Report of the Special Expert Working Group on Chemical Erosion and carbon Transport

1
Report of the Special Expert Working Groupon
Chemical Erosion and (carbon) Transport

• S. Brezinsek

Institut für Energieforschung- Plasmaphysik,
Forschungszentrum Jülich, EURATOM Association,
Trilateral Euregio Cluster, D-52425 Jülich,
Germany
with major contributions from SEWG members from
the following associations
TF-E
CU, CNR
group meeting in JET in July 2008 (joint with
SEWG Fuel Retention and Fuel Removal)
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TASK AGREEMENT WP08-PWI-05
Clarification of Chemical Erosion under ITER
Divertor Relevant Conditions
PWI-08-TA-05/CEA/BS/01 Determination of chemical
erosion yield in Tore Supra, including gaps and
redeposited layers. PWI-08-TA-05/CEA/PS/01 Upgrade
of spectroscopy diagnostic for chemical erosion
characterization PWI-08-TA-05/CEA/BS/02 Molecular
dynamic simulations of graphite
surfaces PWI-08-TA-05/CIEMAT/BS/01 Erosion of
doped graphites and re-deposited
layers PWI-08-TA-05/CU/BS/01 Modelling the
interaction of plasma with wall material (Be or
W) with impurities (Be, B, O) and/or seeding
gases (Ne, N2, Ar) PWI-08-TA-05/CY/BS/01 PWI-08-TA
-05/CY/PS/01 Molecular dynamics (CPMD)
simulations of carbon erosion PWI-08-TA-05/FOM/BS/
01 High ion flux exposure in pilot-PSI and PSI-II
under ITER divertor plasma condition. Impact of
seeding gases on erosion PWI-08-TA-05/FZJ/BS/02 P
WI-08-TA-05/FZJ/PS/02 Impact of ELMs on chemical
erosion in the JET divertor High ion flux
exposure of graphite/ CFC in TEXTOR, pilot-PSI
and PSI-II under ITER-relevant conditions PWI-08-T
A-05/FZJ/BS/03 PWI-08-TA-05/FZJ/PS/03 ERO
modelling of erosion and material transport in
tokamaks and linear devices PWI-08-TA-05/IPP/BS/01
PWI-08-TA-05/IPP/PS/01 Joint MD/DFT simulation
of carbon erosion PWI-08-TA-05/OAW/BS/01 MD
simulations of carbon erosion PWI-08-TA-05/UKAEA/B
S/01 Narrow-band spectroscopic 2D imaging of
carbon erosion during ELMs in MAST
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TASK AGREEMENT WP08-PWI-06
Erosion, Transport and Deposition of First Wall
Impurities
PWI-08-TA-06/CEA/BS/01 Carbon balance in TS
evaluation of sources and sinks, including
gaps PWI-08-TA-06/CEA/PS/01 Modelling of C
erosion /rede-position and fuel retention in
TS PWI-08-TA-06/CEA/BS/02 Monitoring of carbon
deposited layers (IR, ellipsometry, thermal
properties) PWI-08-TA-06/CIEMAT/BS/01 Injection
of hydrocarbons by molecular beam
techniques PWI-08-TA-06/CNR/BS/01 13CH4 tracer
injection in TEXTOR. Post-mortem tile
analysis,spectroscopic analysis, local erosion/
deposition. PWI-08-TA-06/FOM/BS/01 Modelling of
erosion and re-deposition of graphite exposed to
ITER relevant fluxes in Pilot-PSI with
ERO PWI-08-TA-06/FZJ/BS/01 13CH4 tracer injection
in JET and AUG. Post-mortem tile
analysis PWI-08-TA-06/FZJ/BS/02 Development of
