Another dimension: transients
4. Plasma unstable by nature: large instabilities
²  Up to 6% of total stored energy released in ms (>10MJ.m-2)
surface temp.
Plasma only
2.5x1026m-2
melting
Laser
only
-2s1/2)
(20MJ.m
recrystallization
brittle
0
time
450 s
Thermal cycling of the surface
Large number of events (106)
34
Energy Days, Tu/E, 31 May 31 Effect of transients
increasing power density
incident beam
cracking only
Plasma
roughening
2.5x1026m-2
incident beam
homogeneous
melting
After 5 large
ELMs (2MJ/m2)
Laser only
-2s1/2
(20MJ.m
) and
melt
ejection
boiling
droplet formation
ITER min. requirement
W
§
35
Minimum
ITER ELM size
Energy Days, Tu/E, 31 May 31 CFC
Recommended damage
threshold ~0.5 MJm-2 now
adopted by ITER (factor of
10 reduction!)
à Will require ELM mitigation
strategies to keep EELM < 1 MJ
Effect of transients
q  Plastic deformation during heating due to thermal expansion
q  Tensile stress in cooling phase
q  cracking at grain boundary and intergranular if thermal stress> yield
stress
Energy Days, Tu/E, 31 May 31 Effect of transients
q  Experiments in QSPA allow evaluation of material damage under
relevant heat flux densities
q  Mass loss of 1.5g/m-2 for tungsten at 1.5MJ/m-2
Energy Days, Tu/E, 31 May 31 It can get worse… (1/2)
4. Interplays between quiescent and off-normal plasma effects
²  Surface modified by high-flux plasma
²  Temperature excursion and high transient heat/particle source
Plasma only
2.5x1026m-2
38
Energy Days, Tu/E, 31 May 31 Laser only
(20MJ.m-2s1/2)
It can get worse… (2/2)
10x 0.07MJ.m-2
²  Synergistic effect
²  Void formation due to high flux plasma
²  Explosive release of material during transient
%&"'#()'*$+)",'-$
10x 0.15MJ.m-2
%&"'#()'*$
+)",'-$
!"#.$/)*"0$
1"&,20)#.$
&"3(2"0#$
!"#$
4)*"0$
!"#$
10x 0.5MJ.m-2
Plasma-enhanced surface ablation
Possibly a significant concert for divertor
lifetime
39
Energy Days, Tu/E, 31 May 31 Material mixing
D, T, He plasma
Be ‘rain’
from FW erosion
Hot surfaces
T ~ 600-1000 K
W, liner/dome
C strike points
(start-up phase)
W
(DT phase)
²  Multi-material wall and mixed-species plasma
²  High-temperature and plasma fluence
Possible chemical reactions might strongly modify the
material properties e.g. power handling and T retention
[9] C. Linsmaier, et al, Proc. 19th PSI conference, San Diego, 2010
Energy Days, Tu/E, 31 May 31 Be/W system
0
3000
Alloy formation reduces drastically
the melting temperature
Limited database (no tokamak
equivalent)
2100 ± 50o C
~95
o
<1750 C
Be2W
Be22W
0
Energy Days, Tu/E, 31 May 31 100
<2250o C
Be12W
²  PFM properties strongly modified in
particular melting temperature!
90
LIQUID
1000 1287o C 2000
Temperature (o C)
²  ~1500°C (Be12W)
²  Formation of such coumpounds
experimentally observed for
tungsten targets exposed to D2/Be
plasmas
70 80
W
3422o C
²  ~2200°C (Be2W)
²  ~1300°C (Be22W)
Weight percent Tungsten
Be
²  Stable Be-W inter-metallics are:
10
20
30
40
50
60
70
80
90 100
Atomic percent Tungsten
[10] M.J. Baldwin et al, J. Nucl. Mater., 390-391 (2009
The end…?
²  The interaction of a
thermonuclear plasma with
surrounding surfaces represents
a daunting challenge because of
the magnitude of the issues at
stake.
²  Taming those issues is a must on
the way to fusion energy
²  Multi-scale problem requiring
cross-discipline research
Courtesy of D. Whyte, PSFC, MIT
Energy Days, Tu/E, 31 May 31