Report of the Special Expert Working Group on Chemical Erosion

1
Report of the Special Expert Working Groupon
Chemical Erosion
S. Brezinsek
Institut für Plasmaphysik, Forschungszentrum
Jülich GmbH, EURATOM Association, Trilateral
Euregio Cluster, D-52425 Jülich, Germany
SEWG members CEA E. Gauthier, J. Hogan
CIEMAT F. Tabares FZJ A. Pospieszczyk,
A. Kirschner, A. Kreter, D. Borodin, G.
Sergienko, V. Philipps IPP W. Jacob, J. Roth,
Ch. Hopf, M. Mayer, M. Schlüter, T.
Schwarz-Selinger R. Pugno, A. Kallenbach, W.
Bohmeier UKAEA M.F. Stamp
2
Status ITER - tritium inventory limit
WBC, PSI 2002
600 full performance shots!
Roth/Kirschner ERO, PSI 2004
3
Flux dependence of Ychem

• Description of Y as function of
• Ion energy
• Surface temperature
• Ion flux

Ylow(E,T)
Y(E,T,f)
0.54
f
1
61021

• In-situ calibration mass/optical spectroscopy
• Temperature normalisation to Tmax
• Ion energy normalisation to 30 eV
• Flux dependence

Roth et al. EPS2003, PSI 2004
4
ERO modelling for ITER scenarios

• Plasma simulation from B2 Eirene
• Steady-state temperature profiles
• along the divertor target
• (Federici 2003)

standard ITER operation
Roth, Kirschner et al. NF 2004
But No consideration of ELMs, no a-CH layers,
no synergetic effects
5
Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• Mixed layers, Be and W (e.g. PISCES)
• He, Ar (e.g. MAJESTIX), N2 (e.g. JET, AUG)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

6
Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• Mixed layers, Be and W (e.g. PISCES)
• He, Ar (e.g. MAJESTIX), N2 (e.g. JET, AUG)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

7
Spectroscopic data interpretation
TEXTOR pre-heatable graphite limiter
Previous experiments (in Roth data base) Y4.2
at Tmax830 K
New experiments with intensity
corrections Y3.3 at Tmax830 K

• Intensity correction
• Photon efficiency correction of Dg due to D2
  molecules
• - Impurity background in the CD spectrum
  subtracted

Brezinsek, Pospieszczyk et al. 2004
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Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• Mixed layers, Be and W (e.g. PISCES)
• He, Ar (e.g. MAJESTIX), N2 (e.g. JET, AUG)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

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Higher Hydrocarbons
Higher hydrocarbons only partially treated!
JET C2 light from C2H4 injection
YC2H4 3.0
Vignettation l-o-s
t / s
Brezinsek, Stamp 2004
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Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• Mixed layers, Be and W (e.g. PISCES)
• He, Ar (e.g. MAJESTIX), N2 (e.g. JET, AUG)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

11
Plasma regimes H-mode in AUG
There is an ion flux dependence in H-mode
discharges
with ELMs
Data not normalised!
Pugno et al. PSI 2004
Integrated over ELMs!
12
Plasma regimes detachment

• Detached plasmas still large uncertainties, big
  scatter of data
• gt DIII-D practically no chemical erosion
  (Whyte et al NF 2001)
• -gt JET inner divertor 15 erosion (strike point
  on vertical target),
• less on horizontal target
  (Stamp 2003)
• -gt AUG
• -gt TEXTOR test limiter increase of C2 signal!
  Higher erosion
• Effect of rate coefficients?
• (Pospieszczyk EPS 1995)

High flux and low energy should reduce the
erosion yield!
New experiments necessary!
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Geometry and strike-point positions

• Strike-point position important
• Stepwise transport of carbon

Esser et al. PSI 2004
QMB deposition monitor
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Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• He, Ar (e.g. MAJESTIX) on a-CH layers, N2
  (e.g. JET, AUG)
• Mixed layers, Be and W (e.g. PISCES)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

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a-CH layers in the inner divertor of JET