material tracers TMB and SiH4. PWI-08-TA-06/FZJ/BS
/03 Characterisation erosion / re-deposition of
graphite PFCs at TEXTOR and marker probes at
JET PWI-08-TA-06/FZJ/BS/04 Development and
testing of spectroscopic tools for the
observation of carbon and beryllium and of QMBs
for erosion/deposition measurements in TEXTOR and
JET PWI-08-TA-06/FZJ/BS/05 Modelling of erosion /
re-deposition of graphite PFCs at TEXTOR and
marker probes at JET. ERO code modelling of gap
deposition. Coupling of the ERO code to molecular
dynamics simulation codes and study of the
formation of mixed-materials in ITER. Local
erosion/ deposition (ERO) and plasma transport
(EDGE2D, DIVIMP) modelling. PWI-08-TA-06/FZJ/PS/05
ERO modelling of redeposition in gaps
(TEXTOR,JET) 13CH4 tracer injection in JET and
AUG / Characterisation of graphite PFCs at TEXTOR
and marker probes at JET
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TASK AGREEMENT WP08-PWI-06
PWI-08-TA-06/IPP/BS/02 Reflection properties of
hydrocarbon radicals for ITER-like divertor
conditions /Deposition of hydrocarbons in
ITER-like conditions and their interaction with
hydrogenic species PWI-08-TA-06/IPP/PS/01 13CH4
tracer experiments,erosion-redeposition studies,
reflection properties PWI-08-TA-06/IPP.CR/BS/01 Re
active interaction of mol.ions with surfaces.
Reflection properties of hydrocarbon radical for
ITER-like divertor conditions PWI-08-TA-06/IPPLM/B
S/01 Characterization of erosion/re-deposition of
graphite PFCs at TEXTOR SEM, EPMA, XRD, AES, XPS
etc PWI-08-TA-06/MHEST/PS/01 Characterisation of
erosion / re-deposition of graphite PFCs at
TEXTOR (collab FZJ) Improvement of the NRA ion
beam PWI-08-TA-06/MHEST/BS/01 Characterisation of
erosion / re-deposition of graphite PFCs at
TEXTOR and marker probes at JET PWI-08-TA-06/ÖAW/B
S/01 Reactive interaction of molecular ions
(fragmentation, sticking) with Surfaces
/Reflection properties of hydrocarbon radicals
for ITER-like divertor conditions PWI-08-TA-06/ÖAW
/BS/02 Determination of sticking coefficients for
impact of small (deuterated) hydrocarbon and
other molecular ions on ITER relevant surfaces by
using a sensitive Quartz Crystal Microbalance
technique PWI-08-TA-06/TEKES/BS/01 Analysis of
the long-term erosion/deposition marker
samples PWI-08-TA-06/TEKES/PS/01 Analysis of the
long-term erosion/deposition marker samples, work
on hot cells PWI-08-TA-06/TEKES/BS/02 PWI-08-TA-06
/TEKES/PS/02 Local erosion/ deposition (ERO) and
plasma transport (EDGE2D, DIVIMP)
modelling PWI-08-TA-06/VR/BS/01 Extension of
spectroscopic and QMB diagnostics for the JET ILW
project Characterisation of graphite PFCs at
TEXTOR PWI-08-TA-06/VR/PS/01 Tracer experiments.
Development of tracer techniques C-13, TMB, SH4
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Motivation - ITER
ITER
Research goal minimisation of risks and
optimisation of ITER availability!

