136
8 PMI in Large Tokamaks
related to PMI but also to the main plasma which is strongly affected in unexpected
ways, impacting many aspects of tokamak operation. In addition, the increase of the
erosion by the seeded impurities is also concerned.
8.1.2 Exchange of PFM from Carbon to High Z Metals
As introduced in Chap. 1, high Z PFM had been avoided until recently because of
the fear on high radiation from high Z elements accumulated in plasma center. After
finding that source, transport and accumulation of high Z elements in plasma are
separated and controlled by different physics as observed in TEXTOR Mo limiter
experiments [7], and that central accumulation could be avoided with appropriate
plasma control, several plasma machines introduced high Z materials as PFM. For
examples; ASDEX-U gradually increased surface coverage of its PFM with W [8],
Alcator C-mod operated with a first wall and divertor tiles made entirely of Mo [9],
and EAST has changed PFM from initially introduced full C to W coated C [10].
Now, JET is under operation with ITER-Like Wall (ILW) consisting of Be as first
wall and W coated C as divertor. The first summary on comparison of low Z and high
Z used as PFM appeared in [11]. Low Z and high Z elements respond to plasma quite
differently, as described in Chap. 4, and conversion of PFM from low Z to high Z
has resulted in significant change in plasma behavior. Unfortunately, however, owing
to concerns on melt damages of high Z materials, power load to PFM, particularly,
limiter and divertor, was limited, which made it difficult to investigate responses of
high Z elements to reactor like power load. Therefore, high-power load tests have
been done mostly linear plasma machines or high-power ion and electron guns (see
Chap. 3).
Nevertheless, JET ILW is giving important and useful information on possible
PMI in a reactor. In the following is introduced a little detail of different observations
between Carbon (C-wall) wall and ILW in JET.
8.1.3 ITER-Like Wall (ILW) in JET
Conversion of PFM from Carbon to ILW in JET [12] has resulted in significant change
of radiation power load to PFS during plasma break down as shown in Fig. 8.4 [13]
with higher radiation in the C-wall. However, the absence of active cooling forces
limitations on the power handling and requires a much more cautious operation in
JET in comparison with ITER or a reactor equipped with actively cooled PFC. JET
relies simply on inertial cooling. This requires on the one hand maximization of the
power-handling capabilities of the components by design optimization [14] and on
the other hand strict limits on power loads to avoid melting [15].
Therefore, the increase of the power load to PFM is not the main object of the ILW
in JET. Nevertheless, the main PMI processes: (a) material erosion and migration,
8 PMI in Large Tokamaks
related to PMI but also to the main plasma which is strongly affected in unexpected
ways, impacting many aspects of tokamak operation. In addition, the increase of the
erosion by the seeded impurities is also concerned.
8.1.2 Exchange of PFM from Carbon to High Z Metals
As introduced in Chap. 1, high Z PFM had been avoided until recently because of
the fear on high radiation from high Z elements accumulated in plasma center. After
finding that source, transport and accumulation of high Z elements in plasma are
separated and controlled by different physics as observed in TEXTOR Mo limiter
experiments [7], and that central accumulation could be avoided with appropriate
plasma control, several plasma machines introduced high Z materials as PFM. For
examples; ASDEX-U gradually increased surface coverage of its PFM with W [8],
Alcator C-mod operated with a first wall and divertor tiles made entirely of Mo [9],
and EAST has changed PFM from initially introduced full C to W coated C [10].
Now, JET is under operation with ITER-Like Wall (ILW) consisting of Be as first
wall and W coated C as divertor. The first summary on comparison of low Z and high
Z used as PFM appeared in [11]. Low Z and high Z elements respond to plasma quite
differently, as described in Chap. 4, and conversion of PFM from low Z to high Z
has resulted in significant change in plasma behavior. Unfortunately, however, owing
to concerns on melt damages of high Z materials, power load to PFM, particularly,
limiter and divertor, was limited, which made it difficult to investigate responses of
high Z elements to reactor like power load. Therefore, high-power load tests have
been done mostly linear plasma machines or high-power ion and electron guns (see
Chap. 3).
Nevertheless, JET ILW is giving important and useful information on possible
PMI in a reactor. In the following is introduced a little detail of different observations
between Carbon (C-wall) wall and ILW in JET.
8.1.3 ITER-Like Wall (ILW) in JET
Conversion of PFM from Carbon to ILW in JET [12] has resulted in significant change
of radiation power load to PFS during plasma break down as shown in Fig. 8.4 [13]
with higher radiation in the C-wall. However, the absence of active cooling forces
limitations on the power handling and requires a much more cautious operation in
JET in comparison with ITER or a reactor equipped with actively cooled PFC. JET
relies simply on inertial cooling. This requires on the one hand maximization of the
power-handling capabilities of the components by design optimization [14] and on
the other hand strict limits on power loads to avoid melting [15].
Therefore, the increase of the power load to PFM is not the main object of the ILW
in JET. Nevertheless, the main PMI processes: (a) material erosion and migration,