Mixed-material studies in PISCES-B R. P. Doerner, M. J. Baldwin, J. Hanna and D. Nishijima

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

R. Doerner, IAEA CRP on H in Materials, Vienna, Sept. 26, 2006

Mixed-material studies in PISCES-B

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

• Introduction

• Technical results– Temporal behavior of chemical erosion suppression– Response of Be/C to thermal transients– Be/W formation conditions

• Summary of possible mixed-material implications for ITER



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

P-B experiments simulateBe erosion from ITER wall,subsequent sol transport and interaction with W bafflesor C dump plates, as well asinvestigation of codepositedmaterials using witness plates



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



Erosion suppression exhibits a temporal evolution (Be/C)

• Understanding the temporal behavior is critical to determining the fundamental mechanisms responsible for erosion mitigation

• PMI modeling codes should be able to reproduce temporal behavior to provide confidence

10-4

10-3

10-2

10-1

100

101

0 100 200 300 400 500 600

Inte

nsity

[a.u

.]

Time [s]

ID

IBeI/ID

ICD/ID(near)-ICD/ID(far)

20060322

Be/C = 86±1 sec



XPS data shows Be2C formation in resultant mixed-material surface

• Virtually all C remaining at the surface is bound as carbide (t > Be/C)

• Presence of carbide inhibits chemical erosion of C

• Carbide layer reduces sputtering yield of bound Be

• Subsequently deposited Be can be more easily eroded

• Codeposits are primarily Be once carbide layer forms

XPS analysis of Be on C sample surface[M. Baldwin et al., in press JNM]



If Be acts like B doping, then each Be atom should inhibit two C atoms from chemically eroding

• Chemical erosion model [Schenk

et al. JNM 220-222(1995)767] predicts boron reduces sp2 component in favor of sp3 hybridization. In other words, each B atoms inhibits a C=C bond, thereby affecting 2 C atoms

• In-situ Be seeding data shows similar behavior of chemical erosion mitigation, i.e. each Be surface atom is consistent with inhibiting 2 C atoms from chemically eroding

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Y chem vs Be surface fraction

PISCES-B dataB doping modelCoster's PSI equation

Be surface fraction



In-situ Be doping of graphite exhibits similar behavior to boron doping of graphite

• Dopant increases retention

• Dopant shifts hydrogenic release to lower temperature

Temp (K)

Time (sec)

From A. Schenk JNM 220-222(1995)767 (B doping).Solid line Be seedingDashed line no Be seeding



Increased retention due to dopant becomes less significant during higher temperature exposures

Ts (K)

500 1000

Ret

entio

n (D

ato

ms

/m2 )

1020

1021

1022

1023

C target

C target, fBe~0.001

Be target, fCD4~0.02

Be target

Doerner et al. (1999)

Present study

• Solid symbols indicate mixed Be/C surfaces formed during plasma exposure, open symbols are from clean substrate data

• Mixed Be/C layers retain more deuterium than either Be samples or carbon samples exposed to similar plasma

• Difference in retention becomes less pronounced during higher temperature exposure

• Plasma contacting surfaces are not ITER’s main tritium inventory concern



Be first shuts down C chemical erosion, then subsequent Be re-erodes from surface

• Evolution of a mixed Be/C surface– Be oven opens at t = 0 sec.– Be ions arriving at t < 50s

shut down chemical erosion by forming Be2C surface layer [Baldwin JNM 2006 available on-line]

– Once Be2C is formed, subsequent Be arriving (T > 50 s) is more easily eroded and begins coating windows

– Be2C surface thickness saturates after carbide forms 50s in this exposure [Baldwin JNM 2006]

– Resultant codeposited material is primarily Be [Baldwin JNM 337-339(2005)590]

0

0.5

1

1.5

2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000

20060718_all [BeII/ne ~ 0.06%]

BeI/Dg near

Window transmission

CD/Dg near

Time [s]

0

0.5

1

1.5

2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500

20060718_all [BeII/ne ~ 0.06%]

