R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Beryllium and carbon mixed-material studies R....

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, Oct. 18, 2005EU PWI TF meeting, France

Beryllium and carbon

mixed-material studies

R. P. Doerner, M. J. Baldwin, J. Hanna and D. Nishijima

Center for Energy Research, University of California – San Diego

R. Pungo, K. Schmid, and J. Roth

Max-Plank Institute for Plasmaphysics, Garching, Germany

Work performed as part of US-EU Collaboration on Mixed-Material PMI Effects for ITER



Outline

• Technical results– Temporal behavior of chemical erosion suppression– Scaling with Be plasma concentration

• Near term US-EU goals/experiments– Next few months

• Longer term US-EU goals/experiments

• Possible ‘new’ US PFC Program Direction



Motivation for PISCES Mixed Materials Studies

• UC San Diego PISCES and EFDA are investigating the influence of Beplasma impurities on exposed materialsinteractions relevant to ITER.

• The ITER design: Be first-wall,W divertor, C (graphite) strike points.

• Diverted plasma expected to ‘dirty’:Eroded Be impurity conc. up to 10 %.

PISCES-B ExperimentsSimulation This Region



PISCES-B, and Its Associated Surface Analysis Laboratory, Are Compatible with

Beryllium Operations.

- The PISCES-B plasma generator is contained in a separate enclosure to prevent release of beryllium particulates into the general lab.

- All personnel entering the enclosure wear full coverage personal protective equipment and follow strict entry and exit procedures when performing routine maintenance, machine repair, or sample handling/exchange operations.

- The entire enclosure has a specially designed ventilation system that filters all input and output air streams.



PISCES-B has been modified to allow exposure of samples to Be seeded plasma

Radial transport guard

102 mm

153 mm

76 mm

195 mm

45 o

Cooled target holder

Heatable deposition probe assembly

Thermocouple

Thermocouple

Water cooled Mo heat dump

Resistive heating coils

High temperature MBE effusion cell used to seed plasma with evaporated Be

12 °

PISCES-B PlasmaTarget

Depositionprobesample

Axial spectroscopic field of view

Berylliumimpurityseeding

Radial transport guard

102 mm

153 mm

76 mm

195 mm

45 o

Cooled target holder

Heatable deposition probe assembly

Thermocouple

Thermocouple

Water cooled Mo heat dump

Resistive heating coils

High temperature MBE effusion cell used to seed plasma with evaporated Be

12 °

PISCES-B PlasmaTarget

Depositionprobesample

Axial spectroscopic field of view

Berylliumimpurityseeding



A small beryllium impurity concentration in the plasma drastically suppresses carbon erosion

-50 V bias, 200ºC, Te = 8 eV, ne = 3 e 12 cm-3

Chemical erosion Physical sputtering

500

1000

1500

2000

2500

3000

3500

4000

No Be injection0.2% Be ion concentration

Wavelength (nm)

CD band

D gamma Be I

459445431 452438

4000

8000

1.2 104

1.6 104

2 104

2.4 104

2.8 104

No Be injection0.2% Be ion concentration

Wavelength (nm)

C I

941.5940.5939.5938.5937.5



Be rich surface layers form during exposure and shield underlying carbon from erosion

0

20

40

60

80

100

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

T surface ~ 250CT surface ~ 700C

Be plasma concentration (%)

• ITER is expected to have 1-10% Be impurity concentration in the divertor plasma

0.00 0.05 0.10 0.15 0.20-30

-20

-10

0

Bias: 50V~500K

No Be Be seeding

~1000K No Be Be seeding

No

rma

lize

d w

eig

htl

os

s [

10

-26 m

g m

2]

Be1+ plasma concentration [%]

De

cre

as

ing

ero

sio

n

Weight loss dataconfirms reduction in

erosion seen spectroscopically

Equilibrium Be surface concentration after P-B exposure.



Binding energy (eV)

110115

N(E

) (

Arb

units

)

0250

500

750

1000

Be 1s

carb

idic

meta

llic

Be oxideBe, afterD2 plasma

112.2

eV

111.8

eV

Graphite,afterBe seededD2 plasma

280285

35

00

40

00

45

00 C 1s

gra

ph

ite

carb

idic

XPS data shows surface layer is largely Be2C

• Virtually all C remaining at the surface is bound as carbide

• Presence of carbide inhibits chemical erosion of C

• Carbide layer reduces sputtering yield of bound Be

• Subsequently deposited Be can more easily erode

XPS analysis of Be on C sample surface



Inte

nsity

(Arb

. units)

10-3

10-2

10-1

100

103

104

Discharge time (h)

0.0 0.2 0.4

Part

ial pre

ssure

(T

orr

)

10-8

10-7

10-6

10-3

10-2

0.0 0.2 0.4

D2 (4)

CD3 (18)

CD4 (20)

022604 022704RN #

(a)

(b)

