98
6 Material Modification by High-Power Load …
When materials temperature is raised high enough, sublimation becomes appreciable. Although the sublimation removes some loaded power by the heat of sublimation, it is not large enough to cool down the PFS temperature. On the other hand, the
sublimated vapor penetrates in boundary plasma followed by electrical excitation or
ionization of sublimated molecules and atoms. Since the excitation/ionization energy
is much larger than the heat of sublimation, it cools the edge plasma, sometimes
resulting in plasma instability or disruption. However, in large tokamaks, heating
power is large enough to compensate such power loss, disruption due to the sublimation is hardly observed. Instead, with aid of gyration after the ionization of sublimated atoms, most of the sublimated materials are promptly redeposited as shown in
Fig. 5.3 and hardly enter plasma center. Thus, impurity transport and its accumulation
in plasma center can be separated from impurity source (sublimation). Figure 6.2 is
the first clear indication for that [16–18]. A Mo test limiter was exposed to TEXTOR
plasma with the power load of 1.9 kW·cm
−2 (19 MW·m
−2 ). When its temperature
rose above 2100 K, the sublimation of Mo started as observed in MoI line emission.
Nevertheless, other plasma parameters were hardly influenced by the Mo sublimation without the indication of central accumulation of Mo in Mo XXXVII line. This
Fig. 6.2 A Mo test limiter after used in TEXTOR [16], B Cross section of Mo limiter (produced
by powder metallurgy) utilized in a reversed field pinch machine (TPE-1RM15) at the Electrotechnical Laboratory, Japan. Recrystallization to columnar grains from originally granular grains with
cracking between them are clearly seen and a plasma discharge accompanying the melting of Mo
[17]. C Time evolutions of (a) Mo flux and surface temperature near the melting surface, (b) Line
averaged density, (c) deuterium flux on the main graphite limiter, (d) total radiation, and (e) Mo
XXVII line intensity in the core plasma [16]
6 Material Modification by High-Power Load …
When materials temperature is raised high enough, sublimation becomes appreciable. Although the sublimation removes some loaded power by the heat of sublimation, it is not large enough to cool down the PFS temperature. On the other hand, the
sublimated vapor penetrates in boundary plasma followed by electrical excitation or
ionization of sublimated molecules and atoms. Since the excitation/ionization energy
is much larger than the heat of sublimation, it cools the edge plasma, sometimes
resulting in plasma instability or disruption. However, in large tokamaks, heating
power is large enough to compensate such power loss, disruption due to the sublimation is hardly observed. Instead, with aid of gyration after the ionization of sublimated atoms, most of the sublimated materials are promptly redeposited as shown in
Fig. 5.3 and hardly enter plasma center. Thus, impurity transport and its accumulation
in plasma center can be separated from impurity source (sublimation). Figure 6.2 is
the first clear indication for that [16–18]. A Mo test limiter was exposed to TEXTOR
plasma with the power load of 1.9 kW·cm
−2 (19 MW·m
−2 ). When its temperature
rose above 2100 K, the sublimation of Mo started as observed in MoI line emission.
Nevertheless, other plasma parameters were hardly influenced by the Mo sublimation without the indication of central accumulation of Mo in Mo XXXVII line. This
Fig. 6.2 A Mo test limiter after used in TEXTOR [16], B Cross section of Mo limiter (produced
by powder metallurgy) utilized in a reversed field pinch machine (TPE-1RM15) at the Electrotechnical Laboratory, Japan. Recrystallization to columnar grains from originally granular grains with
cracking between them are clearly seen and a plasma discharge accompanying the melting of Mo
[17]. C Time evolutions of (a) Mo flux and surface temperature near the melting surface, (b) Line
averaged density, (c) deuterium flux on the main graphite limiter, (d) total radiation, and (e) Mo
XXVII line intensity in the core plasma [16]