4.3 Emission of Ions and Neutrals
57
PFS are exposed to energetic particles escaping from plasma as well as residual gas
mostly composed of fuels (D 2 , DT, and T 2 ). When the energy of the impinging particles exceeds threshold energy to displace surface atoms, the surface atoms are sputtered to leave the surface, which is referred to as physical sputtering. The threshold
energy is larger for heavier surface atoms and the sputtering yield is larger for heavier
impinging particles. Since plasma always contains impurities like carbon (C), oxygen
(O), and helium (He) produced by D-T reactions, the physical sputtering by these
impurities is significantly larger than that by fuel ions. In Fig. 4.9 is compared the
energy dependence of physical sputtering yields of W by different kinds of ions [20].
One can clearly see the existence of a threshold energy in the physical sputtering
which is larger for higher Z materials. The threshold energy of W sputtering by T
ions is above 100 eV, which is one of the reasons to use W as PFM of divertor in a
fusion reactor. However, lower Z gases like N, Ne, and Ar injection for edge cooling
in a reactor would significantly reduce the threshold as seen in Fig. 4.9. Therefore,
the sputtering by the impurities is the main cause of both erosion of PFS and plasma
contamination by sputtered atoms. Physical sputtering yields change with the angle
of incident ions, showing higher values for lower incident angles; as expected nuclear
stopping region becomes shallower for low angle incidence.
The sputtered atoms are mostly neutral showing the Maxwell–Boltzmann-like
energy distribution with a maximum energy of a few eV. In boundary plasma (or
scrape-off layers) of a reactor, released atoms from PFM by sputtering are immediately ionized and succeedingly gyrated along magnetic field lines. Consequently,
most of them return to be deposited nearby the sputtered location (referred to as
prompt deposition) or transferred along the scrape-off layers to be deposited somewhere on the plasma-facing surface, shadowed area, and/or exhausted from the
reactor. Some of the ionized atoms penetrate deep into plasma and are confined
as impurity ions. Repetitive processes of erosion by sputtering and deposition cause
Fig. 4.9 Sputtering yields
for tungsten by hydrogen and
some impurities at normal
incidence angle [20]
57
PFS are exposed to energetic particles escaping from plasma as well as residual gas
mostly composed of fuels (D 2 , DT, and T 2 ). When the energy of the impinging particles exceeds threshold energy to displace surface atoms, the surface atoms are sputtered to leave the surface, which is referred to as physical sputtering. The threshold
energy is larger for heavier surface atoms and the sputtering yield is larger for heavier
impinging particles. Since plasma always contains impurities like carbon (C), oxygen
(O), and helium (He) produced by D-T reactions, the physical sputtering by these
impurities is significantly larger than that by fuel ions. In Fig. 4.9 is compared the
energy dependence of physical sputtering yields of W by different kinds of ions [20].
One can clearly see the existence of a threshold energy in the physical sputtering
which is larger for higher Z materials. The threshold energy of W sputtering by T
ions is above 100 eV, which is one of the reasons to use W as PFM of divertor in a
fusion reactor. However, lower Z gases like N, Ne, and Ar injection for edge cooling
in a reactor would significantly reduce the threshold as seen in Fig. 4.9. Therefore,
the sputtering by the impurities is the main cause of both erosion of PFS and plasma
contamination by sputtered atoms. Physical sputtering yields change with the angle
of incident ions, showing higher values for lower incident angles; as expected nuclear
stopping region becomes shallower for low angle incidence.
The sputtered atoms are mostly neutral showing the Maxwell–Boltzmann-like
energy distribution with a maximum energy of a few eV. In boundary plasma (or
scrape-off layers) of a reactor, released atoms from PFM by sputtering are immediately ionized and succeedingly gyrated along magnetic field lines. Consequently,
most of them return to be deposited nearby the sputtered location (referred to as
prompt deposition) or transferred along the scrape-off layers to be deposited somewhere on the plasma-facing surface, shadowed area, and/or exhausted from the
reactor. Some of the ionized atoms penetrate deep into plasma and are confined
as impurity ions. Repetitive processes of erosion by sputtering and deposition cause
Fig. 4.9 Sputtering yields
for tungsten by hydrogen and
some impurities at normal
incidence angle [20]