FZK Investigations on Wall Surfaces and Tokamak Plasma

1
FZK EURATOM FUSION ASSOCIATION
FUSION-PL
FZK Investigations on Wall Surfaces and Tokamak
Plasma
I. Landman1, B. Bazylev1, S. Pestchanyi1
with contributions from V. Safronov2, A.
Zhitluckhin2, V. Podkovyrov2 and I. Garkusha3
1 Forschungszentrum Karlsruhe (FZK), Germany 2
Troitsk Institute for Innovation and Fusion
Research (TRINITI), Russia 3 Kharkov Institute of
Physics and Technology (KIPT), Ukraine

• Contents
• 1) Main results on expected consequences of ITER
  transient events
• Surface melting of tungsten divertor armour and
  beryllium first wall
• Evaporation and brittle destruction of carbon
  based materials
• Contamination of the SOL and core plasma after
  ELMs
• 2) Objectives

2

• Main features of FZK PWI activities
• Investigations are carried out for ITER, by
  means of numerical modelling
• (because available tokamaks cannot provide
  required transient loads)
• and engaging the tokamak simulators -
  powerful plasma guns
• We develop own codes to apply to ITER
  predictions
• -- behavior of fusion materials
• -- tolerable sizes of off-normal
  events
• Validations of the codes use mainly plasma
  guns and electron beams
• Current EFDA tasks
• TW3-TPP / MATDAM, TW5-TPP / ITERTRAN, TW5-TPP /
  BEDAM
• Damage to W and CFC ITER divertor materials
  of EU trademark
• (with validation by the plasma guns QSPA-T
  and MK-200UG)
• Damage to beryllium ITER first wall and Be
  coatings
• (with validation by a special plasma gun in
  TRINITI)
• Modelling of damage to ITER divertor target
  (after ITER disruptions and ELMs)
• Modelling of tokamak plasma contamination
  following ITER ELMs

3
Transient energy fluxes expected at the ITER
divertor target
Simulation facilities
4
FZK codes for consequences of ITER off-normal
events

5
Melt motion at ITER ELM conditions

• Multiple ELM relevant loads at QSPA-Kh50 for EU W
• Deposited energy less than 1 MJ/m2 during 0.2 ms
• In 2004 up to 450 shots on one W sample
• Damage below melting threshold is very complex
• Decrease of melting threshold after many shots
• Violent surface cracking of bulk tungsten
• below melting threshold

0.9 mm
after 100 pulses
after 200 pulses
Impact energy 1.20 MJ/m2 Absorbed energy
0.72 MJ/m2 Pulse duration 0.2 ms
1.7 mm
after 250 pulses
after 450 pulses
0.5 mm
after 370 pulses
W cross-section after 1 pulse 30 MJ/m2 0.2 ms
after 450 pulses
6
Simulation of melt motion at ITER ELM conditions
MEMOS calculates melting, resolidification and
evaporation Melt motion is due to 1) ??p, 2)
surface tension, 3) J?B force


Single ELMs and disruptions
(Particular figures significantly depend on the
size of transient event)
Tungsten thresholds as functions of pulse
duration The dependencies Qmelt ??? and Qvap ???
work well
(Simulations with Be are not yet systematic)

• Multiple ELMs and disruptions
• Stochastic separatrix strike positions is
  important
• Stochastic changes of SSP affect favourably
• After a few thousand ELMs vaporization becomes
  dominant
• Multiple ELMs causing melting can significantly
  decrease
• the damage caused by rare disruptions

?0.3 ms
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Simulation of W-brushe with MEMOS

• The complicated profile of W-brushe is
  implemented
• Validation by QSPA-T is carried out
• The depth of W melting and resolidification
  profile
• is rather similar to that of bulk W target
• however melt velocity is less by a factor 0.3
  - 0.5

• Optimization of W macrobrush design
• optimization of inclination of brushes top
  surfaces
• Shadowing of brush edges may decrease melt
  roughness
• Optimal surface inclination angle ?? ? ? / 2

Validation by QSPA-T

• Damage to the dome gaps
• and the divertor cassette gaps
• the melting of copper
• at the W-Cu adjoins is significant
• protective tungsten aprons of the gaps
• may be necessary

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Brittle destruction of CFC
CFC NS31 and NB31 have been developed for ITER ?
CFC have a 3D structure of fibres and a matrix
? At stationary tokamak regime CFC behaves good
? At the transient loads anticipated in ITER
high erosion rates are discovered