• Sweep of the strike point through the corner in
  L-mode

a-CD layer

• Strong C2 light emission from hydrocarbon layers
  gt C2Dy
• Different ratio of C2 to CD light
• Removal within one discharge in H-mode with
  strike-point
• fixed at the location of the layer

Brezinsek et al. PSI 2004
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a-CH layers in the inner divertor of JET

• Strike point fixed at the location of the a-CD
  layer
• Three identical discharges with L- and H-mode
  phase

• Removal of a (soft) a-CD layer / main release
  product C2Dy
• Base value indicates bulk material or a
  different type of a-CD layer
• (gt Planck radiation M.F. Stamp P3-50)

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a-CH layers in the inner divertor of JET

• Repetition of six discharges with strike point
  sweep in L-mode

Different gas injection in the later phase of
the discharge!
Net-erosion?
Brezinsek et al. PSI 2004
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Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• He, Ar (e.g. MAJESTIX) on a-CH layers, N2
  (e.g. JET, AUG)
• Mixed layers, Be and W (e.g. PISCES)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

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Synergetic effects Ion beam and H
UHV experiment with 2 radical beam sources and
one ion beam source
Setup MAJESTIX
W. Jacob, Ch. Hopf, A. von Keudell, M. Meier, and
T. Schwarz-Selinger Particle-beam Experiment
to Study Heterogeneous Surface Reactions Relevant
to Plasma-assisted Thin Film Growth and Etching,
Review of Scientific Instruments 74, 5123-5136
(2003).
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Synergetic effects chemical sputtering
Combined interaction of ions with atomic H leads
to chemical sputtering here ratio H/ion
400 strongly enhanced erosion
atomic H ions
on hard a-CH layer
Model parameters a 0.4 R ? 400
Status TW4-TPP-ERCAR Intermediate Report Jan.
2004
Ch. Hopf, A. von Keudell, and W. Jacob, Chemical
Sputtering of Hydrocarbon Films, J. Appl. Phys.
94, 2373 (2003). Modeling results Ch. Hopf and
W. Jacob, unpublished, N2 data M. Meier et al.,
unpublished
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Energy dependence modelling
Predictions of the chemical sputtering yields for
various fusion relevant species based on the
model by Hopf et al.
Model parameters a 0.4 R ? 400
Status TW4-TPP-ERCAR Intermediate Report Jan.
2004
Ch. Hopf, A. von Keudell, and W. Jacob, Chemical
Sputtering of Hydrocarbon Films, J. Appl. Phys.
94, 2373 (2003). Modeling results Ch. Hopf and
W. Jacob, unpublished
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Synergetic effects new results
Chemical sputtering yield for the simultaneous
interaction of H and Ne Good agreement with
the model by Hopf et al.
Status TW4-TPP-ERCAR New results
M. Schlüter, unpublished
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New results Erosion in N2/H2 plasmas
Erosion was measured in low-temperature ECR
plasmas in mixtures of N2 and H2 (p 1 Pa). The
ion energy was varied by applying an rf bias to
the sample electrode. Steep increase of erosion
rate with increasing N2 admixture (factor
15). Maximum for 20 to 30 N2. Slow decrease
from 30 to 100 N2.
Status TW3-TPP-SCAVOP New results
Sun Chao, Ch. Hopf, T. Schwarz-Selinger, and W.
Jacob, unpublished
Scavenger experiments performed by Tabares in
JET, ASDEX, PISCES gt see CIEMAT
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Synergetic effects Be and C

• Expose C target to Be seeded D plasma, Target
  Bias 50V
• Increase Be concentration from 0.0 to 0.15
  during exposure
• Monitor Be1 line and CD-band
• Target temperature 500K ? high chemical erosion

• CD-band decreases by 80 for a Be plasma
  concentration of 0.15

• Be surface coverage after exposure 87 (AES).

