Power and particle exhaust in

1
Power and particle exhaust in tokamaks
integration of plasma scenarios with plasma
facing materials and components
W.Fundamenski UKAEA, EFDA-JET 18th Int.
Conf. on Plasma Surface Interactions, Toledo,
Spain, 26-30 May 2008
2
with thanks to many contributors
A. Alonso1, P. Andrew2, G. Arnoux3, S.
Brezinsek4, R. Dux, T. Eich6, T.Evans,
M.Fenstermacher, A. Huber4, S. Jachmich9, M.
Jakubowski10, E.Joffrin, A. Kirk, T. Loarer3, B.
LaBombard, P.Lang, Y.Liang, B. Lipschultz, A.
Loarte12, G. F. Matthews5, D. Moulton, V. Naulin,
R. Neu, V. Philipps4, R.A. Pitts8, J.Rapp4, D.
Tskhakaya15 M. Wischmeier and JET EFDA
Contributors
1Associacion Euratom/CIEMAT para Fusion, Madrid,
Spain 2ITER Organization, Cadarache, France,
3Association EURATOM-CEA, DSM-DRFC, CEA
Cadarache, 13108 Saint Paul lez Durance,
France 4Institut für Plasmaphysik,
Forschungszentrum Jülich GmbH, EURATOM
Association, Trilateral Euregio Cluster, D-52425
Jülich, Germany 5Euratom/UKAEA Fusion
Association, Culham Science Centre, Abingdon,
OX14 3DB, UK 6Max-Planck-Institut für
Plasmaphysik, IPP-EURATOM Association, D-85748
Garching, Germany 7FZ Karlsruhe, Postfach 3640,
D-76021 Karlsruhe, Germany 8CRPP-EPFL,
Switzerland, Association EURATOM-Swiss
Confederation 9LPP, ERM/KMS, Association
Euratom-Belgian State, B-1000, Brussels,
Belgium 10Max-Planck-Institut für Plasmaphysik,
Teilinstitut Greifswald, Germany 11VTT Technical
research Centre of Finland, Association
EURATOM-Tekes, Finland 12EFDA-Close Support Unit,
Garching, Boltzmannstrasse 2, D-85748 Garching
bei München, Germany 13Association EURATOM-VR,
Fusion Plasma Physics, Stockholm, Sweden 14PPPL
Princeton University, Princeton, NJ 0854,
USA 15University of Innsbruck, Institute for
Theoretical Physics, Association EURATOM-ÖAW,
A-6020 Innsbruck, Austria See appendix of M.
Watkins et al., Fusion Energy 2006 (Proc. 21st
Int. Conf. Chengdu, 2006) IAEA Vienna (2006)
3
Outline

• Compatibility between the plasma scenarios and
  PFCs
• Ignition vs. exhaust criteria
• Impact of PFCs on fusion gain
• Power balance on ITER
• Steady-state particle and power exhaust
• Limiter vs Divertor exhaust
• Steady plasma loads
• on main chamber PFCs
• on divertor PFCs
• Divertor plasma detachment
• Transient particle and power exhaust
• Edge localised modes (ELMs)
• Plasma loads associated with ELMs
• on divertor PFCs
• on main chamber PFCs
• ELM mitigations techniques
• Magnetic perturbations
• Pellet pacing
• Impurity injection

4
Ignition vs. Exhaust criteria
achieve
maintain
QDT Pfus / Pheat 10
Fuelling
Particle exhaust
gas
pellets
beams
Intrinsic Z
Extrinsic Z
He ash
Heating
Power exhaust
alpha
RF
NBI
neutron
photon
plasma
Confinement
First wall design
equilibrium
mechanical
stability
thermal
current drive, disruptions, tritium, dust,
transport
nuclear
5
max QDT function(reactor design)
Ignition systems
Exhaust systems
Fuelling systems
Control system
QDT Pfus / Pheat pDT tE f(Zeff)
Heating systems
Cooling circuit
Current drive
Maximum achievable QDT determined by the reactor
design, including PFC limits
Cryopumps
Reactor design
PFCs
Impact of a given PFC limit DQ / Q0 1
Q(PFC) / Q0
Magnetic coils
Plasma scenario
Fuelling
Particle exhaust
Heating
Power exhaust
Current profile
PFC loads
Stability transport
Confinement
Impurity influx
Edge plasma conditions
Core plasma conditions
6
Power balance on ITER
Pfus 400 MW
Fusion
Wall area
Heating
QDT 10
Ph 40 MW
Pa 80 MW
Pn 320 MW
Alphas
Neutrons
800 m2
PSOL 60 MW
Pradcore 60 MW
Plasma
Photons (50)
?
800 m2
PELM 20 MW
Pinter-ELM 40 MW
Inter-ELM
ELMs
?
2p x 5 m x 4 mm x 10 1.2 m2
14 MW
inner
6 MW
inner
?
?
?
6 MW
outer
34 MW
2p x 6 m x 4 mm x 10 1.5 m2
outer
Outer divertor losses (CX, ES,
radiation)
25 MW
15 MW
1.5 m2
Total plasma (outer)
Steady-state design limit 10 MW/m2
0.5 MJ / m2
1.2 m2
In reality, must repeat backwards to find maximum
achievable QDT for given PFC limits
ELM frequency 20 Hz
Transient design limit 0.5 MJ/m2 in 250 us
Plasma purity (Zeff 1.7) requires high density
(fGW 0.85) and cold divertor (lt 5 eV)
Need 85 MW total radiation (70 total 50 in
core 20 in SOL), and ELM frequency above 20
Hz (ELM size 1 MJ or 1 of Wped)
7
Steady-state exhaust

