CIPS SEWG FR, JET 2008C. Hopf O 2 /He glow discharge cleaning: Experience at IPP Christian Hopf, Volker Rohde, Wolfgang Jacob Max-Planck-Institut für Plasmaphysik

CIPS

SEWG FR, JET 2008C. Hopf

O2/He glow discharge cleaning: Experience at IPP

Christian Hopf, Volker Rohde, Wolfgang Jacob

Max-Planck-Institut für Plasmaphysik

CIPS


Experience at IPP

ASDEX Upgrade (2005)

He/O2 DC glow discharge

5.4 A, ca. 600 V

2% O2 + 98% He

6 x 10-3 mbar

49 h total O2/He glow time

AUG + laboratory experiments:

(E) Efficiency of He/O2 mixtures as function of the mixture ratio

(A) Accessible locations

(B) Effect of boron in carbon redeposits

(C) Collateral damage: Physical sputtering of various materials and oxidation of tungsten

CIPS


(E) Efficiency of He/O2 mixtures

CIPS


(E) Why He/O2 mixtures?

Use of He/O2 mixtures rather than pure O2 because

• of safety hazards (oil-sealed rotary pumps, dust).

• He has lower physical sputtering yields.

• pure O2 DC glow discharges are hard to ignite/sustain.

→ How does the erosion rate depend on the mixture ratio?

CIPS


(E) Mass spectrometry during discharge in AUG

0.0 0.5 1.0 1.5 2.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

sig

na

l (1

0-5

A)

O2/(O2 + He) (%)

mass 32 28 44

0 2 4 60.0

0.5

1.0

1.5

sig

na

l (1

0-5

A)

discharge current (A)

mass 32 28 44

I = 5.4 A O2/(O2+He) = 0.6 %

erosion limited by ion flux density, not O2 supply

CIPS


(E) Dependence of rate on O2/(He + O2) ratio

0 20 40 60 80 1000.01

0.1

1

ero

sio

n r

ate

(n

m/s

)

R = O2 / (O2

+ He) (%)

hard a-C:H soft a-C:H

Rate saturation already at 10 % O2

ECR He/O2 discharge

Substrate bias: - 400 VPower constant: 150 WTotal pressure constant: 0.5 PaO2/He ratio changed

CIPS


(E) Ion + O2 synergism

Particle beam experiment:

Ar+ (20 to 800 eV) plus thermal O2 (background gas)

200 400 600 8000

5

10

15

20

0

2x1013

4x1013

6x1013

8x1013

Ar++O2

800 eV 400 eV 200 eV 100 eV 50 eV 20 eV

only O2yiel

d (C

/ A

r+)

temperature (K)

rate

(cm

-2 s

-1)

10 100 10000.1

1

10

1012

1013

1014

O2 at 800 K

800 K150 K300 K

yiel

d (

C /

Ar+

)

energy (eV)

rate

(cm

-2s-1

)

C. Hopf,a M. Schlüter, and W. Jacob, Appl. Phys. Lett. 90 (2007) 224106C. Hopf, M. Schlüter, T. Schwarz-Selinger, and W. Jacob, New. J. Phys., submitted

CIPS



0 1 10 100 1000 100000

1

2

3

4

5

0

1x1013

2x1013

400 eV Ar+ + O2 --> a-C:H

T = 300 Kjion = 4 x 1012 cm-2 s-1

yiel

d (C

/ A

r+)

R = jO2/ jAr+

rate

(cm

-2 s

-1)

Ar+ + O2: O2 flux dependence

CIPS


(E) Rate Saturation

Saturation at

Ion energy

jion (cm-2 s-1) p (Pa) jO2 (cm-2 s-1) % O2

AUG 300 eV 1 1014 0.6 < 7 1015 < 0.5

ECR 400 eV 5 1015 0.5 1.2 1017 10

The required oxygen flux densityscales roughly

like the ion (energy) flux density.

