5.1 Introduction
77
This chapter focuses on erosion, deposition, and transport of eroded materials in
tokamaks with separation of carbon and metallic walls.
5.2 Erosion, Transport, and Deposition
Material erosion of PFS is caused by either physical sputtering, chemical sputtering,
radiation-enhanced sublimation, thermal sublimation, or simply evaporation. The
erosion rate changes significantly depending on masses of both incident particles
and PFM, and PFM temperature. Depending on its mechanism, sputter erosion is
divided into physical and chemistry sputtering. In the former, sputtered particles
have energy higher than a few eV given by the incident particles and secondary
particles produced by multi-collisional processes. While the energy of chemically
sputtered particles is lower than a few eV or their temperature is near to the surface
temperature.
In general, sputtering by hydrogen is small and most of the physical sputtering
is caused by impurity ions in plasma, like C and O. Sputtering by seeded ions such
as N, Ne, and Ar introduced for edge cooling is concerned. In addition, once PFM
are eroded, the eroded materials penetrate in boundary plasma and contribute to
sputtering of PFM (self-sputtering), which was concerned with the utilization of
high Z materials as PFM. For C and Be used as PFM, chemical sputtering results in
significant erosion of PFM which could limit their lifetimes.
Energies and species of eroded materials (either by physical or chemical sputtering, radiation-enhanced sublimation, or evaporation) are distributed in wide
ranges. However, except those released with very high energy and penetrating deep
into the plasma, most of the released particles are neutrals and they are subsequently
ionized in the boundary plasma. After the ionization, owing to the strong magnetic
field in a reactor, they gyrate along magnetic field lines and mostly returned to PFS
as schematically shown in Fig. 5.3a. Since the gyro-radius of heavier ions is larger
than that of lighter ones, the heavier ions are easier to be deposited resulting in less
net erosion. Figure 5.3b shows trajectories of 50 typical sputtered Mo in divertor
computed by WBC code; for particles launched near the middle of ALCATOR CMOD outer vertical divertor made of Mo, for 800 kA shot in ohmic (OH) phase
[2]. As seen in the figure, since the angle of the magnetic field line to the plasmafacing surface is very shallow, most of the ions return to the surface to be deposited.
This is referred to as “prompt deposition”. With the increasing ionization state from
Mo
+ to Mo
3+ , the gyro-radius becomes smaller and transported longer, as seen in
Fig. 5.3b. In addition, because sputtering of heavier or higher Z materials is less than
that of lighter materials, their total erosion is much less than that of lighter or lower
Z materials.
Once deposited somewhere on PFS, the deposits are subjected to plasma again.
Depending on the locations of the deposits, either eroded area exposed higher incident particle flux or deposited area exposed less particle flux, they are re-eroded
77
This chapter focuses on erosion, deposition, and transport of eroded materials in
tokamaks with separation of carbon and metallic walls.
5.2 Erosion, Transport, and Deposition
Material erosion of PFS is caused by either physical sputtering, chemical sputtering,
radiation-enhanced sublimation, thermal sublimation, or simply evaporation. The
erosion rate changes significantly depending on masses of both incident particles
and PFM, and PFM temperature. Depending on its mechanism, sputter erosion is
divided into physical and chemistry sputtering. In the former, sputtered particles
have energy higher than a few eV given by the incident particles and secondary
particles produced by multi-collisional processes. While the energy of chemically
sputtered particles is lower than a few eV or their temperature is near to the surface
temperature.
In general, sputtering by hydrogen is small and most of the physical sputtering
is caused by impurity ions in plasma, like C and O. Sputtering by seeded ions such
as N, Ne, and Ar introduced for edge cooling is concerned. In addition, once PFM
are eroded, the eroded materials penetrate in boundary plasma and contribute to
sputtering of PFM (self-sputtering), which was concerned with the utilization of
high Z materials as PFM. For C and Be used as PFM, chemical sputtering results in
significant erosion of PFM which could limit their lifetimes.
Energies and species of eroded materials (either by physical or chemical sputtering, radiation-enhanced sublimation, or evaporation) are distributed in wide
ranges. However, except those released with very high energy and penetrating deep
into the plasma, most of the released particles are neutrals and they are subsequently
ionized in the boundary plasma. After the ionization, owing to the strong magnetic
field in a reactor, they gyrate along magnetic field lines and mostly returned to PFS
as schematically shown in Fig. 5.3a. Since the gyro-radius of heavier ions is larger
than that of lighter ones, the heavier ions are easier to be deposited resulting in less
net erosion. Figure 5.3b shows trajectories of 50 typical sputtered Mo in divertor
computed by WBC code; for particles launched near the middle of ALCATOR CMOD outer vertical divertor made of Mo, for 800 kA shot in ohmic (OH) phase
[2]. As seen in the figure, since the angle of the magnetic field line to the plasmafacing surface is very shallow, most of the ions return to the surface to be deposited.
This is referred to as “prompt deposition”. With the increasing ionization state from
Mo
+ to Mo
3+ , the gyro-radius becomes smaller and transported longer, as seen in
Fig. 5.3b. In addition, because sputtering of heavier or higher Z materials is less than
that of lighter materials, their total erosion is much less than that of lighter or lower
Z materials.
Once deposited somewhere on PFS, the deposits are subjected to plasma again.
Depending on the locations of the deposits, either eroded area exposed higher incident particle flux or deposited area exposed less particle flux, they are re-eroded