• Lifetime issues
• Erosion, transport, and deposition
• of divertor/first wall material (Be/C)
• Qualification of W as PFM
• Mixed material systems (Be, C, W)
• Control of transient heat loads
• Safety issues
• Retention of tritium via co-deposition
• Methods to release the trapped tritium
• Dust formation

Beryllium
Tungsten
All topics are related to each other!
Graphite
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Material Migration in Tokamaks with low-Z PFCs

• Present view
• Erosion and deposition is a
• question of flux balance
• Main chamber is the dominant
• erosion source of C (Be) caused by ion
• and neutral bombardment
• Material is transport to the inner
• divertor due to flows
• Multiple step process (C) and transport
• to remote areas
• Outer divertor erosion or deposition zone

• This SEWG deals primarily with
• Measurement and modelling of chemical
• erosion
• C and Be migration/transport in fusion
• devices (measurement and modelling)
• Deposition and sticking of (hydro)carbons
• Balance of erosion and deposition in fusion
• devices
• Erosion and deposition diagnostics

7
Motivation - Tasks
We have to understand

• Where are the species eroded from? How much is
  eroded?
• Spatial distribution of the erosion yield
• What is the impact of seeding impurities on the
  erosion process?
• Synergetic effects with Nitrogen, Argon
• What does the plasma do with the eroded
  hydrocarbons ?
• Hydrocarbon catabolism and C transport.
• What does the plasma do with the eroded Be?
• Be transport.
• Where are the eroded species deposited?
• Inner divertor, gaps, remote areas .
• Amount of deposited particles? Which hydrocarbon
  films are produced?
• Hard layers, soft layers, mixed layers .
• Which species is re-eroded and how?
• Chemical sputtering Methane-, ethane-family
• ELM-induced erosion material clusters .

Erosion
Migration
Deposition
Re-erosion
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ITER Predictions / SEWG
Predictions for ITER
Tokamak experiments JET, AUG
Laboratory experiments MAJESTIX
Linear experiments pilot-MAGNUM, PSI-II PISCES
ERO modelling
Plasma background B2-Eirene or EDGE2D
Data base, AM data, Material data HYDKIN,
MD, ADAS, TRIM
Code and data base validation benchmark
experiments TEXTOR, AUG, TJ-II, Tore Supra
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Outline

• Measurement and modelling of chemical erosion
• C and Be migration/transport in fusion devices
• (measurement and modelling)
• Deposition and sticking of (hydro)carbons
• Balance of erosion and deposition in fusion
  devices
• Erosion and deposition diagnostics

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Measurement and Modelling of Chemical Erosion

• Examples
• Spectroscopic measurement of chemical erosion und
  ITER-like detached divertor conditions FZJ,
  TF-E
• Impact of ELMs on chemical erosion in the JET and
  MAST divertor FZJ, UKAEA, TF-E
• High ion flux exposure of different CFC materials
  in pilot-MAGNUM and PSI-II under ITER-divertor
  plasma conditions FOM, IPP, FZJ
• Understanding of hydrocarbon break-up in He/H
  plasmas with molecular beams CIEMAT

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JET Chemical Erosion in Cold Divertor Plasmas
L-mode Reduction of chemical sputtering at low
Te (energetic threshold)
Density ramp discharges with transiently detached
outer divertor leg
Decrease of intrinisic CD and CII photon flux
emission in recombining plasmas low Telt2 eV
and high ne21020m-3
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ELM-induced Enhanced Erosion in JET
Thermal decomposition of carbon layers under ELM
impact
Processed exp. data
16
10
15
10
10x less
C deposition per ELM atoms/cm2
Arrhenius-type equation
14
10
Physical sputtering (Y1.5)
Carbon deposition on QMB reflects erosion of
divertor target, mainly from ISP position
13
10
0
100
200
300
400
500
ELM energy ?WELM kJ
A. Kreter et al. accepted for PFR
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Impurity Production during ELMs in MAST
Time resolved measurements of divertor CI, CII
and CIII emission during an ELM

• Motivation characterisation of material
  transport due to transients
• Photron camera optics from the slow filtered
  camera ? fitler imaging speed
• low spatial resolution ?1 cm, 256x256 pixels ?
  high time resolution 30-60 kHz
• insufficient time resolution to capture
  dynamics, but getting closer

CI filter at 910 nm, 5 nm FWHM, 30 kHz, 20 ms
shutter shot 20627
LOWER DIVERTOR VIEW
t0
33 ms
66 ms
99 ms
297 ms
627 ms
1320 ms
132 ms
S. Lisgo presented at IAEA 2008
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Erosion yields at high ion fluxes in pilot-PSI

