Net-erosion

The ratio Rero = rCTO// i..p determines whether we have a net-erosion or a net-deposition zone... 

Fig. 1.7. Twin-limiter made of graphite und tungsten showing net-erosion and net-deposition of C on W after exposure to TEXTOR...

The conditions where net erosion turns into net deposition for carbon irradiation may further be complicated by effects of surface temperature, influencing the self-sputtering yield by radiation-enhanced sublimation (RES) as well as the diffusion and surface segregation of implanted impurities. An example is introduced in Fig. 9.4b, where the erosion of W due to C+ bombardment... 

Deposition in the remote areas of the ITER divertor (see Fig. 12.2) is expected to occur because of the sticking of hydrocarbon radicals produced directly in areas of net erosion near the strike points and transported across the low-temperature plasma (few eV) at the divertor leg or from the interaction and transformation of species with the plasma. 

Plasma conditions and wall materials must also enable a sufficient lifetime of the first wall components for economic reasons. Chemical erosion of graphite leads to significant erosion yields even under low-temperature, cold plasma conditions and can seriously limit the lifetime. Since the tokamak is a fairly closed system, most of the eroded material will be re-deposited somewhere inside the machine. The question of tritium retention and overall inventory in the device is closely connected to the chemical erosion and to possible co-deposition as well [6,7]. In order to minimize the net-erosion and optimize the lifetime of wall components, the re-deposition should be concentrated in areas of major erosion. Another way to minimize chemical erosion is the use of mixed materials, which - in laboratory experiments - display a reduced erosion yield in comparison to pure graphite. 

Multiple techniques exist to measure sediment erodibility, each with advantages, disadvantages, and potential artifacts. The devices that have been used include annular flumes, Sedfiume, and straight flumes. Annualar flames, which can be employed either in situ or ex situ, apply rotation of an overlying water column to a bed of in-place or reconstituted sediments in a closed circular system. Because they are closed systems, potential entrance and entry effects are avoided. Prior to the development of Sedfiume, the annular flume was the leading method of erodibility and critical shear stress measurement for sediment transport studies (Lick et al., 1995). At each velocity and associated shear stress, an experiment was run to establish a steady-state suspended solids concentration in the water column from which net erosion could be inferred. With the annular flume, there is a finite amount of net erosion at any given shear stress. 

With the annular flume, the net erosion at any given shear stress is interpreted as an event indicative of steady state. The annular flume provides little or no direct information on the rate of erosion. The annular flume provides information on critical shear stress, but use of the annular flume is limited to shear stresses less than about lOdyn/cm. Higher shear stresses result in preferential erosion near the outer wall and a buildup of sediments near the inner wall, thereby tilting the bed surface and further affecting erosion within the device (Lick, 2009). Additional limitations of the device are that it measures only suspended load and can only be used to measure near-surface erosion. 

The EEDC and SEDZLJ models are based on similar theory for cohesive and nonco-hesive sediment transport and representation of the sediment bed. Both models have the capability to simulate multiple cohesive and noncohesive particle size classes, as well as bed load transport and suspended load transport. There are some key differences between how these models implement sediment transport, however. EFDC calculates net erosion or deposition for a given particle size class at a given simulation timestep. In the case of noncohesive sediments, the net rate of water-bed sediment exchange is defined by the suspended and bed load transport capacities of the water... 

Long-term changes in sediment bed elevations and bed surface particle size distributions, indicating time trends of net deposition, net erosion, or dynamic equilibrium. 

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