium and beryllium at 45 incidence. Deuterium saturation levels for lithium are 50% D/li, and for beryllium, 33% D/be. These are levels of saturation, which mimic those expected from PMI interactions in magnetic fusion devices at the wall boundary. (b) Energy dependence of 45 incidence he<sup>+</sup> bombardment on non-D-treated and D-treated solid lithium measurements and VFTRIM-3D simulation.

the lightest component and for the least bound species. The deuterium is sputtered preferentially and the surface, in time, is enriched in lithium. However, at doses in IIAX and doses found in typical plasma-facing conditions in tokamaks, the one-to-one ratio of lithium matrix atoms and saturating-deuteride species are kept over at least the depth of origin of sputtered species as a constant flux of deuterium atoms impinges on the lithium sample and a source of implanted deuterium atoms segregates to the surface over the time of dose [34–36].

Another factor is the competition between preferential sputtering on the one hand, and mixing or segregation on the other [37, 38]. These latter effects are less pronounced here since we have a surface that is "soaked" with deuterium atoms and not an alloy composed of deuterium and lithium constituents. Therefore, preferential sputtering mechanisms are justified as a viable interpretation. The binding of deuterium and lithium atoms is less likely than deuterium atoms penetrating and sitting at interstitial sites in bcc lithium.

### 3.1.4. Self-sputtering

The effect of deuterium saturation on beryllium sputtering is a bit stronger for helium bombardment (not shown here) than for deuterium bombardment. This is due to the effective energy transfer from the incoming deuterium to the embedded deuterium atoms in beryllium and lithium. This effect, however, is lessened with a lower amount of deuterium in the beryllium or lithium lattice. Therefore, the energy dependence for a given deuterium saturation level in a material cannot be simply determined from the energy dependence at 0% saturation by a constant multiplication. Similar results are found for lithium sputtering except for larger yields and a lower-sputtering threshold mostly due to a better energy transfer from

Figure 5b further shows the importance of deuterium treatment on the measured absolute sputtering yield of solid lithium (i.e., IIAX experiments). Figure 5b shows experimental and VFTRIM-3D simulation results for He<sup>+</sup> bombardment on D-treated and non-D-treated lithium at 45 incidence. The figure plots the energy dependence of the absolute sputtering yield of lithium in atoms per incident ion. The lithium-sputtering yield functional behavior of the non-D-treated lithium target is shifted toward a maximum at higher energies (~1000 eV) for one of the experimental cases. The VFTRIM-3D results begin to diverge the experimental data at energies above 500 eV. In addition, the computational model used for the non-D-treated data is based on a mechanism for channeling energy from subsurface layers to the top layer [23]. The simulation model used to predict the D-treated data does not utilize this mechanism. This result implies that the absence of deuterium atoms at interstitial sites of the lithium bcc lattice allows for atoms from deeper in the sputtering cascade to transfer their momentum up to

The D-treated lithium-sputtering yield is measured to be significantly lower than bombardment with no deuterium treatment. As explained earlier, preferential sputtering is expected for

Figure 5. (a) VFTRIM-3D simulation of deuterium sputtering of D-treated and non-D-treated lithium and beryllium at 45 incidence. Deuterium saturation levels for lithium are 50% D/li, and for beryllium, 33% D/be. These are levels of saturation, which mimic those expected from PMI interactions in magnetic fusion devices at the wall boundary. (b) Energy dependence of 45 incidence he<sup>+</sup> bombardment on non-D-treated and D-treated solid lithium measurements and

deuterium and helium atoms to lithium target atoms.

58 Plasma Science and Technology - Basic Fundamentals and Modern Applications

surface layer atoms, thus contributing to more sputtering.

VFTRIM-3D simulation.