• Main results from plasma guns MK-200UG and QSPA-T
• CFC NB31 and NS31 were exposed to 200 shots 15
  MJ/m2
• Both CFC behaved similarly (regime with vapour
  shield)
• Maximum erosion rate is proportional to pulse
  duration
• PAN fibres max. erosion rate is of 20 ?m/ms
• pitch fibres max. erosion rate is of 3 ?m/ms
  (evaporation)
• Graphite particles of sizes of 1 to 102 ?m are
  collected
• Now investigations for EU trademark CFC at
  0.5-1.5 MJ/m2
• in frame of the EFDA task MATDAM started
• Start of vaporization Qmin0.3 MJ/m2 for 0.05
  ms (MK-200UG)
• (Qmin???? at 0.5 ms would be Qmin 1 MJ/m2)

CFC surface after 150 shots at QSPA-T
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CFC brittle destruction simulation using
PHEMOBRID and PEGASUS
PEGASUS works on microscopic scale (weeks of
running) PHEMOBRID works on macroscopic
scale (BD threshold of CFC (10 KJ/g) is like
melting point of W) (PHEMOBRID 3D code also but
only a few hours of running)
Value of thermal conductivity is important Pulse
shape is also important (?? and ? 50)
PHEMOBRID results
The PEGASUS model 3?106 cells of 1 ?m represent
CFC 3D structure ? Thermal- and mechanical
bonds between the grains ? Anisotropic heat
transport through grain boundaries ? Stress due
to anisotropy and temperature gradients ?
Cracking of the bonds above elasticity threshold
? The crack interrupts connection between grains
Simulation 0.8 MJ/m2 0.5 ms Experiment 0.3 MJ/m2
0.05 ms (data for the emissivity 0.9)
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CFC simulations using PEGASUS
The CFC erosion is due to preferential
cracking on the surfaces of PAN fibres erosion
depth 30 um, 4000 K at the boundary This
simulation only tried to discover BD erosion
features but not the scale Validation is
necessary
PEGASUS BD damage to a standard CFC structure
New CFC structure is suggested The PAN fibres
are inclined under 45 deg to the pitch fibres
In PEGASUS simulations BD erosion rate has
decreased significantly ( 5 times) Experiments
at MK-200 UG to proof this qualitative
prediction are set up (the CFC is to be cut as
111)
PEGASUS BD damage to improved CFC structure
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Modelling of ELM-induced SOL contamination

• Development of FOREV-2D
• magnetic toroidal geometry of ITER and JET are
  available
• multi-fluid SOL plasma description (ions of D,
  T, He, C)
• radiation transport in toroidal geometry for C
  is implemented
• Results obtained with upgraded FOREV-2D
• radiation load of the first walls in ITER and
  JET
• a rough validation by JET was carried out (20
  versus 35 MW)
• SOL contamination by carbon impurity after Type I
  ITER ELMs
• For Q???1?MJ/m2, carbon ions fill SOL for several
  ms
• The density up to 1021?m?3, thus DT is dissolved
  in C
• In few ms SOL is cooled down to a few eV by
  radiation losses.
• Influx of carbon impurity into the pedestal after
  ELM 3?1017 m-2

12
Contamination of ITER core after ELMS (first
simulations with the new code TOKES)

• Main Features of TOKES
• The Grad-Shafranov equation is solved at each
  time step
• (2D magnetic field evolves together with
  plasma)
• Multi-fluid plasma, Pfirsch-Schlüter cross
  transport so far
• (now D, T, He and C ion species are available)
• Poloidal field coils automatically control
  plasma boundary
• D- and T beams heat and feed, radiation cools
• DT ? He n reaction produces burning by alphas

First preliminary result Whole ITER confinement
of 500 s was simulated Tolerable ELM size 1 MJ/m2
for ELM frequency 0.5?Hz Rather uncertain
implications have still been used (plasma
fraction dumped out in ELM burst assumed 0.5) We
see that ELMs do not clean the plasma of
impurities
Carbon impurity propagation into the core after
ELM (TOKES)
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Objectives Up to now mainly carbon transport in
SOL was simulated (W and Be not) Therefore we
will develop tungsten impurity transport in SOL
and the core Radiation transport also for
tungsten impurity Further material investigations
(CFC, W, Be) with PEGASUS and MEMOS in
particular, aiming impurity influxes into
SOL Main future activities are going to be
devoted for ITER transients Further
quantification of the heat fluxes to ITER
divertor and first Quantification of ELM size
threshold for radiation collapse caused by the
impurities Lifetime prediction for CFC, W and
Be Theoretical support of ongoing experiments
with EU materials for ITER Continue W-O-H
chemical erosion with MD code CADAC