Member of the Helmholtz Association Carbon based materials: fuel retention and erosion under...

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Carbon based materials: fuel retention and erosion under ITER-like mixed species plasma

conditions

Arkadi Kreter et al

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July 2009 2

CFC in ITER

ITER divertor cassette mock-up

Carbon Fibre Composite (CFC)foreseen for strike plates – regions of highest heat loads

Tungsten

Investigation of fuel retention in CFC for ITER-relevant conditions is necessary

ITER-relevant divertor conditions High D flux (up to D~1024 m-2s-1) High D fluence (>1026 m-2 for 1 ITER pulse at SP) Low incident energy (Ei~10 eV)

Range of wall temperatures (Ts = 500 K - 1000 K) Impurities

• Beryllium from the main wall

• He from D-T reactions

• Ar for divertor cooling


Exposures in PISCES-A/-B linear plasma devices

ErodedMaterial

PISCES (-A, -B) schematic view

Steady-state plasma

ne = (2-3)·1018 m-3 ; Te = 7-15 eV

= (3-6)·1022 D/m2s

Variations of:

Ei = 20 - 120 eV

= 1·1025 - 5·1026 D/m2 (~1 ITER pulse at strike point)

Ts = 370 K ('cold' ITER wall) - 820 K (ITER strike point)

Controlled Be, He and Ar seeding

All experiments in erosion-dominated conditions

PISCES-A plasma and target

Ex-situ analysis of samples for retention

• Thermal desorption spectrometry (TDS)• Nuclear reaction analysis (NRA) with 3He

beam

B

Optical spectroscopy as the main tool to quantify carbon erosion


Wide database for carbon materials exposed to pure D plasma

Ret

enti

on

[m

-2]

Ion fluence [m-2]

NB41 PISCES-A N11 PISCES-A NB31 TEXTOR DMS780 TEXTOR EK98 TEXTOR

1024 1025 1026 1027

1021

1022

0.35

ATJ PISCES-A

Retention vs incident D fluence,exposures at Ts = 470 K, Ei = 120 eV

Ret

enti

on

[D

/m2 ]

Ion fluence [D/m2]1025 1026 1027

1021

1022

Total D retention for exposures at different temperatures in PISCES-A and -B

[1] R. Pugno et al., JNM 375 (2008) 168

Saturation

NB41 Ts=370K NB41 Ts=470K NB41 Ts=820K DMS701 Ts=1070K [1]


Retention in CFCs and FGGs for pure D plasma

In-bulk retention is similar in different CFCs and fine-grain graphites CFCs and fine-grain graphites are porous

In-bulk retention is higher for lower exposure temperatures Additional trapping sites at lower temperatures Higher population of available trapping sites at lower temperatures

In-bulk retention scales as fluence with depending on temperature:

<~0.5 for low Ts (surface diffusion along pores)

=0 for Ts>~800K – saturation of retention for < 31025 D/m2 (few sec of ITER pulse)

In-bulk retention is higher for higher incident ion energies Higher D concentration in implantation layer [Staudenmaier JNM79] leads to higher D

amount in bulk

In-bulk retention is higher for lower fluxes Amount of diffused deuterium t, longer time available for D to diffuse in for the same

fluence


Influence of Be on retention

D2 TDS spectra for ATJ exposed w/o and with Be (Ts=720K, Ei=35 eV)

Be carbide layer appears to prevent increase of retention with fluence

Sensitive to exposure parameters

Total Deuterium Retention

With Be, =0.5e26 D/m2 before Be, =2e26 D/m2 total:

1.9e21 D/m2

Pure D, =2e26 D/m2:

2.3e21 D/m2

Pure D, =0.5e26 D/m2:

1.6e21 D/m2400 600 800 1000 12000.0

0.1

0.2

0.3

0.4

0.5

pure D =0.5e26m-2

pure D=2e26m-2

Be containingplasma

D2 d

eso

rpti

on

flu

x [x

1019

D/m

2s]

Temperature [K]

Scenario of Be experiment

1. Establishing background plasma (=0.5e26 D/m2)

2. Be injection from oven (total =2e26 D/m2)

0.5 K/s


400 600 800 1000 12000.0

0.1

0.2

0.3

0.4

0.5 D D+Be D+Be+He

D2 d

es

orp

tio

n f

lux

[x

10

19 D

/m2 s

]

Temperature [K]

Influence of Be+He on retention – low energy case

D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+He (Ts=720K, Ei=35 eV, fHe=16%)