• Compatibility between the plasma scenarios and
  PFCs
• Ignition vs. exhaust criteria
• Impact of PFCs on fusion gain
• Power balance on ITER
• Steady-state particle and power exhaust
• Limiter vs Divertor exhaust
• Steady-state plasma loads
• on main chamber PFCs
• on divertor PFCs
• Divertor plasma detachment
• Transient particle and power exhaust
• Edge localised modes (ELMs)
• Plasma loads associated with ELMs
• on divertor PFCs
• on main chamber PFCs
• ELM mitigations techniques
• Magnetic perturbations
• Pellet pacing
• Impurity injection

8
Limiter vs divertor exhaust
Scrape-off layer (SOL) plasma
SOL
SOL
Edge
Edge
SOL
Core
upstream nu , Tu
Core
Limiter
LCFS
Separatrix
Vessel walls
SOL width determined by competition between
parallel and perpendicular transport
Private plasma
Target nt , Tt
Steady-state plasma loads determined largely by
Edge/SOL turbulence !
Divertor targets
9
Plasma turbulence in the Edge-SOL
Collisionality scan
Current scan
Density scan
ESEL
Wall flux 1/current
Wall flux density2
TCV
O.E.Garcia et al, PPCF 48 (2006) L1
10
Density profiles in the Edge-SOL
TCV
C-mod
ESEL
n
n
SOL density profile broadens with increasing
collisionality
ESEL
Such broadening observed on many tokamaks
Density fluctuations increase with radius,
approaching unity in the far-SOL
11
Radial flow profiles in the Edge-SOL
TCV
Radial plasma flux increases with collisionality
Such increase with radius observed on many
tokamaks
ESEL
Effective radial velocity is roughly constant
with radius and increases with collisionality in
the near-SOL
ESEL
12
PDFs of fluctuations in the far-SOL
TCV
Similar result for PDF of velocity fluctuations
Temporal pulse shape of density blobs reveals
leading front trailing wake
PDF of density fluctuations in the far-SOL
universal and highly intermittent
13
Interchange motion of plasma blobs
pulse shape
interchange drive
Dynamics of plasma filaments, or blobs, is
determined by charge conservation balance of
divergences of polarization, diamagnetic and
parallel currents
sheath dissipation
acceleration
O.E.Garcia et al, PoP (2006)
As collisionality increases, plasma filaments
become electrically isolated from the sheath at
the divertor target, making the interchange drive
more effective
R-4 Boedo
O.E.Garcia et al, PPCF 48 (2006) L1
14
Edge-SOL turbulence not anomalous
Edge/SOL turbulence is no longer anomalous.
Predictive capability in sight.
Recall that anomalous abnormal, irregular, not
understood
Ironically, it is the absence, rather then
presence of turbulence which now appears
anomalous.
Theres just one more thing that bothers me
How do I get this H-mode ??!
We know who did it. We still dont know how.
flow shear poloidal toroidal rotation
magnetic shear, X-point geometry, ion orbit
losses, bootstrap current,
The Holy Grail of tokamak theory ! and main
obstacle in predicting tokamak plasma exhaust
(ITER)
15
Divertor heat loads in ELMy H-mode
Averaged heat load profiles in natural density,
ELMy H-mode
JET
Integral power width decreases with field,
current and power
Narrowest profile ion poloidal gyro-radius at
pedestal temperature
Narrow inter-ELM profile confirmed by high
resolution IR system on JET
Most of the energy arrives at the outer target
Pouter Pinner 2.5 1
W.Fundamenski et al, NF 45 (2005) 950
16
Turbulence reduction in the near-SOL
Obtained scaling is best explained by
neo-classical ion conduction
ITER prediction 4 mm
Recall in wetted area
?
Consistent with partial extension of the ETB
into the near-SOL
Earlier analysis confirmed with multi-fluid
(EDGE2D) simulations
17
Divertor operating regimes
Low recycling (sheath limited) nt
nu, Tt Tu, pt pu
high recycling (conduction limited) nt
nu3, Tt nu2, pt pu
Can this narrow heat load profile be broadened by
a divertor buffering ?
upstream nu , Tu
Fully detached (radiation limited)
X-point MARFE
Partially detached (CX-ES limited) nt
decreases, Tt lt 5 eV, pt ltlt pu
Target nt , Tt
18
Divertor plasma detachment
A.Loarte et al, PPCF (2001)
Loss of plasma pressure and energy by CX/ES
line radiation
JET
Reduction of target plasma flux
Inner target typically detaches earlier, i.e. at
lower upstream density, than the outer target
This asymmetry is consistent with power flow into
divertor volume (ExB drifts, geometry, etc.)
Detachment of the outer divertor is needed for
steady-state load reduction
O-25 Wischmeier
19
Thermal instability X-point MARFE
Outer target detachment typically accompanied by
an X-point MARFE
Results in substantial cooling of the edge
plasma, reduction of pedestal stored energy and
degradation of energy confinement
At higher densities transforms into an inner wall
MARFE density limit nGW Ip/a2
JET
A. Huber et al, NF (2007)
20
Energy confinement degradation
JET
Density (fuelling) scan
50
Normalised energy confinement (H98) reduced with
line average density as it approaches the density
limit (nGW)
H98 also reduced by 15 after a Type-I to
Type-III ELM transition
Radiation (impurity seeding) scan
H98 reduced with radiative fraction
Caused by reduction of pedestal temperature and
pressure
50
Since Wped 1/3 W, hence a 50 drop in Wped
means a 15-20 drop in H98
M.Beuskens et al, submitted to NF
21
Impurity accumulation in the core
with ITB
without ITB
Zeff 1 Prad / ne2
Impurity density roughly uniform in the absence
of an ITB
JT-60U
ITB acts as a barrier for impurity transport as
well as for transport of fuel ions and energy
Inward velocity of impurities (neoclassical and
turbulent pinch) overcomes outward diffusion
Impurity accumulation increases with ion charge
Cause for concern for both medium and high-Z
impurities
H.Takenaga et al, NF 43 (2003) 1235
22
Transient exhaust