Self-sputtering at the plasma boundary in fusion devices will occur for impurities, which have been injected into the plasma, are ionized, and return to strike solid surfaces at the wall or divertor regions. The momentum transfer between like masses is extremely effective due to maximum energy transfer, leading to increased sputtering and potentially diluting the plasma with impurities. There are two components in self-sputtering that must be considered. One is the erosion component due to physical sputtering and the other is the reflection of the incident particle into the plasma. Since both sources cannot be distinguished from each other, except their average particle energy, these sources must be added to obtain the total amount of particles injected into the plasma. Figure 6a shows the self-sputtering yield (sputtering +

Figure 6. (a) Self-sputtering experimental and simulation yields (total = reflection and sputtering for TRIM-SP simulations) of tungsten, tin, beryllium, and lithium at 45 incidence. Normal incidence data are adjusted to 45 using the Yamamura formula for oblique particle incidence. (b) VFTRIM-3D, TRIM-SP and experimental data of 2 keV D<sup>+</sup> bombardment of graphite versus particle angle of incidence.

reflection) for the cases of tin, lithium, tungsten, and beryllium at 45 incidence. Normal incidence data are adjusted to 45 incidence by Yamamura's empirical formula (Eq. (1)) for oblique particle incidence.

data are predicted successfully for VFTRIM-3D modeling roughness. The scatter in the experimental data at high angles of incidence is due to a variety of roughness levels in the materials

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The lithium self-sputtering yield for deuterium-treated and non-deuterium-treated surfaces is shown in Figure 7b. The dependence of the lithium-sputtering yield on the angle of incidence is also shown for two incident energies, 100 and 500 eV. Due to the importance of selfsputtering runaway, a thick black (red) line indicates unity lithium-sputtering yield. Unity self-sputtering means the sputtered flux is greater than the incident-particle flux and thus the potential for a runaway condition, which results in enhanced amounts of target material being eroded, re-ionized, and re-deposited with an impact on fusion device operation. The dependence of the lithium-sputtering yield on the angle of incidence is strong for higher oblique angles. This is due to the decrease in the penetration length of the incident bombarding atom and consequently greater energy deposition near the surface, increasing the probability to sputter. For deuterium-treated surfaces, the lithium-sputtering yield is lower than nondeuterium-treated surfaces at all angles of incidence shown and the given incident energies. The lithium yield due to 500-eV incident lithium atoms on deuterium-treated lithium is slightly lower than the case for 100-eV lithium on non-deuterium-treated lithium. This is due to preferential sputtering of deuterium atoms playing a large role in reducing the lithiumsputtering yield even at higher energies. At 500 eV, lithium penetrates farther and thus the resulting lithium yield is lower. However, at larger angles of incidence, the energy deposition increases even for the case of deuterium-treated lithium, thus the yield crosses the low energy curve for non-deuterium-treated lithium. For the high-energy case with no deuterium coverage, we find that self-sputtering runaway is reached at a lower angle of incidence than for deuterium-treated lithium. This occurs at an angle of incidence of about 55, while with

Figure 7. (a) VFTRIM-3D, TRIM-SP, and experimental data of 1 keV be<sup>+</sup> bombardment of beryllium against nominal particle angle of incidence. (b) Self-sputtering yield from lithium surface at about 200C, for deuterium-treated and nondeuterium-treated lithium. TRIM-SP simulations are done for 45 incidence and 100 and 500 eV incident-particle energies.

as well as the creation of surface roughness in the course of ion bombardment.

Self-sputtering of tungsten and tin is simulated by VFTRIM-3D and compared with experimental data in Figure 6a. Due to the highly efficient transfer of energy process in selfsputtering, tungsten results in the largest yield still increasing at 3 keV. Tin self-sputtering [34–39] is shown to have a slightly lower-sputtering yield close to beryllium self-sputtering for energies up to 1 keV within experimental uncertainty. Beryllium self-sputtering is elaborated on in several papers [20–22]. The beryllium self-sputtering yield is found to have a maximum at about 1.2 keV. The data are predicted quite successfully as surface roughness is modeled by VFTRIM-3D. This is an important point that will be elaborated on in the next section regarding the dependence of the sputtering yield on the angle of incidence of the bombarding particle and the fact that in fusion devices PMI results in evolving surfaces that can roughen over the time scale of plasma-induced modification and operation.

