Global migration of carbon impurities in the ASDEX Upgrade tokamakEuratom-Tekes Fusion Seminar

Tartu, 29-30 May, 2012

Antti HakolaVTT Technical Research Centre of FinlandCollaborators:

S. Koivuranta, J. Likonen: VTTM. Groth, T. Kurki-Suonio, V. Lindholm, T. Makkonen, J. Miettunen: Aalto UniversityA. Herrmann, K. Krieger, M. Mayer, H. W. Müller, R. Neu, V. Rohde: IPP-GarchingP. 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

• 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

How can the migration mechanisms in tokamaks be elucidated?

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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

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 W-coated graphite tiles since 2007

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Campaign Discharges/ configuration

Gas Bt (T) Ip (MA) ne (×1019 cm-3)

Paux (MW) 13C/15N injected

(×1022 at.)

Injection rate

(×1021 s-1)

Flat-top time (s)

2010-2011 27382-27392 L-mode, LSN

H -2.5 0.8 5.7 1.8 (NBI) 0.9 (ECRH)

4.6 + 4.6 2.0 (mixture)

5.0

2007 22573-22585 L-mode, LSN

D -2.5 0.8 3.3 2.6 (NBI,< 1 s) 0.9 (ECRH)

2.7 1.0 3.5

2005 20646-20659 L-mode, LSN

H -2.5 0.8 6.0 2.9 5.0 1.0 7.0

2004 19535-19546 H-mode, USN

H -2.0 0.8 9.0 5.1 0.21 0.20 4.5

2003 18190-18202 H-mode, LSN

H -2.0 1.0 8.5 6.8 3.2 1.0 4.5

See A. Hakola et al., Plasma Phys. Control. Fusion 52, p. 065006, 2010A. Hakola et al., Journal of Nuclear Materials (submitted)

Master table of the AUG global 13C/15N experiments

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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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5-keV O2+ primary ion beam, current 500 nA,

analysis area 300×430 mm2

Tiles analyzed using SIMS at VTT – first extensive analysis program after the 2007 experiment

SIMS = Secondary Ion Mass Spectrometry

Si samples

Si samples

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)

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Region DepositionInner 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

Why to bother anymore: everything was clear after the 2007 experiment!?

Totally different deposition behavior on W and on C, especially at the outer divertor

Is the applied assumption of toroidally symmetric deposition valid?

Well, not really…

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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!

17 mm

Gap samples

Plasma-facingsamples

Tile A1/2RIGHT

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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

• UD: rather uniform deposition, results in line with the 2007 profile

• HS: situation dramatically changed from 2007 to 2011 plasma flowsdifferent?

• 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

Deposition at (a) upper divertor (UD)(b) heat shield (HS)(c) inner divertor (ID), roof baffle

(RB), and outer divertor (OD)(d) tile gaps

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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%(4% of 13C found)

0.3%(1% of 13C found)

0.4%(1% of 13C found)

4%(11% of 13C found)

15%(41.5% of 13C found)

15%(41.5% 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

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SOLPS modelling: range of background plasmas and flow profiles obtained

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

ne,sep = 2.25 × 1019 m-3

ne,sep = 1.5 × 1019 m-3

ASCOT

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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