CIPS


(A) Accessible Locations

CIPS


(A) a-C:H samples in AUG

sectors 6, 7, 8 sectors 3, 11, 15

C removedNo C removed

Glow discharge

accesses large surface area

does not access shielded places, such as tile gaps, behind first wall, and deep in the divertor

See T. Schwarz-Selinger, tile gap experients

CIPS


(B) Effect of Boron in Redeposited Layers

CIPS


(B) “Real layers”

a-C:H → Tokamak redeposits

Laboratory glow discharge:

Layer removed

AUG:

No signs of erosion

Ion beam analysis of 2005 layers:

Dominated by boron

B/C >> 1

CIPS


(B) Effect of boron on a-C:H erosion

0.0 0.2 0.4 0.6 0.8 1.0

1013

1014

1015

1016

B + C

rem

oval

rat

e (a

tom

s cm

-2 s

-1)

B / (C+B)

a-B:C:H films (varying B content)

ECR oxygen discharge

RF substrate bias -60 eV

CIPS



0.0 0.2 0.4 0.6 0.8 1.0

1013

1014

1015

1016

B + C

rem

oval

rat

e (a

tom

s cm

-2 s

-1)

B / (C+B)




Erosion rate drops by factor of 50.

20 % B → factor 10 reduction

CIPS



0.0 0.2 0.4 0.6 0.8 1.0

1013

1014

1015

1016

B + C B C

rem

oval

rat

e (a

tom

s cm

-2 s

-1)

B / (C+B)






CIPS



0.0 0.2 0.4 0.6 0.8 1.0

1013

1014

1015

1016

B + C B C

rem

oval

rat

e (a

tom

s cm

-2 s

-1)

B / (C+B)



RF substrate bias -60 V

Boron erosion rate constant:

• Enrichment of boron at surface

• B eroded by physical sputtering

• Carbon eroded according to stoichiometry



CIPS


(B) Arcing during DC glow discharge

Layers’ conductivity decreased during venting prior to AUG GDC cleaning.

Surface charged positively. → Local arcs burnt holes through layer, tungsten, and graphite.

No energetic ion bombardment on major parts of the films.→ Main reason for finding no erosion of the layers.

CIPS


(B)

Conclusion:

High B concentrations in AUG layers would have significantlyreduced their erosion rates

but

as they had become insulating they experienced no energetic ion flux

and were not eroded at all.

CIPS


(C) Collateral Damage

CIPS


(C) Sputtering of C, Al, Fe, and W by He+ and O+

10 100 1000 1000010-4

10-3

10-2

10-1

100

O on Al (this work) He on Al (Eckstein, Bohdansky) He on Al2O3 (Eckstein, Bohdansky)

O on Al, TRIM.SP (this work) He on Al, TRIM.SP (Eckstein)

sput

terin

g yi

eld

energy (eV)

Target: Al and Al2O3

100 1000 1000010-3

10-2

10-1

100

sput

terin

g yi

eld

energy (eV)

O on C (Hechtl, Eckstein) He on C (Eckstein) O on C, TRIM.SP (this work) He on C, TRIM.SP (Eckstein)

Target: graphite

100 1000 1000010-3

10-2

10-1

100

sput

terin

g yi

eld

energy (eV)

He on Fe (Eckstein) O on Fe, TRIM.SP (this work) He on Fe, TRIM.SP (Eckstein)

Target: Fe

100 1000 1000010-4

10-3

10-2

10-1

100

sput

terin

g yi

eld

energy (eV)

O on W (Hechtl, Eckstein) O on W (this work) He on W (Eckstein) O on W, TRIM.SP (Eckstein) He on W, TRIM.SP (Eckstein)

Target: W

Reactivity of oxygen can

increase (carbon)

or

decrease (Al, W)

the sputtering yield.

CIPS


(C) Collateral damage: Selectivity

Erosion yields for 300 eV ion bombardment:

to be compared to a yield of 1 C/ion for a-C:H or 0.1 C/ion for B-impurified films

Selectivity: Keep ion energy low!

