4.4 Emission of Electrons and Photons
63
The radiation or emitted photons can be used as a diagnostic tool to measure
surface temperature. However, since radiation power is proportional to the fourth
power of absolute temperature, the radiation becomes appreciable only when the
target temperature increases over around 2000 K. In a divertor target area where
power load by plasma particles is very large, radiated power from the target could
have some influence on power valance. Transition heat load such as ELM and arching
makes the local hot spot which can be detected from the radiation. The hot spot
enhances photoelectron emission which, in turn, attracts more incident ions resulting
in temperature escalation to make the hot spots [27].
4.5 Energy Reflection
As already described above, part of energetic particles and photons incident to the
material surface are reflected with energy distributed from the same energy of incident
to zero depending on the incident energy, incident angle, and target materials. As
examples, reflection of ions and energy carried by reflected ions are given in Figs. 4.5
and 4.6, respectively for C and W, as reflection coefficients of ion and energy for
incident ions of H, D, and He. In energy reflection, not only energy carried by primary
ions and photons but also energy carried by secondary particles and photons are
included. Furthermore, as discussed in the previous section, radiation from the target
materials becomes high if deposited energy by primary particles and photons are high
enough to make the target temperature very high. As described in the previous section,
the radiation from the target material becomes appreciable above around 2000 K. In
the divertor target area, owing to very high heat load, the effect of energy reflection
and radiation from the target materials would become significant. In material power
load tests using electron or ion beam, the energy reflection is taken into account.
4.6 Reemission of Incident Ions
Initially injected ions except reflected ones are retained in a target material. Accordingly, the concentration of the injected ions in the target increases. When their concentration in the subsurface region becomes the saturation concentration, reemission of
the incident ions starts. After long implantation, although the depth of saturated
region gradually broadened, most of the injected ions are reemitted, in other words,
reemitted flux becomes nearly the same as the incident flux. Different from the
reflected ones, reemitted particles are mostly thermalized at the target temperature.
Reemission behavior is quite different between fuels and inert gases such as He
produced by fusion and Ne and Ar seeded for edge cooling. In the following sections,
they are discussed separately.
63
The radiation or emitted photons can be used as a diagnostic tool to measure
surface temperature. However, since radiation power is proportional to the fourth
power of absolute temperature, the radiation becomes appreciable only when the
target temperature increases over around 2000 K. In a divertor target area where
power load by plasma particles is very large, radiated power from the target could
have some influence on power valance. Transition heat load such as ELM and arching
makes the local hot spot which can be detected from the radiation. The hot spot
enhances photoelectron emission which, in turn, attracts more incident ions resulting
in temperature escalation to make the hot spots [27].
4.5 Energy Reflection
As already described above, part of energetic particles and photons incident to the
material surface are reflected with energy distributed from the same energy of incident
to zero depending on the incident energy, incident angle, and target materials. As
examples, reflection of ions and energy carried by reflected ions are given in Figs. 4.5
and 4.6, respectively for C and W, as reflection coefficients of ion and energy for
incident ions of H, D, and He. In energy reflection, not only energy carried by primary
ions and photons but also energy carried by secondary particles and photons are
included. Furthermore, as discussed in the previous section, radiation from the target
materials becomes high if deposited energy by primary particles and photons are high
enough to make the target temperature very high. As described in the previous section,
the radiation from the target material becomes appreciable above around 2000 K. In
the divertor target area, owing to very high heat load, the effect of energy reflection
and radiation from the target materials would become significant. In material power
load tests using electron or ion beam, the energy reflection is taken into account.
4.6 Reemission of Incident Ions
Initially injected ions except reflected ones are retained in a target material. Accordingly, the concentration of the injected ions in the target increases. When their concentration in the subsurface region becomes the saturation concentration, reemission of
the incident ions starts. After long implantation, although the depth of saturated
region gradually broadened, most of the injected ions are reemitted, in other words,
reemitted flux becomes nearly the same as the incident flux. Different from the
reflected ones, reemitted particles are mostly thermalized at the target temperature.
Reemission behavior is quite different between fuels and inert gases such as He
produced by fusion and Ne and Ar seeded for edge cooling. In the following sections,
they are discussed separately.