Mixed-material studies in PISCES-B · 10/4/2006 · Mixed-material studies in PISCES-B R. Doerner...

U C S DUniversity of California San Diego

R. Doerner, VLT call, Oct. 4, 2006

Mixed-material studies in PISCES-B

R. Doerner for the PISCES Research TeamPart of the US-EU collaboration on

Mixed-Material Effects on ITER

• Introduction

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

• Summary of possible mixed-material implications forITER



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

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



Erosion suppression exhibits atemporal evolution (τBe/C)

• Understanding the temporalbehavior is critical todetermining thefundamental mechanismsresponsible for erosionmitigation

• PMI modeling codes shouldbe able to reproducetemporal behavior toprovide 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)- I

CD/I

D!(far)

20060322

"Be/C

= 86±1 sec



XPS data shows Be2C formation in resultantmixed-material surface

• Virtually all C remaining atthe surface is bound ascarbide (t > τBe/C)

• Presence of carbide inhibitschemical erosion of C

• Carbide layer reducessputtering yield of bound Be

• Subsequently deposited Becan be more easily eroded

• Codeposits are primarily Beonce 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 Be should inhibit Catoms 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-situ Be seeding datashows similar behavior ofchemical erosionmitigation

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Be surface fraction



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

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

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

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

– Be2C surface thicknesssaturates after carbide forms50s in this exposure [Baldwin JNM2006]

– Resultant codepositedmaterial is primarily Be [BaldwinJNM 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 nearWindow 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 nearWindow transmission

CD/Dg near

Time [s]



WPM samples show collection of beryllium-richcodeposits 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

positio

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 (τBe/C) depends on severalquantities that can be varied almost independently

101

102

103

104

105

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

!i ~ 1e22 m-2

s-1

, T s ~ 690±10 K

!i ~ 3e22 m-2

s-1

, T s ~ 600±50 K, Ref. [2]

!i ~ 3e22 m-2

s-1

, T s ~ 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 ~ 1.7-4.5x10

22 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 ~ 1-3x10

22 m

-2s

-1

cBe = 10-3, # = -2

!Be/C = 1.0 exp(4.6x103/T

s)

Be concentrationin plasma

Incident ionenergy

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



PISCES chemical erosion mitigation time scalingpredicts 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 ITERconditions (τBe/C

ITER ~ 6 msec)

[cBe = 0.05, Ei = 20 eV, Ts = 1200 K and Γi = 1023 m-2s-1]100

101

102

103

104

105

100 101 102 103 104 105

!B

e/C

exp [

s]

!Be/C

scale [s]

Closed: Ts ~ 550-700 K

Open: Ts ~ 800-970 K

Ei ~ 15 eV

Ei ~ 30-40 eV

Ei ~ 85 eV

"

3e# 4 $ cBe $1e# 2

15 $Ei [eV]$ 85

550$ Ts [K]$ 970

1$ %i [e22 m#2s#1]$ 4.5

τBe/Cscale [s] = 1.0x10-7 cBe

-1.9±0.1 Ei0.9±0.3 Γi

-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 suppresscarbon target erosion at temperatures up to 1000°C

• ITER will experience large temperature excursions (up to3800°C) at the carbon dump plates during periodic ELMs

• Will the thin, surface Be, Be/C layers survive such dramatictemperature 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 positivesample biasing during plasma discharges.



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

• During 1.5 MW/m2 powerpulse graphite surfacetemperature rises to~2000°C (by pyrometers)

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

• Surface temperature rise islimited by power supplies(IPP has supplied a newpower 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 Be2Cformation leading to shorter mitigation times

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

• Pulsing surface temperatureto the 1200°C range resultsin faster chemical erosionsuppression– Be2C disassociates at

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

2471°C at 1 atm• D retention during transient

surface heating alsoincreases by ~50% bothwith and without Be plasmaseeding



How might mixed materials impact ITER?• Due to elevated temperature of C dump

plates, carbides will likely form and limitC erosion

• If a full C divertor were employed, carbideformation on regions of the baffles, wherethe temperature is lower, would takelonger, resulting in more C erosion andthereby more hard-to-remove tritium

• Be deposition on W baffles will likely notresult in significant beryllide formation(TW ~ 400°C)

• If a full W divertor were used, beryllideformation near the strike points would be aconcern (perhaps an issue for the JETITER-like wall experiments)

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

Mixed-material studies in PISCES-B · 10/4/2006 · Mixed-material studies in PISCES-B R. Doerner...

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