cips sewg fr, jet 2008c. hopf o 2 /he glow discharge cleaning: experience at ipp christian hopf,...
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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
SEWG FR, JET 2008C. Hopf
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
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SEWG FR, JET 2008C. Hopf
(E) Efficiency of He/O2 mixtures
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SEWG FR, JET 2008C. Hopf
(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?
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SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
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SEWG FR, JET 2008C. Hopf
(E) Ion + O2 synergism
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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(A) Accessible Locations
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SEWG FR, JET 2008C. Hopf
(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
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SEWG FR, JET 2008C. Hopf
(B) Effect of Boron in Redeposited Layers
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SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
Erosion rate drops by factor of 50.
20 % B → factor 10 reduction
CIPS
SEWG FR, JET 2008C. Hopf
(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 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
Erosion rate drops by factor of 40.
20 % B → factor 10 reduction
CIPS
SEWG FR, JET 2008C. Hopf
(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 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 V
Boron erosion rate constant:
• Enrichment of boron at surface
• B eroded by physical sputtering
• Carbon eroded according to stoichiometry
Erosion rate drops by factor of 40.
20 % B → factor 10 reduction
CIPS
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(C) Collateral Damage
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SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
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
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SEWG FR, JET 2008C. Hopf
CIPS
SEWG FR, JET 2008C. Hopf
(E) Ion + O2 synergism
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
SEWG FR, JET 2008C. Hopf
(E) Ion + O2 synergism
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
SEWG FR, JET 2008C. Hopf
(E) Ion + O2 synergism
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
SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
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
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SEWG FR, JET 2008C. Hopf
(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
SEWG FR, JET 2008C. Hopf
(E) Ion + O2 synergism
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
SEWG FR, JET 2008C. Hopf
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