Chapter 5
Erosion and Deposition, and Their
Influences on Plasma Behavior (Material
Transport in Tokamak)
5.1 Introduction
Very high photon and particle fluxes to plasma-facing surfaces (PFS) in a fusion
reactor cause materials’ erosion like physical and chemical sputtering and sublimation. The eroded materials which are mostly neutral particles enter into boundary
plasma. Subsequently, they are ionized and gyrated along magnetic field lines. Since
in the boundary plasma, the magnetic field lines attach to PFS with tiny contact
angles, most of the gyrated ions return to the surface to make deposited layers close
to the eroded location. (referred as prompt deposition). Therefore, direct long-range
transfer of the eroded materials does not likely occur, but repetitive processes of
erosion and deposition transport the eroded materials in long distance as shown in
Fig. 5.1 in large tokamaks. In present divertor tokamaks, the inner first wall and the
outer divertor are mostly eroded, while the inner divertor is deposited. The outer first
wall is either eroded or deposited depending on location. In tile gaps particularly for
surface eroded tiles, some of the eroded materials penetrate to make deposited layers.
In carbon wall tokamaks, significant deposition appears on locations not directly
facing to plasma but on the line of sight such as divertor opening for pumping as
shown in Fig. 5.1.
Figure 5.2 is an example of detailed erosion and deposition profiles of CFC
tiles used in the W-shaped divertor of JT-60U [1]. Because plasma-facing materials
(PFM) are subjected to boundary plasma, erosion and deposition simultaneously
occur. Hence, net erosion and gross erosion can be distinguished. In this particular
case, deposition on the inner divertor is larger than the erosion at the outer divertor.
However, the mass balance between the erosion of the outer divertor and deposition
at the inner divertor was missed with excess deposition at the inner divertor, which
was attributed to the erosion of the first wall.
Erosion/deposition on PFS (first wall and divertor) are heterogeneous in both
poloidal and toroidal directions. Without making a detailed investigation of
erosion/deposition profiles on full PFS in tokamaks, the unbalance between the
erosion and deposition is hard to understand.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
T. Tanabe, Plasma-Material Interactions in a Controlled Fusion Reactor, Springer Series
in Plasma Science and Technology, https://doi.org/10.1007/978-981-16-0328-0_5
75
Erosion and Deposition, and Their
Influences on Plasma Behavior (Material
Transport in Tokamak)
5.1 Introduction
Very high photon and particle fluxes to plasma-facing surfaces (PFS) in a fusion
reactor cause materials’ erosion like physical and chemical sputtering and sublimation. The eroded materials which are mostly neutral particles enter into boundary
plasma. Subsequently, they are ionized and gyrated along magnetic field lines. Since
in the boundary plasma, the magnetic field lines attach to PFS with tiny contact
angles, most of the gyrated ions return to the surface to make deposited layers close
to the eroded location. (referred as prompt deposition). Therefore, direct long-range
transfer of the eroded materials does not likely occur, but repetitive processes of
erosion and deposition transport the eroded materials in long distance as shown in
Fig. 5.1 in large tokamaks. In present divertor tokamaks, the inner first wall and the
outer divertor are mostly eroded, while the inner divertor is deposited. The outer first
wall is either eroded or deposited depending on location. In tile gaps particularly for
surface eroded tiles, some of the eroded materials penetrate to make deposited layers.
In carbon wall tokamaks, significant deposition appears on locations not directly
facing to plasma but on the line of sight such as divertor opening for pumping as
shown in Fig. 5.1.
Figure 5.2 is an example of detailed erosion and deposition profiles of CFC
tiles used in the W-shaped divertor of JT-60U [1]. Because plasma-facing materials
(PFM) are subjected to boundary plasma, erosion and deposition simultaneously
occur. Hence, net erosion and gross erosion can be distinguished. In this particular
case, deposition on the inner divertor is larger than the erosion at the outer divertor.
However, the mass balance between the erosion of the outer divertor and deposition
at the inner divertor was missed with excess deposition at the inner divertor, which
was attributed to the erosion of the first wall.
Erosion/deposition on PFS (first wall and divertor) are heterogeneous in both
poloidal and toroidal directions. Without making a detailed investigation of
erosion/deposition profiles on full PFS in tokamaks, the unbalance between the
erosion and deposition is hard to understand.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
T. Tanabe, Plasma-Material Interactions in a Controlled Fusion Reactor, Springer Series
in Plasma Science and Technology, https://doi.org/10.1007/978-981-16-0328-0_5
75