• Compatibility between the plasma scenarios and
  PFCs
• Ignition vs. exhaust criteria
• Impact of PFCs on fusion gain
• Power balance on ITER
• Steady-state particle and power exhaust
• Limiter vs Divertor exhaust
• Steady plasma loads
• on main chamber PFCs
• on divertor PFCs
• Divertor plasma detachment
• Transient particle and power exhaust
• Edge localised modes (ELMs)
• Plasma loads associated with ELMs
• on divertor PFCs
• on main chamber PFCs
• ELM mitigations techniques
• Magnetic perturbations
• Pellet pacing
• Impurity injection

23
Edge localized modes (ELMs)
Difference between ELM and pre-ELM infra-red
images
JET
MAST
Growth stage
Linear instability (e.g. ideal/resistive MHD
mode) forms 10-20 flute-like ripples in
pedestal quantities
Transport stage
These develop into 10-20 filaments during the
non-linear phase of the instability (beginning of
transport)
AUG
Exhaust stage
Filaments move outward, driven by interchange
(curvature pressure), while draining to the
divertor targets
P1-24 Jakubowski
I-5 Kirk
24
Pedestal changes during an ELM
Pedestal plasma eroded during the ELM
Density drop convective losses
Temperature drop conductive losses
Small ELMs are mostly convective
ELM size decreases with collisionality
25
In-out energy asymmetries
Steady-state power deposited mostly on the outer
target (factor of 2.5).
ELM energy deposited mostly on the inner target
(factor of 2).
What is the reason for these opposite in-out
energy asymmetries?
?
JET
JET
EOUTER / EINNER
Eheat - Erad (MJ)
26
In-out energy asymmetries
Radial electric field in the edge and SOL regions
points in opposite directions !!!
For normal field direction
Electric drifts in the SOL increase the
convective power flow to the outer target
Electric drifts in the edge increase the
convective energy flow to the inner target
Parallel motion of ions and electrons convects
energy towards both targets
Inner target
Net poloidal velocity determines the in-out
energy asymmetry
VE in the edge
V
V
Poloidal angle
VE in the SOL
Link to plasma rotation ?!
Outer target
Toroidal angle
27
Parallel transport of ELM energy
Agrees with the heat pulse at both inner and
outer divertor targets and with the fraction of
energy arriving before the peak.
ELM heat load can be explained by arrival of
free-streaming ions from an initially Maxwellian
distribution with pedestal ion temperature
When M 0, the energy deposition is symmetric.
However, M 0.2 towards the inner target can
account for the observed in-out energy asymmetry
AUG
JET
18
O-17 Eich
O-18 Tskhakaya
28
Transient heat load limits in ITER
TRINITI plasma gun
CFC
40
250 us
W
ITER adopted 0.5 MJ/m2 for the maximum allowed
ELM energy load in 250 us
29
Max. permitted ELM size in ITER
Combining the above estimates for the ELM wetted
area, in-out energy asymmetry and PFC transient
energy limits one finds
DWELM QELM x Sin x (1 Pout/Pin) 0.5 MJ/m2 x
1.2 m2 x 1.5 0.9 MJ
Assuming W 400 MJ, Wped/W 1/3, then DWELM /
Wped lt 1
This requires a decrease in the natural ELM
size by a factor of 20 !
Some caveats
Difference in temporal pulse shape and absolute
plasma pressure between plasma gun and ELM
Not all ELMs are equal. Amplitude and temporal
PDFs are intermittent. Large ELMs cause most