He appears to change the retention mechanism and reduce retention for low incident energies


D+Be, =0.5e26 D/m2 before Be, =2e26 D/m2 total:

1.8e21 D/m2

Pure D, =0.5e26 D/m2:

1.6e21 D/m2

D+Be+He, =0.4e26 D/m2 before Be, =1.7e26 D/m2 total:

0.5e21 D/m2

0.5 K/s


400 600 800 1000 12000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 D D+Be D+Be+He Ei=35eV D+Be+He Ei=80eV

D2 d

es

orp

tio

n f

lux

[x

10

19 D

/m2 s

]

Temperature [K]

Influence of Be+He on retention – high energy case

D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+He (Ts=720K) Ei=80 eV vs Ei=35 eV

No effect of reduced retention due to He at high incident energy

Retention is higher presumably due to higher incident energy



1.8e21 D/m2

Pure D, =0.5e26 D/m2:

1.6e21 D/m2


0.5e21 D/m2


3.6e21 D/m2

0.5 K/s


400 600 800 1000 12000.0

0.1

0.2

0.3

0.4

0.5 D Ei=35eV D+Be Ei=35eV D+Be+Ar Ei=35eV D+Be+Ar Ei=120eV

D2 d

es

orp

tio

n f

lux

[x

10

19 D

/m2 s

]

Temperature [K]

Influence of Be+Ar on retention

D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+Ar (Ts=720K, Ei=35 eV, fAr=10%)

Ar appears to change the retention mechanism and reduce retention for both low and high incident energies



1.8e21 D/m2

Pure D, =0.5e26 D/m2:

1.6e21 D/m2

D+Be+Ar, =0.1e26 D/m2 before Be, =0.5e26 D/m2 total:

0.8e21 D/m2

D+Be+Ar, =0.1e26 D/m2 before Be, =0.5e26 D/m2 total:

0.5e21 D/m2

0.5 K/s


Influence of impurities on retention in carbon materials

1. Incident deuterium ions saturate the implantation layer (~10-100 nm)2. The level of saturation is defined by the balance between adsorption and ion-induced

desorption for a given number of available trapping sites3. From the implantation layer, D 'diffuses' further in-bulk along surfaces of the pores

With addition of Be no further increase of in-bulk retention Be carbide layer appears to suppress the in-bulk penetration of deuterium (Be2C layer

thickness is a few 100 nm) However, Be itself can cause fuel uptake

He and Ar impurities decrease the in-bulk retention for certain exposure conditions Presumably due to depletion (ion-induced detrapping) of the implantation layer, from

where it otherwise moves deeper in the bulk


Erosion of carbon by mixed species plasma: background

Attributed to build up of Be carbide surface layer

CD band light emission in front of FGG target as a measure of chemical erosion

Scaling [Nishijima JNM 2007] obtained for D plasma with various cBe, Ts, Ei, I

How the addition of other impurities (Ar, He) would influence the effect of reduced erosion?

Reproduce the experiments from [Nishijima JNM 2007] with addition of Ar and He

Chemical sputtering of carbon materials due to combined bombardment by ions and atomic hydrogen

[W. Jacob et al, Phys. Scr. T124 (2006) 32]

Chemical sputtering: Simultaneous interaction of low-energy ions and atomic hydrogen

It causes a significantly higher erosion than the sum of the individual processes - chemical erosion due to atomic hydrogen alone and physical sputtering due to ions

Does it mean that impurities in ITER (e.g. Ar) will cause enhanced erosion?

Check for the effect of Ar and He on erosion of graphite target in PISCES

Mitigation of chemical erosion by Be seeding[M.J. Baldwin, R.P. Doerner, Nucl. Fusion 46 (2006) 444, D. Nishijima et al, J. Nucl. Mater. 363–365 (2007) 1261]


0 100 200 300 400 5001E-3

0.01

0.1

D+Be+Ar D+Be

CD

/Dg

Time [s]

Ar and He do not influence C erosion significantly

Initial value (before Be injection) similar Chemical sputtering due to Ar not visible

Decay time similar with and without Ar No influence on Be carbide build up

Addition of Argon (and Helium) does not appear to affect carbon erosion

Normalized CD light emission (Ts=700K, Ei=35 eV, fAr=10%)

Similar behaviour w/ and w/o Ar also for high Ei;

Also the case for He injection instead of Ar

Member of the Helmholtz Association Carbon based materials: fuel retention and erosion under...

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