BeI/Dg near

Window transmission

CD/Dg near

Time [s]



Be2C layer thickness saturates

• Be layer observed after ~3E22 Be+/m2 (i.e 1600 s) accounts for virtually all incident Be

• Be layer after 1E23 Be+/m2 (i.e 4800 s) accounts for only ~30% of incident Be

• Be/C under these plasma exposure conditions would be ~ 2000 sec

Scattered energy (keV)

0.5 1.0

Cou

nts

(A

rb. u

nits

)

01

C

~ Depth (m)

0.00.5

x103

Be+fluence ~

3 x 1022 m-2

STD

Be+fluence ~

1 x 1023 m-2

O

Be

From M. Baldwin et al., in press JNM

RBS spectra of P-B samples exposed to unseeded and Be-seeded plasma



WPM samples show collection of beryllium-rich codeposits during Be seeding runs

Carbon target : 300ºC target exposure Carbon target : 700ºC target exposure

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

100500

com

po

sitio

n (

%)

Depth (nm)

Be% C% O% Ta%

0 20 40 60 80 100 120

0

10

20

30

40

50

60

70

80

90

100

100500

Be% C% O% Ta-%

Ato

mic

%

Depth (nm)

More C is detected in codeposits during lower C target temperature exposure (possibly due to a combination of lower chemical erosion yield and/or quicker beryllium carbide layer formation)



Chemical erosion suppression time (Be/C) depends on several variables that can be varied almost independently

101

102

103

104

105

10-4 10-3 10-2 10-1

i ~ 1e22 m-2s-1, Ts ~ 690±10 K

i ~ 3e22 m-2s-1, Ts ~ 600±50 K, Ref. [2]

i ~ 3e22 m-2s-1, Ts ~ 955±20 K

Be/C

[s]

cBe

= 1.3e-3 = -2.1

= 1.1e-4 = -2.0

= 5.4e-4 = -2.1

Be/C = cBe

Ei ~ 40 eV 101

102

103

101 102

Be

/C [

s]

Ei [eV]

cBe = 10-3, = -2

Ts ~ 852-973 K, i ~ 1.7-4.5x1022 m-2s-1

Be/C = 1.5xEi1.2

102

103

0.001 0.0012 0.0014 0.0016 0.0018

Be/

C [

s]

1/Ts [K-1]

Ei ~ 30-40 eV, i ~ 1-3x1022 m-2s-1

cBe = 10-3, = -2

Be/C = 1.0 exp(4.6x103/Ts)

Be concentrationin plasma

Incident ionenergy

Surface temperatureof target From D. Nishijima et al., PSI17.



PISCES chemical erosion mitigation time scaling predicts suppression between ELMs in ITER

From D. Nishijima et al., PSI17. • Surface temperature effects reaction rate

• Be plasma concentration effects arrival rate at surface

• Ion energy effects erosion rate

• Ion flux impacts through redeposition

• Type of graphite does not seem to play a significant role (ATJ vs. CFC)

• Scaling law using these variables has been developed to allow extrapolation to ITER

conditions (Be/CITER

~ 6 msec) [cBe = 0.05, Ei = 20 eV, Ts = 1200 K and i = 1023 m-2s-1]

100

101

102

103

104

105

100 101 102 103 104 105

Be

/Ce

xp [

s]

Be/Cscale [s]

Closed: Ts ~ 550-700 KOpen: Ts ~ 800-970 K

Ei ~ 15 eVEi ~ 30-40 eVEi ~ 85 eV

3e 4 cBe 1e 2

15Ei [eV]85

550Ts [K]970

1i [e22 m 2s 1]4.5

Be/Cscale [s] = 1.0x10-7 cBe

-1.9±0.1 Ei0.9±0.3 i

-0.6±0.3 exp((4.8±0.5)x103/Ts)

x

X =CFC



Thermal transient experiments:Motivation for positive pulse biasing

• PISCES has shown that Be plasma impurities suppress carbon target erosion at temperatures up to 1000°C

• ITER will experience large temperature excursions (up to 3800°C) at the carbon dump plates during periodic ELMs

• Will the thin, surface Be, Be/C layers survive such dramatic temperature excursions?