Figure 6a also shows the experimental and computational results for lithium self-sputtering at 45 incidence. Computational data using TRIM-SP for incident-particle reflection are also included in the self-sputtering yield calculated. The experimental results are surprising. The self-sputtering yield of solid lithium maximizes at 700 eV to a value of 0.245 0.100 atoms/ion. This is considerably lower than the values predicted by László and Eckstein [40]. There are two main reasons why the calculated values by László and Eckstein are significantly greater than our measured results. The computational model used by J. László et al. used the TRIM-SP, which assumes a smooth surface and neglects surface roughness. Second, the model does not utilize a compositional component to incorporate the effect of deuterium implantation at interstitial sites of the lithium sample as discussed in the previous section.

Within the experimental error, the simulation predicts the functional behavior of the sputter yield, except at higher energies (E ≥ 800 eV) where the two begin to slightly diverge. This is due to a shorter mean range of the incident lithium ion in solid lithium compared to D or He. Thus, a large percentage of its kinetic energy is distributed among the top-most surface deuterium atoms, which lead to a preferentially larger deuterium erosion and a reduction of lithium sputtering. The fact that lithium self-sputtering is significantly reduced due to deuterium saturation of the surface is encouraging for its use as a potential plasma-facing component.

### 3.2. Sputtering yield dependence on angle of incidence

As discussed in the introductory sections of this review, the effect of roughness on the sputtering yield of materials is enhanced with an increase in the angle of incidence. Figure 6b shows the angle of incidence dependence of graphite sputtering by 2-keV incident deuterium ions. As the angle of incidence is increased, the effect of surface roughness is enhanced and thus simulation using TRIM-SP better predicts the data very well compared to using a smooth surface simulation with TRIM-SP [41]. Figure 7a demonstrates this effect for beryllium selfsputtering as well. Two sets of experimental data [10, 21] are plotted with VFTRIM-3D and TRIM-SP simulations for 1-keV beryllium self-sputtering. Again in this case, the experimental data are predicted successfully for VFTRIM-3D modeling roughness. The scatter in the experimental data at high angles of incidence is due to a variety of roughness levels in the materials as well as the creation of surface roughness in the course of ion bombardment.

reflection) for the cases of tin, lithium, tungsten, and beryllium at 45 incidence. Normal incidence data are adjusted to 45 incidence by Yamamura's empirical formula (Eq. (1)) for

Self-sputtering of tungsten and tin is simulated by VFTRIM-3D and compared with experimental data in Figure 6a. Due to the highly efficient transfer of energy process in selfsputtering, tungsten results in the largest yield still increasing at 3 keV. Tin self-sputtering [34–39] is shown to have a slightly lower-sputtering yield close to beryllium self-sputtering for energies up to 1 keV within experimental uncertainty. Beryllium self-sputtering is elaborated on in several papers [20–22]. The beryllium self-sputtering yield is found to have a maximum at about 1.2 keV. The data are predicted quite successfully as surface roughness is modeled by VFTRIM-3D. This is an important point that will be elaborated on in the next section regarding the dependence of the sputtering yield on the angle of incidence of the bombarding particle and the fact that in fusion devices PMI results in evolving surfaces that can roughen over the

Figure 6a also shows the experimental and computational results for lithium self-sputtering at 45 incidence. Computational data using TRIM-SP for incident-particle reflection are also included in the self-sputtering yield calculated. The experimental results are surprising. The self-sputtering yield of solid lithium maximizes at 700 eV to a value of 0.245 0.100 atoms/ion. This is considerably lower than the values predicted by László and Eckstein [40]. There are two main reasons why the calculated values by László and Eckstein are significantly greater than our measured results. The computational model used by J. László et al. used the TRIM-SP, which assumes a smooth surface and neglects surface roughness. Second, the model does not utilize a compositional component to incorporate the effect of deuterium implantation at

