1

Effects of tungsten surface condition on carbon deposition

Y. Ueda, M. Fukumoto, A. Yamawaki, Y. Soga, 　 Y. Ohtsuka (Osaka U.)S. Brezinsek, T. Hirai, A. Kirschner, A. Kreter, A. Litnovsky, V. Philipps, A. Pospieszczyk, B. Schweer, G. Sergienko (FZJ)T. Tanabe (Kyushu U.), K.Sugiyama (Max-Planck-nstitute) K. Ohya (Tokushima U.), N. Ohno (Nagoya U.) the TEXTOR team

18th International Conference on Plasma Surface Interaction May 26-30, 2008

Beatriz Hotel, Toledo, Spain

2Topics in this talk

Roughness effects on C deposition on W and C Pre-irradiation effects of W on C deposition

High density He plasma H & C mixed ion beam

C deposition on W at elevated temperatures T ~300 ºC, ~550 ºC, ~850 ºC

3Background and purpose of this study

Use of CFC in ITER DT phase CFC : T retention problem greatly reduces DT shots number Tungsten : several concerns Melting, high DBTT, Helium

embrittlement Importance of Tungsten and Carbon material mixing

Plasma facing wall, in gaps, (remote area) D(T) & C mixed ion irradiation to tungsten Many basic studies have been done : C+DW

Complicated processes : Chemical erosion, C diffusion in W+C , RES Issues : Actual surface condition, Mechanism based modeling

Purpose of this study Effects of surface roughness on C deposition Effects of pre-treatment (He plasma exposure, H&C ion irradiation) C deposition at elevated temperature

Detailed study on the mechanism of C and W mixing

4Erosion and deposition of carbon : basics

Carbon deposition is more pronounced on graphite Reflection coefficient is

lower than that on W R~0.6 (50eV C to W)

R~10-4(50 eV C to C)

Carbon ML is easily re-sputtered by reflected H from W substrate

A. Kreter, et al., Plasma Phys. Control. Fusion 48 (2006) 1401

Difference in reflection

Enhancement of sputtering of surface C

5Evolution of deposition/erosion (EDDY code)

D+ + C4+ mixed ion irradiation to tungsten

Simulated by EDDY code D : 96%, C : 4%

As deposition proceeds, Yc and Rc drastically decrease.

Thickness change

Sputtering of C :YC

Reflection of C : Rc

613CH4 puff exp. with graphite limiter (TEXTOR)

C deposition on graphite test limiter (TEXTOR exp.) Deposition Efficiency

Deposited 13C /injected 13CH4

C on unpolished C (Ra ~ 1 µm)

~9% C on polished C (Ra ~ 0.1 µm)

~1.7%

Surface roughness significantly affects C deposition Similar or larger than substrate

effects (W or graphite)PolishedRa ~0.1 µm

A. Kreter, et al., submitted　（２００８）

UnpolishedRa ~1 µm

Ohmic discharge

~9%

~1.7%

7Experimental conditions for this study

Effects of surface roughness on C deposition Tungsten

Roughness Ra ~ 9 nm, ~18 nm, ~180 nm

Graphite (fine grained graphite) Roughness Ra ~ 70 nm, ~350 nm, ~700 nm

C deposition on pre-treated tungsten High density He plasma exposure

Nano-structure formed H + C ion beam pre-irradiation

C surface concentration : ~60%, ~40%, ~10%

C deposition on heated tungsten Temperature range

~300 ºC : ~ITER wall ~550 ºC : ~Chemical Sputtering peak ~850 ºC : Thermal diffusion + RES

8Experimental setup for test limiter exposure

IR thermometer Roof limiter system

Samples on graphite roof limiter Position : 46 cm (LCFS) ~ 47.5 cm Base temperature : ~300 ºC Standard ohmic plasma

Ip = 350 kA, ne = 2.5 x 1019 m-3

Bt = 2.25 T, Ohmic Power ~0.3 MW Edge plasma Parameter (r =48cm)