• How will Be-W react during temperature excursions?

• It is possible to simulate the large temperature excursions associated with ITER ELMs in PISCES-B using positive sample biasing during plasma discharges.



Large power loads can be drawn to P-B sample during positive biasing

• During 1.5 MW/m2 power pulse graphite surface temperature rises to ~2000°C (by pyrometers)

• Bulk graphite temperature rise at back of sample ~20°C during 0.1 s. pulse (thermocouple)

• Surface temperature rise is limited by power supplies (IPP has supplied a new power supply as part of US-EU collaboration)



10-3

10-2

10-1

100

0 100 200 300 400 500 600

w/o H.P. (20060322)exponential fit for w/o H.P.w/ H.P. (20060323)exponential fit for w/ H.P.

Time [s]

BeII/ne ~ 0.13 %

Be/C ~ 17 sec

Be/C ~ 83 sec

Transient surface heating promotes Be2C formation leading to shorter mitigation times

Surface temperature during heat pulse ~ 1200°C[from R. Pungo et al., PSI17]

• Pulsing surface temperature to the 1200°C range results in faster chemical erosion suppression– Be2C disassociates at

~2200°C at 1 atm– Beryllium boiling point =

2471°C at 1 atm

• D retention during transient surface heating also increases by ~50% both with and without Be plasma seeding



0

500

1,000

1,500

2,000

0 5,000 10,000 15,000

Rea

ctio

n La

yer T

hick

ness

(nm

)

Time (seconds)

850 C

D 58 x 10-14

cm2sec

-1

750 C

D 0.43 x 10-14

cm2sec

-1950 C

Tungsten beryllide (BexW) formation may plague hot W plasma facing components

• Be2W and Be12W appear preferred (Be22W not seen)

• Beryllides only form in high temperature W surfaces (> 600°C)

• Be diffusion rate into W becomes significant above ~800°C

• At high temperature, Be availability (high vapor pressure of Be) on the surface can limit growth rate

Measurements from SNLL



Plasma conditions will play a dominant role in determining the impact of beryllides

• At high surface temperature, Be sublimation may prevent significant beryllide formation

• Sputtering at higher incident ion energies also tends to prevent significant beryllide formation

• Higher incident plasma flux tends to push energy, and temperature, necessary to avoid beryllides to larger values

T surf (K )

1100 1200 1300 1400 1500 1600

f Be+

10-3

10-2

10-1

10 eV

25 eV

50 eV

100 eV

D +=2 x1018 cm -2s -1

PISCES

D +=2 x1019 cm -2s -1

ITER

Uncoated Wsurface

Be coveredW surface

M. J. Baldwin et al., PSI17

fBeplasma(1–Rf) = YD-Beplasma(1–Rd) + fBeYBe-Beplasma(1–Rd) +evap(1–Re) + Dbulk



How might mixed materials impact ITER?

• Due to elevated temperature of C dump plates, carbides will likely form and limit C erosion

• If a full C divertor were employed, carbide formation on regions of the baffles, where the temperature is lower, would take longer, resulting in more C erosion and thereby more hard-to-remove tritium

• Be deposition on W baffles will likely not result in significant beryllide formation (TW ~ 400°C)

• If a full W divertor were used, beryllide formation near the strike points would be a concern (perhaps an issue for the JET ITER-like wall experiments)

• Beryllide formation in ITER only appears to be a concern on the W cassette liner ‘louvers’ (that are designed to be hot surfaces)

Date post:	07-Jan-2016
Category:	Documents
Upload:	afi
View:	30 times
Download:	1 times

Mixed-material studies in PISCES-B R. P. Doerner, M. J. Baldwin, J. Hanna and D. Nishijima

Documents