Within the experimental error, the simulation predicts the functional behavior of the sputter yield, except at higher energies (E ≥ 800 eV) where the two begin to slightly diverge. This is due to a shorter mean range of the incident lithium ion in solid lithium compared to D or He. Thus, a large percentage of its kinetic energy is distributed among the top-most surface deuterium atoms, which lead to a preferentially larger deuterium erosion and a reduction of lithium sputtering. The fact that lithium self-sputtering is significantly reduced due to deuterium saturation of the surface is encouraging for its use as a potential plasma-facing component.

As discussed in the introductory sections of this review, the effect of roughness on the sputtering yield of materials is enhanced with an increase in the angle of incidence. Figure 6b shows the angle of incidence dependence of graphite sputtering by 2-keV incident deuterium ions. As the angle of incidence is increased, the effect of surface roughness is enhanced and thus simulation using TRIM-SP better predicts the data very well compared to using a smooth surface simulation with TRIM-SP [41]. Figure 7a demonstrates this effect for beryllium selfsputtering as well. Two sets of experimental data [10, 21] are plotted with VFTRIM-3D and TRIM-SP simulations for 1-keV beryllium self-sputtering. Again in this case, the experimental

oblique particle incidence.

time scale of plasma-induced modification and operation.

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3.2. Sputtering yield dependence on angle of incidence

interstitial sites of the lithium sample as discussed in the previous section.

The lithium self-sputtering yield for deuterium-treated and non-deuterium-treated surfaces is shown in Figure 7b. The dependence of the lithium-sputtering yield on the angle of incidence is also shown for two incident energies, 100 and 500 eV. Due to the importance of selfsputtering runaway, a thick black (red) line indicates unity lithium-sputtering yield. Unity self-sputtering means the sputtered flux is greater than the incident-particle flux and thus the potential for a runaway condition, which results in enhanced amounts of target material being eroded, re-ionized, and re-deposited with an impact on fusion device operation. The dependence of the lithium-sputtering yield on the angle of incidence is strong for higher oblique angles. This is due to the decrease in the penetration length of the incident bombarding atom and consequently greater energy deposition near the surface, increasing the probability to sputter. For deuterium-treated surfaces, the lithium-sputtering yield is lower than nondeuterium-treated surfaces at all angles of incidence shown and the given incident energies.

The lithium yield due to 500-eV incident lithium atoms on deuterium-treated lithium is slightly lower than the case for 100-eV lithium on non-deuterium-treated lithium. This is due to preferential sputtering of deuterium atoms playing a large role in reducing the lithiumsputtering yield even at higher energies. At 500 eV, lithium penetrates farther and thus the resulting lithium yield is lower. However, at larger angles of incidence, the energy deposition increases even for the case of deuterium-treated lithium, thus the yield crosses the low energy curve for non-deuterium-treated lithium. For the high-energy case with no deuterium coverage, we find that self-sputtering runaway is reached at a lower angle of incidence than for deuterium-treated lithium. This occurs at an angle of incidence of about 55, while with

Figure 7. (a) VFTRIM-3D, TRIM-SP, and experimental data of 1 keV be<sup>+</sup> bombardment of beryllium against nominal particle angle of incidence. (b) Self-sputtering yield from lithium surface at about 200C, for deuterium-treated and nondeuterium-treated lithium. TRIM-SP simulations are done for 45 incidence and 100 and 500 eV incident-particle energies.

deuterium-treated surfaces, the lithium yield remains under unity up to about 70 incidence at 500-eV incident-particle energy. This has some important implications. If one can maintain a one-to-one deuterium to lithium coverage, self-sputtering runaway of lithium could be dramatically reduced even for incident particles at high energies. This is important since deuterium-absorbed lithium plasma-facing surfaces give rise to low-recycling plasma regimes at the edge [42]. The comparison of the sputtering yield dependence on the incident angle and differences of incident-particle energy are illustrated in Figure 8. Figure 8 shows the lithium sputter yield for both D-saturated and non-D-saturated surface conditions sputtered by 100 and 1000 eV D atoms. Notice that the enhancement with an incident angle decreases with a decreasing incident energy and in fact become equal at 60 incidence, which means that the energy deposition is predominant on the surface and only D saturation could significantly decrease the sputtering.