• Exposures have been performed in the flux range
  1023-1024 /m2s
• Calibration of the chemical erosion has been
  verifies by
• absolute measurements with CH4 injection (D/XB
  500)
• Comparison with ERO calculation showed impact of
  small plasma
• diameter (mean free path) at Telt2eV
• Flux determination is currently being
  re-examined ( factor 2)
• Impact of surface temperature distribution not
  yet included

J. Westerhout et al.
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Preliminary Flux Dependence Analysis
Te and Eion normalised
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Injection of hydrocarbons in TJ-II by molecuar
beams

• Methane and ethene break-up in hydrogen and
  helium plasmas

• The set-up for He beam diagnostic was used to
  inject methane and ethene in He and H ECRH
  plasmas
• The emission profiles of Ha and CH (A-X) were
  recorded
• Ratios of Ha/CH emission are evaluated by
  relative calibration
• 3H per CH are emitted from methane break-up (He
  plasmas)
• Penetration of H from methane much larger than
  from H2 in He plasmas
• Same penetration for CH from methane/ethene ?
  Common precursor

P. Tabares et al. PSI 2008
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Carbon Migration/Transport in Fusion Devices

• Milestones
• 13CH4 tracer injection in JET. Post-mortem tile
  analysis, spectroscopic analysis, local erosion/
  deposition (ERO ) and plasma transport (EDGE2D)
  modelling
• - TF-E, UKAEA, VR, TEKES, FZJ
• 13CH4 tracer injection in AUG. Post-mortem tile
  analysis, spectroscopic analysis, local erosion/
  deposition (ERO) and plasma transport (DIVIMP)
  modelling IPP, TEKES
• 13CH4 tracer and hydrocarbon injection modelling
  (ERO) in TEXTOR FZJ

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JET 13C deposition in the Centre of the MKII-GB
divertor
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JET 13C Deposition in the MKII-SRP Divertor
M. Rubel et al.
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Local Erosion/deposition Modelling of 13CH4
Injection in AUG with the ERO code
Measured deposition (AUG divertor)

• Extensive modelling of the 2003 AUG
• divertor puffing experiment was carried out
• Shape of deposition reasonably well
• reproduced
• Locally deposited amount significantly
• smaller than in experiment (shadowing)
• The deviation of deposition tail from B
• direction due to E x B drift can be
• reproduced by applying a uniform E-field
• Results suggest that the injection might
• have a local perturbation in plasma
• Exact gemoetry with shadowing effects
• not yet applied

Shape of deposition (ERO)
M. Airilia et al. presented at PSI 2008
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Plasma Transport (EDGE2D, DIVIMP) Modelling (AUG)
Outer divertor target plate

• L-mode plasma for the 2007 AUG methane injection
  experiment was modelled with SOLPS
• A realistic plasma background for the local
  injection was obtained
• A DIVIMP model for the global carbon transport
  was set up using an OSM background plasma

13C deposition pattern
L. Aho-Mantila et al.
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Modelling of CH4 Injection at TEXTOR
Example hydrocarbon injection through gas inlet
higher hydrocarbons
C2D4 injection radial penetration of C2 and CII
light EXP vs. ERO
Effective D/XB values EXP vs. ERO
R. Ding et al. submitted to ppcf
Good agreement of observed and modelled profiles
Modelled and observed D/XB agree well for C2, but
differ for CD by a factor of 2
Next Check for influence of local injection on
local plasma parameters (cooling and increase of
ne) ? coupling of ERO with fluid model from M.
Tokar
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Deposition and Sticking of Hydrocarbons

• Examples
• Deposition and re-erosion of hydrocarbons in
  castellated structures FZJ, VR, SFA,TF-E, VR,
  UKAEA
• Reactive interaction of molecular ions
  (fragmentation, sticking) with surfaces ÖAW,
  CR-IPP
• Erosion and deposition in remote areas and the
  impact of N2 IPP, CIEMAT

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Carbon Deposition in Be Limiter Tile Gaps in JET
M. Rubel et al.
25
Ion Survival Probability
Experimental setup
ION SURVIVAL PROBABILITY Sa() HYDROCARBON IONS
ON CARBON INCIDENT ENERGY 3 45 eV