Te ~ 40 eV, ne ~ 2.5 x 1018 m-3

60 mm

59 m

m

Ion drift side

TEXTOR

Te ne

35

55

46 48 46 48

ALT-II limiter

cm cm0.1

0.8

LCFS LCFS

9Postmortem analysis (NRA, SIMS , XPS)

Profilometer Surface roughness measurement Stylus type (~10 µm : radius of curvature) (DEKTAK)

NRA (Nuclear Reaction Analysis) Analysis beam: 2.5 MeV 3He+

Protons produced by D(3He, p)4He & 12C(3He, p)14N nuclear reactions were detected.

SIMS (Secondary Ion Mass Spectroscopy)XPS (X ray Photoelectron Spectroscopy)Colorimetry

Thickness of C deposition layer estimated by color

10Setup for study on surface roughness effects

Pure W samples Ra~9 nm, ~22 nm, ~180 nm Difference in surface polishing

Graphite (fine grained) Ra~70 nm, ~350 nm, ~700 nm

Experimental conditions 37 shots of OH discharge Radial position of 46 cm.

Deposition mechanism Higher carbon density deeper into

SOL Lower Te deeper into SOL

Edge effects C deposition on W edge adjacent

to graphite

Ra~180 nm

Ra~9 nm

Ion drift side

11C deposition and D retention on W

C deposition Roughness enhances C

deposition Ra~180 nm : Long tail Sharpe boundary between

erosion and deposition D retention

similar to C deposition no surface retention in

erosion zone D/C = 0.1~0.15

NRA measurementW Graphite

12D retention (C deposition) on graphite

C deposition on graphite D retention was mainly in C

deposition layer D/C ~ const in deposition

layer D retention ~ C deposition

Characteristics of C deposition on graphite Roughness enhanced C

deposition also on graphite No sharp transition between

erosion and deposition different from W

NRA

W Graphite

Measured position

13C deposition on pre-treated tungsten H&C mixed ion beam pre-irradiation

(1) – (3) H+C ion beam pre-irradiation 1 keV H3

+ + C+

Fluence: 5 x 1024 m-2

~0.9% C in ion beam Surface C : ~60%

~0.3%~40%, ~0.1%~10%

(1) C : ~0.1%in ion beam

(2) C : ~0.3%in ion beam

(3) C : ~0.9%in ion beam

Atomic concentration of each pre-irradiated W

W

CO

W W

C CO O

Before TEXTOR plasma exposure

14C deposition on pre-treated tungsten He plasma pre-exposure

He plasma pre-exposure High density pure He plasma

exposure in NAGDIS-II (Nagoya U.) Black surface after ~1h exposure at

1300 ºC (flux ~1023 m-2s-1) Sudden change of surface color He bubble and nanostructure

formation Surface structure removed before

TEXTOR plasma exposure Loosely bound nano-structure was

wiped out mechanically

Roughness of He exposed W Roughness ~15 nm (after exp.)

Small pits could be missing due to stylus type measurement

M. Baldwin et al., I-20, PSI18

Before TEXTOR exposure

W surface in this work

T~1600 K

15

10

8

6

4

2

0

C a

real

den

sity

(x1

01

7 c

m-2

)

49.549.048.548.047.547.046.546.0

Radius (cm)

He roughened W Surface C ~60%: W Surface C ~40%: W Surface C ~10%: W Reference W

C deposition on pre-treated W

Carbon deposition

H+C pre-irradiated W C deposition speed relates to

surface C concentration only 10% initial C affects

deposition No deposition on pure W (0%C)

Ra ~ 10 nm for each W

He pre-exposed W Enhancement of C deposition C profile : long tail

increase in deposition area large enhancement of

deposition despite small roughness (~15 nm)

Before After

He pre-exposure

H+C pre-irradiation

60%40%

10%

0%

46 shots (Ohmic plasma)r = 46 cm (same as LCFS)