Indicates shielding effect through Be layer
formation
Schmidt et al. PSI 2004
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Synergetic effects Be and C

• At target temperatures from 500K to 1280K

• Be plasma concentrations from 0 to 0.35

• Measure (AES) Be surface concentration after
  exposure

• Be surface concentration increases quickly with
  Be plasma concentration

• At temperatures 1200K diffusion and thermally
  enhanced erosion reduce Be surface coverage

Almost full surface coverage for Be plasma
concentrations lt 1
Schmidt et al. PSI 2004
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Topics to be discussed

• Improvement in the data evaluation
• Better spectroscopic data interpretation
• Photon efficiencies
• Higher Hydrocarbons
• Influence of
• ELMs (e.g. AUG) detached plasmas (e.g. DIII-D,
  JET)
• Geometry and strike-point positions (e.g. JET,
  AUG)
• Erosion of redeposited a-CH layers (e.g. JET)
• Synergetic effects
• He, Ar (e.g. MAJESTIX) on a-CH layers, N2
  (e.g. JET, AUG)
• Mixed layers, Be and W (e.g. PISCES)
• ERO modelling
• Benchmark experiments (e.g. PSI-2, TEXTOR)

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ERO modelling
- Parallelisation of ERO code - Improvement of
surface model for material mixing (TriDyn) -
Merging of various versions (limiter divertor
linear devices) - Including global transport for
divertor devices, code coupling?? - Modelling
transport of injected hydrocarbons TEXTOR,
JET, AUG (D/XB, deposition efficiency,
re-erosion of deposits) chemical erosion
studies (Roth formula, JET, ITER) PISCES (Be-C
layers, tungsten) formation of mixed layers
(dedicated experiments at TEXTOR)
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Example ERO modelling for ITER
Erosion re-deposition along outer target Roth
vs. 1 yield
Sticking 0 for hydrocarbons, MolDyn for atoms
with Roth yield 94 re-deposition 6 loss to PFR
with fixed yield 80 re-deposition 20 loss to
PFR
? Gross erosion 10 times larger with fixed 1
than Roth yield. ? Local re-deposition
significantly smaller with fixed yield.
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ERO modelling PSI-2 experiments

• comparison of databases (Erhardt Langer
  Alman, Ruzic Brooks,
• Janev Reiter)
• check of sticking coefficients
• in-situ study of erosion and deposition

Bohmeyer et al. PSI2004
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Future work

• Improvement in the data evaluation
• Erosion yield for higher hydrocarbons
• Extension of the plasma parameter regime
  (detached plasmas)
• erosion of redeposited layers (a-CH layer)
• Synergetic effects
• effect of mixed layers, Be and C
• Chemical erosion He, N2 - H mixtures
• ERO Modelling
• TEXTOR and JET methane puffs an hydrocarbon
  transport,
• D/XB values, higher hydrocarbons, PISCES
• sticking coefficients, data base, PSI-2

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Online manual on ERO webpage
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Further plans

• TW4-TPP-ERCAR
• Measurement of Chemical Sputtering yields for He
  H (experiment MAJESTIX)
• TW3-TPP-SCAVOP
• Measurement of Chemical Sputtering yields for N2
  H (experiment MAJESTIX)
• Investigation of gas phase chemistry for the
  plasma surface interaction of N2/H2 gas
  mixtures with hydrocarbon surfaces in
  low-temperature laboratory plasma experiments
  (PLAQ and PAUKE)
• Investigation of deposition in CH4 plasmas with
  increasing N2 admixture (experiment PLAQ)

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Normalisation Tmax
TEXTOR Y4.2 at Tmax830 K
Roth et al. EPS2003, PSI 2004
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Spectroscopic data interpretation
TEXTOR pre-heatable graphite limiter

• Intensity correction
• Photon efficiency correction of Dg due to D2
  molecules
• - Background in the CD spectrum

Brezinsek, Pospieszczyk 2004
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IPP contributions to SEWG Chemical Erosion
Plasmatechnology group Ch. Hopf, W. Jacob, M.
Schlüter, and T. Schwarz-Selinger Max-Planck-Insti
tut für Plasmaphysik, 85748 Garching

• Content
• The experiment MAJESTIX
• Chemical Sputtering of hydrocarbon layers
• Ar H (older data and model predictions)
• Ne H (new results)
• Erosion in N2/H2 plasmas
• Further Plans