contribute to the sputtering yield. The secondary ion sputtering fraction does not vary significantly in the range of 500–1000 eV. The fraction of sputtered atoms in the ionic state is measured to be about 65% or two out of three sputtered atoms come out as ions. The dependence of the secondary ion fraction has been linked with the combination of chemical potential

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An introduction to plasma-material interactions in fusion devices was provided in this chapter. The effects of varying surface roughness were described and the use of fractal dimensions as a viable model for simulating PMI of realistic surfaces. Physical sputtering and particle reflection were selected as primary mechanisms of PMI. Simulations and experimental data of low-Z and high-Z materials were provided. Fusion relevant ion-surface interactions for candidate materials were presented. These included combinations of D, T, and He on Li, Be, C, Sn, and W with

Results show that surface roughness is an important effect that must be accounted for in reflection and sputtering measurements, especially at low-incident-particle energies and oblique incidence. Low-Z materials such as lithium and beryllium suffer from low-sputtering thresholds, however, maintaining fairly low self-sputtering yields. High-sputtering thresholds on the other hand characterize high-Z materials but maintain high self-sputtering yields even at low bombarding energies. Oblique incidence is important to consider due to the strong dependence of sputtering on the incident-particle angle. Deuterium saturation of low-Z materials such as lithium or beryllium effectively reduces sputtering. Moreover, lithium has a high

We acknowledge our funding sources DOE-OFES grant DE-SC0010717. To learn more about the Center for Plasma-Material Interactions and the Radiation Surface Science and Engineering Laboratory, visit our sites: (http://rssel.engineering.illinois.edu) and (http://cpmi.illinios.edu).

Department of Nuclear, Plasma and Radiological Engineering, Center for Plasma-Material

Interactions, University of Illinois at Urbana-Champaign, Urbana, IL, USA

secondary ion-sputtering fraction, thus leading to an even lower-sputtering yield.

and work function of a surface with alkali metals having the highest yields.

4. Conclusion

and without D-saturation of the surfaces.

Acknowledgements

Author details

Jean Paul Allain\* and David N. Ruzic

\*Address all correspondence to: allain@illinois.edu

#### 3.3. Secondary ion sputtering fraction in lithium sputtering

Another very important property in PMI is the surface charge density and the role of charge dynamics when sputtered atoms are released from the surface. The secondary ion sputtering fraction, defined as the fraction of ions to neutrals sputtered from the incident ions, has been measured for lithium sputtering by bombardment of D<sup>+</sup> , He+ , and Li<sup>+</sup> at low energies and oblique incidence [13]. Such a measurement is important since in a fusion device, plasmasputtered ions will immediately return to the surface due to the sheath potential and thus not

Figure 8. Lithium-sputtering yield versus angle of incidence using the Bohdansky-Sigmund-Yamamura (BSY) model. Open circles: 100 eV D on solid pure (100%) lithium; filled circles: 100 eV D on solid LiD (50% D-li); dashed line: 1 keV D on solid pure li; solid line: 1 keV D on solid LiD. As a deuterium-treated sample loses D near the li surface, the lithiumsputtering yield begins to increase and approaches the pure li yield. This is shown by the large arrow pointing in the direction of li-sputtering increase.

contribute to the sputtering yield. The secondary ion sputtering fraction does not vary significantly in the range of 500–1000 eV. The fraction of sputtered atoms in the ionic state is measured to be about 65% or two out of three sputtered atoms come out as ions. The dependence of the secondary ion fraction has been linked with the combination of chemical potential and work function of a surface with alkali metals having the highest yields.
