Physical sputtering results from the elastic transfer of energy from incoming projectiles to atoms on the surface of the target material. Target atoms can be sputtered when the energy they receive from the collisional cascade of the projectile exceeds the bind­ing energy of the atom to the surface. The physical sputtering rate is usually referred to as the sputtering yield, Y, which is defined as the ratio of the number of atoms lost from a surface to the number of incident energetic particles striking the surface. The lower the binding energy of surface atoms, the larger the physical sputtering yield. As physical sputtering can be approximated using a series of binary collisions within the surface, it is relatively easy to estimate

the physical sputtering yield of given projectile-target scenarios. Monte-Carlo based simulation codes (such as transport of ions in matter (TRIM))53 have been used to generate extensive databases of sputtering yields based on incident particle angle, energy, and mass, for a variety of targets54 including beryllium.

Measurement ofthe physical sputtering yield from a beryllium surface is complicated by the natural affinity of beryllium for oxygen. A beryllium surface will quickly form a thin, stable, passivating oxide surface layer when exposed to atmosphere. In ion beam devices, it is possible to clean any oxides from the beryllium surface before a measurement and with careful control of the residual gas pressure, make the measurements before the oxide reforms on the surface and alters the measurement.55 It has also been shown that it is possible to deplete the beryllium surface of oxide by heating the sample to tempera­tures exceeding 500 °C, where the beryllium below the oxide can diffuse through the oxide to the surface,56 thereby allowing measurements on a clean beryllium surface. The comparison between the calcu­lated sputtering yield and measurements made using mass-selected, monoenergetic ion-beams devices impinging on clean beryllium surfaces is fairly good.57

Measurements of sputtering yields in plasma devices, however, are complicated by several factors. In plasma devices, the incident ions usually have a temperature distribution and may contain different charge state ions. Each different charge state ion will be accelerated to a different energy by the elec­trostatic sheath in the vicinity of the surface. When hydrogenic plasma interacts with a surface, one must also account for a distribution of molecular ions striking the surface. In the case of a deuterium plasma, for example, the distribution of molecular ions (D+, Dj, D3) must be taken into account as the incident molecule disassociates on impact with the surface and a Dj ion becomes equivalent to the bombardment of two deuterium particles with one-half the incident energy of the original Dj ion. Figure 3 shows the change to the calculated sputter­ing yield when one includes the effects of molecular ions in a plasma, compared to the calculated sputter­ing yield from pure D+ ion bombardment.

The trajectory of the incoming ions can also be altered by the presence of electrostatic and magnetic sheaths. Plasmas also contain varying amounts of impurity ions, originating either from PWIs in other locations of the device, or ionization of residual back­ground gas present in the device and these impurity ions, or simply neutral gas atoms, may interact with
the surface. Finally, the incident flux from the plasma is usually so large that the surface being investigated, and its morphology, becomes altered by the incident flux and a new surface exhibiting unique character­istics may result.

Nevertheless, the physical sputtering yield from beryllium surfaces exposed to plasma ion bombard­ment has been measured in several devices. Unfortu­nately, there is little consensus on the correct value of the physical sputtering yield. In JET, the largest confinement device to ever employ beryllium as a PFC sputtering yield measurements range from values far exceeding47 to values less58 than one would expect from the predictions of TRIM. In the Plasma Interaction with Surface Components Experimental Station B (PISCES-B) device, systematic experiments to measure the physical sputtering yield routinely show values less59-61 than those expected from TRIM. This difference is shown in Figure 3, where the energy dependence of the calculated yield is com­pared to experimental measurements.

Another primary difference between the condi­tions in an ion beam device and those encountered in a plasma device has to do with the neutral density near the surface being investigated. In an ion beam experiment, the background pressure is kept very low

Figure 3 Calculated sputtering yields from pure D+ bombardment at normal incidence compared to that calculated fora (0.25,0.47,0.28) mix of D+, Dj, and Dj; also shown is the measured yield from such a plasma.

so that the surface being probed maintains its clean properties. On the other hand, the incident flux in a plasma device is usually several orders of magnitude larger than in an ion beam device, ensuring that the surface remains clean because of the large incident flux. However, this plasma-facing surface undergoes not only energetic ion bombardment, but also bom­bardment by neutral atoms and molecules.

The neutral density in plasma generators is typi­cally on the order of 1020 m~3 (a few millitorr) which is necessary for breakdown of the plasma. The esti­mated neutral atom flux is approximately equal to the incident ion flux to the surface61 and it is often not possible to alter significantly this flux ratio. In the case of a beryllium surface which can form a hydride (see Section 4.19.3.1.3), the presence of adsorbed deuterium on the surface could affect the measured sputtering yield by decreasing the beryllium concen­tration at the surface and altering the binding energy of surface beryllium atoms.

Some evidence of this effect may be discerned in data from JET measurements of the beryllium sputtering yield. Two sets of sputtering yield mea­surements have been reported from JET; one from beryllium divertor plate measurements and the other from beryllium limiter measurements. In the divertor region, one expects a neutral density similar to that encountered in plasma generators (1020 m~3 or more) and the measured sputtering yield is lower than that predicted by TRIM calculations.58 When sputtering measurements are made on the limiter, where the neutral density is typically lower, the sputtering yield agrees with, or exceeds, the calculated value.47 Of course, other issues such as impurity layers on the divertor plate and angle ofincidence questions tend to confuse the results. However, the data sets from JET are consistent with the impact of neutral absorption on the beryllium plasma-facing surface.

Effects associated with plasma operation will need to be taken into account when predicting sputtering yields from different areas of confinement devices. In addition to the low-energy neutral atom flux and higher-energy charge exchange neutral flux, the impact of small impurity concentrations in the inci­dent plasma flux will also have a large impact on the expected sputtering yield. Some of the implications of the formation of a mixed-material surface are discussed in the next section and in Section 4.19.3.3.