CIPS


(C) Redeposition of sputtered metals

O2/He DC GDC in stainless steel vessel:

Analysis (chemical, XRF, XPS): Fe, Cr, Ni, O → redeposited and oxidized SS

CIPS


(C) Collateral damage II: Oxidation of W

0 20 40 60 800

1

2

3

4

5

6

7

8

polished solid W W coatings

10

16 O

/ c

m2

exposure time (min)

28 min H2

after 17 min O2

O2 plasma -200 V

0 50 100 150 200 2500

10

20

30

40

50

60

10

16 O

/ c

m2

exposure time (min)

O2 plasma -400 V

H2 plasma -400 V

Oxidation of W saturates

Rapid oxygen removal in H discharge

CIPS


Summary

• Accessible locations: Critical issue: gaps

• O2/He mixtures: Required oxygen concentrations depend on ion energy flux density (understood by modelling)Typically 1–10% O2 required

• Effect of impurities:Boron: Huge reduction of erosion rate even at low concentrationsOther impurities that do not form volatile oxides:Experiments under well defined laboratory conditions needed

• Collateral damage: Selectivity: Keep ion energy low (problem for DC discharges)Oxidation: rapidly reversible, no problems reported after tokamak experiments

CIPS


CIPS



1013 1014 1015 1016 1017 10181012

1013

1014

1015

1016

1017T = 400 KE = 300 eVHe+ + O2

jion = 1 x 1013

jion = 1 x 1014

jion = 1 x 1015

jion = 1 x 1016

eros

ion

rate

(cm

-2 s

-1)

O2 flux density (cm-2 s-1)

jion = 1 x 1017

CIPS



1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 10231012

1013

1014

1015

1016

1017

1018

T = 400 KE = 300 eVHe+ + O2

jion = 1 x 1013

jion = 1 x 1014

jion = 1 x 1015

jion = 1 x 1016

eros

ion

rate

(cm

-2 s

-1)

O2 flux density (cm-2 s-1)

jion = 1 x 1017

CIPS



Particle beam experiment:

Ar+ (20 to 800 eV) plus thermal O2 (background gas)

C. Hopf,a M. Schlüter, and W. Jacob, Appl. Phys. Lett. 90 (2007) 224106C. Hopf, M. Schlüter, T. Schwarz-Selinger, and W. Jacob, New. J. Phys., submitted

200 400 600 8000

5

10

15

20

0

2x1013

4x1013

6x1013

8x1013

Ar++O2

400 eV 200 eV 50 eV

only O2

yiel

d (C

/ A

r+)

temperature (K)

rate

(cm

-2 s

-1)

10 100 10000.1

1

10

1012

1013

1014

O2 at 800 K

800 K150 K300 K

yiel

d (

C /

Ar+

)

energy (eV)

rate

(cm

-2s-1

)

CIPS


(3) a-C:H erosion samples: toroidal scan

123

4

5

6

7

8

9 1011

12

13

14

15

16

No a-C:H erosion where arcing occurred

C removedNo C removed

CIPS


0

100

200

300

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

100

200

300

Thi

ckne

ss [

nm]

Sector

a-C:H erosion samples: toroidal scan

Anodes:

Roof Baffle

Outer Divertor

Film thickness left of initial 350 nm

No a-C:H erosion where arcing occurred

CIPS


(B) Composition of AUG layers in 2005

Layers on all investigated tiles are boron and oxygen dominated

No significant erosion

No significant reduction of C or D concentration

HS IUD OUD0

10

20

30

40

50

60

70

80

90

Ato

mic

Fra

ctio

n (%

)

B

12C D O

13C

before O2 glow

HS IUD OUD0

10

20

30

40

50

60

70

80

90

B

12C D O

13C

after O2 glow

minimum and maximum concentrations on tiles

CIPS



0 1 10 100 1000 100000

1x1013

2x1013

400 eV Ar+ + O2 --> a-C:H

T = 300 Kjion = 4 x 1012 cm-2 s-1

yiel

d (C

/ A

r+)

R = jO2/ jAr+

rate

(cm

-2 s

-1)

Ar+ + O2: O2 flux dependence

CIPS


Where do the removed 25 g C come from?

Eroded carbon: 25 g = 1.2 x 1024 C atomsIons: 50 h x 5.4 A = 6 x 1024 Ions

Yield: 0.2 (in lab experiment: 0.1)

Arc pits: 40 holes / cm2

1 mm x 0.1 mm x 7 µmassumption: 20 m2 affected

8 g C eroded

Not W-covered surface: 34 %

Carbon dust

Not analyzed tiles with C-rich layers

Documents

CIPS SEWG FR, JET 2008C. Hopf O 2 /He glow discharge cleaning: Experience at IPP Christian Hopf, Volker Rohde, Wolfgang Jacob Max-Planck-Institut für Plasmaphysik