• neutralization of ions (survival pobability)

• surface-induced dissociations (energy
  partitioning)

• chemical reactions at surfaces (H-atom,
  CHn-transfer)

Z. Hermant et al.
26
Reactive Interaction of Molecular Ions with
Surfaces
W. Schustereder et al. Nucl. Instr. Meth. B

• CD2 on plasma sprayed tungsten (PSW) and CFC
  (AUG tiles)
• very stable beam with D flux of 1011 cm-2 s-1
• long time exposure (BESTOF)
• very low incident energy ( 0 eV), narrow energy
  distribution (100 meV FWHM)
• sticking coefficient of D from CD2
• CFC S 0.1 - 0.4 PSW S 0.05 - 0.1

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PSI-2 Experimental Setup Pumping Duct
Measurements
experimental setup in PSI-2
JET MKII divertor
C, H
pumping duct
to the pumps
W. Bohmeyer et al. PSI 2008
28
PSI-2 Impact of Nitrogen on Erosion Deposition
Pattern in Remote Areas
Film thickness in the pump duct vs. time In pure
H2 plasma and co-injection of CH4 and / or N2
Collector temperature 330 K. Pressure 1 Pa
W. Bohmeyer et al. PSI 2008
No synergetic effect of enhanced erosion with H2
and N2 gt volume process not a surface process
29
PSI-II Impact of Nitrogen on Erosion
Deposition Pattern in Remote Areas
No impact of Ne on the erosion and deposition
process!
W. Bohmeyer et al. PSI 2008
30
Balance of Erosion and Deposition in Fusion
Devices

• Examples
• In-situ layer disintegration in the inner
  divertor of JET TF-E, FZJ, UKAEA, FOM
• Carbon balance in Tore Supra Carbon sources and
  sinks - CEA
• Characterisation and modelling of erosion and
  re-deposition of graphite PFCs at TEXTOR FZJ,
  VR, SFA, IPPLM

31
Dynamics in Material Migration in the Inner
Divertor of JET

• Strongest deposition observed in the inner
  divertor pump duct area
• Post-mortem analysis, spectroscopy and deposition
  monitors (QMB) used The (step-wise) local
  migration is mainly determined by
• Strike-point configuration (line-of-sight
  transport)
• History effect (soft layer appearance and
  destruction)
• Power to the target (ELM strength)
• Cleaning discharges in H-mode with strike-point
  sweeping over the horizontal target led to the
  strongest deposition in the pump duct area

Deposition on the QMB in the inner divertor
pump duct entrance
S. Brezinsek et al. EPS 2008
32
JET Inner vs. Outer Divertor Deposition and
Erosion

• Direct comparison between inner and outer
  divertor deposition with QMBs

VT
HT
inner divertor QMB
outer divertor QMB

• Fuelled ELMy H-mode
• Similar conditions

72372
72376
72377
72371

• Outer divertor close to erosion/deposition
  balance
• Different surface conditions in inner and outer
  divertor leg
• Investigation is ongoing (impact of other
  configurations, gas injections

33
TS Measurement of Chemical Erosion on the TPL
4 fibers linked to a Czerny-Turner spectrometre
CCD
CCD
Filters wheel (Da, C2, C, CD)
Filters wheel (Da, C2, C, CD)
Splitter cube
Splitter cube
? optical fibres validate filtered images
CCD and spectrometer system both calibrated in
situ with labsphere
E. Delchambre et al. PSI 2008
34
Erosion Yield Measurements in TS
CD band observed on TPL since Chemical erosion
experiment (Dec 06)? (sensitivity 10x lower in
this region ! ? integration time 3 s) ?Ychem
CD(431 nm)/Dg

• S/XBHg 1000
• S/XBCD 65
• Erosion experiment
• Ychem 2 30 of Ytot (CII426nm/D?)