16Explanation of roughness effect on deposition

He roughened W surface

Roughness (0.01-1 µm) << Ion Lamor radius (0.1-1mm) D ion flux and C ion flux did not change locally local shading effect of D ions may not occur

Some of sputtered or reflected particles redeposited immediately. Trapping rate depends on the morphology He roughened surface was very fine and complicated structure

He induced roughness could have high trapping rate (C deposition)

M. Kunster et al., Nucl. Instrum. Meth.B145 (1998)320.

17Partially heated limiter exp. for C deposition on W

770 930 ºC

Deposition by edge plasma exposure

Deposition due to “gas puff” (CO)

No deposition on the heated sample.

520 600 ºC

Deposition by edge plasma exposure

No deposition on the heated sample.

280 340 ºC240290 ºC

A

A’

A-A’ cross sectionCO gas : desorbed above ~700 ºC

EXP-A EXP-B

HeatedHeatednon-Heated

non-Heated

18

Partially heated limiter exp. (heated W : 520 ºC) non-heated W (240 ºC~280 ºC)

Beltlike C deposition (asymmetry) D retention only on C deposition D/C ratio ~ 0.3

consistent with previous results Alimov, et. al. Physica Scripta T108 (2004)

46. Heated W (520 ºC~600 ºC)

no C deposition no near surface D retention near peak T of chemical sputtering

Bulk diffusion and trapping (permeation) could occur in erosion area for both W Issue : Role of WC mixed layer on D

retention and permeation

Heated520600 ºC

non-heated240290 ºC

0 mm

56 mm

19

Heated770930 ºC

non-heated280340 ºC

non-heated W (280 ºC~340 ºC) Beltlike C deposition (asymmetry) Dense deposition by CO gas puff D/C ratio ~ 0.25

Heated W (770 ºC~930 ºC) No C deposition

No deposition of C originated from CO gas

indication of bulk diffusion in some area

Partially heated limiter exp. (heated W : 770 ºC)

NRA

SIMS 2SIMS 1

20C depth profiles (heated W : 770 ºC)

A small amount of C only near the surface (SIMS 1) No C in the bulk

Diffusion length is consistent with previous diffusion results (SIMS 2) C concentration near surface : ~30% (X

PS) Diffusion length

Experiment : ~ 45 nm Estimation : ~37 nm ( )

K.Schmid et al., J. N. M. 302 (2002) 96. Concentration dependent diffusion D = 4 x 10-20 m-2s-1 (1030 K)

C diffusion mainly between shots(a)

(b) ~ 75 nm

Dtx 2

~30%

212D carbon distribution

2D Carbon surface density (NRA)

Ion energy could cause this difference C in plasma : highly charged (~ +4), thermalized

impact energy E ~ 580 eV (Te~Ti~40 eV)

C+ or CO+ from CO gas : singly charged, not thermalized impact energy E ~120 eV (Te~40 eV, Ti~0 eV) Ion range ~ less than a few ML Implantation segregation sputtering, sublimation more study needed

Heated sample non-heated sample

In area A (heated W) No C observed near

CO gas puff In area B (heated W)

C diffusion in bulk W

22Summary

Roughness effect on C deposition Roughness significantly affects C deposition for both W and

graphite substrates Increase in amount of C deposition Extension of C deposition area

significant for large Ra (engineering surface : Ra~180 nm : W) Dependence on surface morphology

significant deposition on He exposed W surface despite low Ra (~15 nm) Carbon deposition at elevated temperature

Carbon deposition hardly occurred at least above ~520 ºC under TEXTOR edge plasma conditions

C behavior at elevated temperatures (~850 ºC) depends on incident carbon energy Sophisticated modeling needed

C deposition on W & C mixed layer Increase in C deposition with C concentration in tungsten (up to

60%C) in substrates. Only 10% of C in W enhance C deposition Its effect is less than roughness effect