188
10 Selection of Plasma-Facing Materials
Table 10.1 Summary of behavior of Carbon material, Beryllium, and High Z as PFM armor
Carbon-based material
Be
High Z
General
Much experience in
large tokamaks, JET,
JT-60U, TFTR, ASDEX,
DIII-D TEXTOR, Tore
Supra, and others
Experience only in
JET
Some tokamak
experience in JET,
ASDEX and EEST
Dedicated experiments
in TEXTOR and
ALCATOR
Radiation
Allowable level in
burning plasma
Low
10 −4
Low
10 −3
High
10 −6
Thermal response
No melting allows
highest Temp. use
Low thermal
conductivity at high
Temp.
Low melting Temp.
limits <1000 K
High melting Temp.
Crystalline growth
limits maximum
Temp. <1600 K
Embrittlement limits
low Temp. >800 K
Thermal fatigue limits
long time use
Erosion
High
Low physical sputtering
threshold
High chemical erosion
RES given by impurity
ions limits maximum
Temp. <1600 K
High
physical/chemical
sputtering
Evaporation
Low
High physical
sputtering threshold
Sputter erosion by
impurity
Melting/crack erosion
Deposition
Large on plasma
shadowed area
Large on plasma
shadowed area
Prompt deposition
He effect
Little
Blistering
Blister erosion
Tritium retention
Large in deposited layers
Significant reduction
above 800 K
Significant trapping
in defects
T trapping near surface
T dissolution in bulk
Neutron damage
Low activation.
Degradation of thermal
conductivity
Swelling
Low activation
He transmutation
Medium activation
Embrittlement and
DBTT in crease
Transmutation to Re
and Os
maximum allowable concentration is around 10
−3 and 10
−6 for C and W, respectively.
However, it should be noted that some impurities are required for edge cooling or
power exhaust and impurity seeding in which low Z to medium Z materials like N,
Ne, and Ar are intentionally introduced in the boundary plasma. In this respect, Cwall has advantage because eroded C works as a radiator without additional impurity
seeding.
Although high radiation of high Z materials accumulated in plasma center is
serious concern, their high radiation in the boundary plasma is attractive for cooling
if they were localized in the boundary plasma. In recent tokamak experiments using
10 Selection of Plasma-Facing Materials
Table 10.1 Summary of behavior of Carbon material, Beryllium, and High Z as PFM armor
Carbon-based material
Be
High Z
General
Much experience in
large tokamaks, JET,
JT-60U, TFTR, ASDEX,
DIII-D TEXTOR, Tore
Supra, and others
Experience only in
JET
Some tokamak
experience in JET,
ASDEX and EEST
Dedicated experiments
in TEXTOR and
ALCATOR
Radiation
Allowable level in
burning plasma
Low
10 −4
Low
10 −3
High
10 −6
Thermal response
No melting allows
highest Temp. use
Low thermal
conductivity at high
Temp.
Low melting Temp.
limits <1000 K
High melting Temp.
Crystalline growth
limits maximum
Temp. <1600 K
Embrittlement limits
low Temp. >800 K
Thermal fatigue limits
long time use
Erosion
High
Low physical sputtering
threshold
High chemical erosion
RES given by impurity
ions limits maximum
Temp. <1600 K
High
physical/chemical
sputtering
Evaporation
Low
High physical
sputtering threshold
Sputter erosion by
impurity
Melting/crack erosion
Deposition
Large on plasma
shadowed area
Large on plasma
shadowed area
Prompt deposition
He effect
Little
Blistering
Blister erosion
Tritium retention
Large in deposited layers
Significant reduction
above 800 K
Significant trapping
in defects
T trapping near surface
T dissolution in bulk
Neutron damage
Low activation.
Degradation of thermal
conductivity
Swelling
Low activation
He transmutation
Medium activation
Embrittlement and
DBTT in crease
Transmutation to Re
and Os
maximum allowable concentration is around 10
−3 and 10
−6 for C and W, respectively.
However, it should be noted that some impurities are required for edge cooling or
power exhaust and impurity seeding in which low Z to medium Z materials like N,
Ne, and Ar are intentionally introduced in the boundary plasma. In this respect, Cwall has advantage because eroded C works as a radiator without additional impurity
seeding.
Although high radiation of high Z materials accumulated in plasma center is
serious concern, their high radiation in the boundary plasma is attractive for cooling
if they were localized in the boundary plasma. In recent tokamak experiments using