• S/XBHg 1000
• S/XBCII (426nm) 20
• Erosion experiment
• Ytot (CII/Da) (S/XBCII/S/XBHa) 6

E. Delchambre et al. PSI 2008
However, other line combinations Da and CII at
658 nm suggest lower yield
35
TEXTOR Long-term Deposition in Gaps of the
Toroidal Limiter
Composition from EPMA, RBS B C O 2
1 1 Mass density 1.3 g/cm3
Deposition profile tile 20 toroidal gap (1.5 mm)
Deposition profile tile 20 poloidal gap (1.8 mm)
? 0.54 mm
? 0.75 mm
A. Kreter, P.Wienhold et al.
Larger thickness in toroidal than in poloidal
gaps (factor of gt2) Decay length comparable to
previous experiments despite larger gap
36
Modelling of Deposition in Gaps

• 3DGAP code has been developed and simulations
  started
• reflection at inside walls of gap
• chemical erosion of layers deposited inside gaps
• elastic collisions with neutral gas inside gaps
• various particle sources
• Coupling of 3DGAP code with PIC modelling and
  ERO (plasma penetration, electrical field, usage
  of ERO infrastructure, ) in preparation

First comparison with TEXTOR experiments
(castellated test limiter)
RNC 0.5

• Profile shape is similar to the experimental one
• Absolute values are not recovered
• Deposition at the bottom (not shown here) can be
  partially recovered (nbottom/nedge 20 for RND
  0.9)

A. Kirschner et al.
37
Summary
Progress in both Task Agreements clearly
visible Tasks cover a wide range of physics and
chemistry from basic research to tokamak
discharges

• Main points
• benchmark of modelling codes with experiments gt
  code verification
• machine comparison gt general trends
• basic understanding of sticking coefficients gt
  input for codes
• experiments under ITER-like conditions

38
Outlook 2009
New Task Agreement Erosion, transport and
deposition of first wall impurities focuses on
the material migration part of the SEWG

• Objectives for 2009/2010
• Global transport investigations using 13CH4,
  SiH4 tracer and Be-evaporation
• in divertor tokamaks and associated plasma
  transport modelling
• Global transport investigations tokamaks and
  associated plasma transport
• modelling
• Local transport investigations with associated
  plasma transport modelling
• Deposition and re-erosion in gaps
• Measurements and modelling of first wall
  material erosion under high
• particle fluxes

39
MAJESTIX Experimental Set-up
UHV experiment with 2 radical beam sources and
one ion beam source
W. Jacob, Ch. Hopf,
A. von Keudell, M. Meier, and T.
Schwarz-Selinger Review of Scientific
Instruments 74, 5123-5136 (2003).
40
A New Model for Chemical Sputtering by N2

• in contrast to Ne TRIM ( physical sputtering)
  cannot describe the results for pure N2
• yield ? 1 for Eion gt 50 eV
• threshold between 20 and 50 eV
• almost no energy dependence in range 50 to 900 eV
• new model yields excellent description of
  experimental results Ytot Ychem Yphys
  (N-on-C model)
• at high E physical sputtering as calculated by
  TRIM.SP
• at low E chemical sputtering, i.e., formation of
  volatile species due to reaction of N (at end of
  ion range) with C from the layer

W. Jacob , C. Hopf, and M. Schlüter APL 86,
204103 (2005). M. Schlüter, C. Hopf, and W.
Jacob submitted to NJP.
41
A New Model for Chemical Sputtering by N2 H0

• chemical sputtering for combined bombardment with
  N2 and H0
• yield increases from about 1 at 30 eV to 7 at
  900 eV (R 380)
• yields higher than predicted by Hopf model (red
  dash-dotted line)
• new model yields excellent description of
  experimental results Ytot YHopf YN-on-C
• impinging energetic N causes chemical erosion as
  described by the Hopf model (bond breaking by
  energetic species and reaction with H)
• decelerated N at end of ion range reacts with C
  to form volatile CxNy species this lead to an
  additional contribution to chemical sputtering

M. Schlüter, C. Hopf, and W. Jacob submitted to
NJP.