(d)

(c)

baseline

Be I/DCD/DD

CD/D baseline

Be2C layer formation time is measured by decrease in CD band intensity

• CD band intensity near C target drops w/ time as Be erosion signal from target increases

• CD signal approaches background device level (walls)

• RGA signals during plasma exposure shows similar drop in m/q = 20 & 18 (CD4 & CD3)

• Identical exposure of Be coated sample on the following day show changes persist and are not associated with plasma start-up or wall changes



• Increasing Be plasma concentration decreases the Be layer formation time

• Solid lines are exponential decay fits ICD = exp(-t/fBe

)

• Decrease in CD band intensity is correlated to concentration of C in the surface layerTime (s)

0 500 1000 1500 2000

Nor

m. C

D B

and

stre

ngth

(A

rb. u

nits

)

0.1

1

Sur

face

car

bon

conc

entr

atio

n

0.1

10.18 % Be0.41 % Be

0.13 % Be

1.10 % Be

0.03 % Be

0.16% Be

0.04% Be

PISCES-B conditions: fBe ~ 0.001, pl = 3e18 cm-2s-1, Be = 3e15 Be/cm2s or 1 Be monolayer/sec

Chemical erosion mitigation time depends of Be plasma concentration



Rate of chemical erosion decay (1/fBe)

• ICD = exp(-t/fBe)

• We can obtain the scaling of

the erosion decay rate by fitting

data with a power law

expression: 1/fBe = fBe

,

obtaining = 785 s-1, = 2.07

• Assuming this holds for ITER

with 1%< fBe (ITER)<10%, then

16 sec < fBe (ITER)

< 0.1 sec

.

increases with plasma beryllium concentration

fBe x100

0.01 0.10 1.00

1/ f B

e (s-1

)

10-5

10-4

10-3

10-2

10-1

10 s

100 s

1000 s

10000 s

(Plasma Be concentration %)



0

20

40

60

80

100

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

T surface ~ 250CT surface ~ 700C

Be plasma concentration (%)

‘Equilibrium’ Be surface concentration after P-B exposure.

Time (s)

0 500 1000 1500 2000

Nor

m. C

D B

and

stre

ngth

(A

rb. u

nits

)

0.1

1

Sur

face

car

bon

conc

entr

atio

n

0.1

1

0.18 % Be0.41 % Be

0.13 % Be

1.10 % Be

0.03 % Be0.04% Be

0.16% Be

From K. Schmid et al., Nucl. Fusion 44 (2004)815.

Recall data from previous viewgraphs and ICD = exp(-t/fBe

)



fBe

0.000 0.002 0.004

¨ I

CD

(A

rb u

nits

)

0.0

0.5

1.0

¡ A

ES

C

(%)

050

100

texp = 0.167 h

Data fromRef [9]

(c)

Erosion mitigation time scaling is also valid for previous PISCES data sets

• Surface composition AES data (solid circles) from K. Schmid et al., NF (2004) were measured after 0.55 – 2.8 hrs. Grey shaded region corresponds to those exposure times

• Open circles correspond to AES data collected after 0.167 hr plasma exposures

• Open squares correspond to CD intensity measurements made at 0.167 hrs. during plasma exposure

K. Schmid et al., NF (2004).



Near term collaboration goals

• Extend CD decay scaling toward ITER– Incident Ion Flux– Ion Energy

• Quantify results of Be injection with simultaneous CD4 puffing into plasma

– Target behavior– Witness Plate Deposition

• Be/W alloy formation conditions



Longer term collaboration goals

• Heat pulse deposition to mixed-material targets– Pulse positive bias to targets (collect electrons)

• Undergoing circuit debugging on PISCES-A

– Measure surface temperature evolution• Fast (~1 sec) response with filtered photodiodes• NIR (0.9 m – 1.6 m) spectra acquisition in 0.5 msec

timescale for blackbody temperature fit (EU camera)

– Measure surface layer response during & after pulses

• Be seeding and CD4 puffing on W targets

• Role of oxygen in Be-rich codeposits



Possible New Direction for the US-PFC Community

• Carbon may lead to insurmountable tritium accumulation in ITER

• Removal of hydrogenated carbon codeposits at sufficient rates to benefit ITER has not been demonstrated

• ITER technology phase uses a tungsten divertor target

• Metal PFCs also still have a variety of problems– Alloy formation, melting, tritium accumulation

and removal, fatigue, etc.



US PFC Community is proposing to focus its efforts towards an ‘all-metal’ ITER scenario

• De-emphasize C work both experimentally and in modeling, some critical C investigations would continue

• Modeling will stress identifying ITER edge, SOL and divertor plasma scenarios compatible with metal PFCs

• Experiments will study mixed-materials, tritium retention and high heat load response of metals

• Direction is seen as complementary to the JET ITER-like wall project

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Beryllium and carbon mixed-material studies R....

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