Transcript Title in bold letters

Global migration of carbon impurities in
the ASDEX Upgrade tokamak
Euratom-Tekes Fusion Seminar
Tartu, 29-30 May, 2012
Antti Hakola
VTT Technical Research Centre of Finland
Collaborators:
S. Koivuranta, J. Likonen: VTT
M. Groth, T. Kurki-Suonio, V. Lindholm, T. Makkonen, J. Miettunen: Aalto University
A. Herrmann, K. Krieger, M. Mayer, H. W. Müller, R. Neu, V. Rohde: IPP-Garching
P. Petersson: KTH
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Material migration: why is it important?
”Material migration is important because it is net, rather
than gross, erosion which is of practical consequence”
(P. C. Stangeby)
In other words, necessary step between
erosion of plasma-facing components and
deposition of the eroded material & retention of
plasma fuel (particularly T) in fusion reactors
How can the migration mechanisms in tokamaks
be elucidated?
•
•
•
Carry out tracer injection experiments (e.g.
13C) just before venting the vessel for
maintenance
Analyze a comprehensive set of first-wall
components for their surface densities of the
tracer elements
Try to obtain – and predict – the resulting
deposition profiles numerically
2
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This presentation concentrates on migration studies
in ASDEX Upgrade (AUG)
AUG is an ITER- and DEMO-relevant environment for
migration studies  all the first-wall structures Wcoated graphite tiles since 2007
Migration can be studied both globally and locally: Here
the focus in on the global scale
• isotopically labelled methane (13CH4) (and recently
also 15N2) injected into the torus from one valve at the
outer midplane
• results modelled using the ASCOT, DIVIMP, and
SOLPS codes
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Master table of the AUG global 13C/15N experiments
Campaign
Discharges/
configuratio
n
Gas
2010-2011
27382-27392
L-mode, LSN
H
-2.5
0.8
5.7
1.8 (NBI)
0.9 (ECRH)
2007
22573-22585
L-mode, LSN
D
-2.5
0.8
3.3
2005
20646-20659
L-mode, LSN
H
-2.5
0.8
2004
19535-19546
H-mode, USN
H
-2.0
2003
18190-18202
H-mode, LSN
H
-2.0
See
Bt (T)
Ip (MA)
ne
cm-3)
Paux (MW)
13C/15N
Injection
rate
(×1021 s-1)
Flat-top
time (s)
4.6 + 4.6
2.0
(mixture)
5.0
2.6 (NBI,< 1 s)
0.9 (ECRH)
2.7
1.0
3.5
6.0
2.9
5.0
1.0
7.0
0.8
9.0
5.1
0.21
0.20
4.5
1.0
8.5
6.8
3.2
1.0
4.5
(×1019
injected
(×1022 at.)
A. Hakola et al., Plasma Phys. Control. Fusion 52, p. 065006, 2010
A. Hakola et al., Journal of Nuclear Materials (submitted)
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The latest injection experiment took place in 2011:
global injection of both 13C and 15N
• High-density, lower single-null L-mode discharges in hydrogen
• Altogether 11 shots (#27382-#27392), two reference shots (#27366, #27371)
• 13C and 15N injected simultaneously, atomic ratio of 13C and 15N was 1:1
• Midplane manipulator in operation during the reference shots
 Te, ne, and flow velocities at the outer midplane
• Additional data for modelling purposes:
 Te and ne distributions at the divertor from Balmer emission lines
 jsat, Te, and ne data from fixed Langmuir probes (LP) at the strike-point zones
 thermography data at the divertor plates (temperature and power density)
 bolometer data to estimate radiated power in different regions
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Tiles analyzed using SIMS at VTT – first extensive
analysis program after the 2007 experiment
SIMS = Secondary Ion Mass Spectrometry
5-keV O2+ primary ion beam, current  500 nA,
analysis area 300×430 mm2
In 2007, 13C distribution measured from
• selected W-coated graphite tiles (3-5 mm or 200 mm)
• uncoated graphite regions of marker tiles (divertor)
• small Si samples (remote areas)
Si samples
Si samples
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Why to bother anymore: everything was clear after
the 2007 experiment!?
