4. Conclusion

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

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

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

, He+

, and Li<sup>+</sup> at low energies and

decrease the sputtering.

direction of li-sputtering increase.

3.3. Secondary ion sputtering fraction in lithium sputtering

62 Plasma Science and Technology - Basic Fundamentals and Modern Applications

measured for lithium sputtering by bombardment of D<sup>+</sup>

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 and without D-saturation of the surfaces.

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