Well, not really…
Region
Deposition
Inner divertor
0.8%
Roof baffle
0.1%
Outer divertor
0.1%
Limiter region
0.3%
PSL
0.04%
Upper divertor
1 - 6%
Heat shield
0.6%
Remote areas
1.3%
Only 4 - 9% of
injected 13C found
experimentally
Totally different deposition behavior on
W and on C, especially at the outer
divertor
Is the applied assumption of
toroidally symmetric deposition
valid?
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Predictive ASCOT modelling indicates strong
deposition hot spots at certain toroidal locations
Is this really the case? Must be checked experimentally!
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39 wall tiles removed for post mortem analyses after
the 2011 injection experiment
Tiles removed for analyses marked in red
Especially:
• tiles removed from many different toroidal locations at
the outer midplane: two ICRH antennas, two different
poloidal limiters
• samples taken from the side faces of the removed tiles
 deposition in tile gaps!
Tile A1/2RIGHT
Plasma-facing
samples
Gap samples
17 mm
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Results (1): remarkable deposition at the outer
midplane
• largest surface densities (up to
1018-1019 at/cm2) localized to the
vicinity of the injection valve
• deposition decays from the peak
values to 1016 at/cm2 within 100 mm
• large differences in 13C levels
between tiles from different ICRH
antennas and poloidal limiters
 toroidally symmetric deposition
not a valid assumption
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Results (2): deposition profiles in different regions of
the torus
Deposition at
(a) upper divertor (UD)
(b) heat shield (HS)
(c) inner divertor (ID), roof baffle
(RB), and outer divertor (OD)
(d) tile gaps
• UD: rather uniform deposition, results
in line with the 2007 profile
• HS: situation dramatically changed
from 2007 to 2011  plasma flows
different?
• lower divertor minor deposition
region
• OD: deposition profiles in 2011
qualitatively different from the 2007
case  due to different magnetic
configuration? or due to eroded tile
surfaces?
• tile gaps account for considerable
13C surface densities
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Results (2): deposition profiles in different regions of
the torus
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Approximately 35% of the injected 13C found
Inner divertor
Roof baffle
Outer divertor
Upper divertor
Heat shield
Outer
midplane
1.5%
0.3%
0.4%
4%
15%
15%
13C
13C
13C
13C
(4% of
•
•
found)
(1% of
found)
(1% of 13C found)
toroidally symmetric deposition assumed
for the inner divertor, roof baffle, outer
divertor, upper divertor, and heat shield
at the outer midplane, the average 13C
surface density multiplied the total surface
area of the different limiter and ICRH antenna
tiles
Main chamber is the main deposition region
for 13C
(11% of
found)
(41.5% of
found)
(41.5% of 13C found)
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SOLPS modelling: range of background plasmas and
flow profiles obtained
ne,sep = 2.25 × 1019 m-3
ASCOT
ne,sep = 1.5 × 1019 m-3
SOLPS = 2D plasma fluid code
• shot #27385 with pure H plasma, ne,sep as a free parameter
• decent match for ne and Te at the OMP and OD – but not
simultaneously
• background plasma corresponding to a fit for ne at OMP
selected for ASCOT simulations (top)
• SOLPS predicts weak plasma flow and stagnation point at
the OMP  in contradiction with typical situations in
tokamaks
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ASCOT modelling: imposed flow profile required
ASCOT = 3D Monte Carlo code
• shot #27385, magnetic equilibrium at 2.8 s
• test particles (300,000) followed until their deposition
• ASCOT predicts strong, localized deposition at OMP
• imposed flow profile required to shift deposition away
from outer divertor
 SOLPS flow profile predicts 50% at the outer divertor
(experimentally: 1% of the 13C found)
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Conclusions
•
Global 13C and 15N injection experiment successfully carried out in AUG in 2011
•
A comprehensive set of toroidally and poloidally distributed tiles analyzed for their 13C
content
•
Experimental highlights:
 Main chamber of AUG is the most prominent sink for 13C: almost 35% of the injected 13C
found there
 Gaps between tiles contain significant 13C inventories
 Lower divertor is a minor deposition region for 13C
•
Status of numerical simulations
 SOLPS simulations provided a set of background plasmas and poloidal flow profiles but
flows generally rather weak and stagnation points occur at wrong places
 ASCOT simulations with the weak SOLPS plasma flow would deposit 50% of the particles
at the outer divertor
 imposed flow profile required to reproduce the observed localized deposition peaks
at the outer midplane