damage.
1
P2-35 Moulton
30
Small ELMs less energy to the wall
Smaller ELM filaments travel slower, consistent
with interchange dynamics
Predicted power width scaling on ELM filament
energy in the far-SOL
ITER
For a natural (unmitigated) ELM on ITER, expect
10 of its energy to main wall PFCs
For a small (mitigated) ELM expect only a tiny
fraction (ltlt1) of ELM energy to main wall PFCs
O-8 Pitts
Maximum ELM size on ITER determined by divertor
PFCs !!!
31
Impurity seeding no ELM buffering
Only a small fraction (10-20 ) of the ELM
energy radiated during the ELM
Consistent with multi-fluid edge/SOL simulations,
which indicate that ELM energy buffering occurs
only for very small ELMs (below 20 kJ on JET).
However, impurities released from the targets by
the ELM, lead to large post-ELM radiation
(comparable to the ELM energy itself) !!!
JET
20
JET
10
For sufficient large ELMs, the cooling of the
X-point can result in a back transition to L-mode
(loss of ETB)
WELM 0.45 MJ
WELM 0.85 MJ
32
ELM induced impurity inflows
Inter-ELM W influx from outer divertor is
strongly reduced as outer divertor plasma is
cooled, consistent with the physical sputtering
threshold
With the outer divertor detached, the average W
influx is dominated by ELMs !
Transient W influx increases with ELM energy
In all cases, dominated by impurity sputtering
Argon seeding increases W erosion during ELMs,
decreases erosion between ELMs
AUG
Optimum seeding rate (smallest W influx) is
determined by competition between erosion by Ar
ions and cooling by Ar radiation !!!
hot
cold
I-6 Dux
33
ELM control Type-III ELMs
ELM frequency can be increased substantially (gt
factor of 10), by cooling the pedestal and thus
replacing Type-I, by Type-III, ELMs
At present, the only scenario compatible with all
ITER exhaust requirements frad
75, fGW 0.85, q95 3, Zeff lt 2, divertor
detachment, DW/Wped lt 1
QDT 10 can be recovered by increasing the
current by 13 (to 17 MA)
However, pedestal pressure reduced by 50,
energy confinement (H98) by 15-20, so that QDT
reduced by 50 (QDT 5 at 15 MA).
ITER
Pheat, Prad 10 MW
nGDL, ne 1019 m-3
Zeff
Q10 domain
Ha
O-16 Rapp
H98(y,2)
Time s
34
ELM control pellets
Type-I ELM frequency can be increased by
injection of small fuel ice pellets, provided
that pellet frequency gt 1.5 times the natural ELM
frequency
In AUG, stored energy and energy confinement
(H98) reduced by 10-20, due to increased
convective losses. Pellets are too big !!!
Pellet provides a perturbation, correlated to
penetration depth, which leads to an MHD
instability. It triggers a Type-I ELM at any
point in the ELM cycle !
20
AUG
Can the effect of plasma fuelling and ELM pacing
be decoupled ? Can pellet pacing be demonstrated
at high density (fGW 0.85)?
fPel gt 1.5 f0ELM
Can pellet injection produce a strong enough
perturbation on ITER to trigger edge MHD (Type-I
ELM) but not core MHD (e.g. NTM) ?
35
ELM control RMPs
Type-I ELMs can be suppressed entirely by
resonant magnetic perturbations
DIIID
Edge plasma density is reduced due to magnetic
field ergodization, which increases parallel
convection to the divertor
This effect, known as magnetic pump-out, is
well documented with ergodic divertors, e.g.
Tore-Supra. It represents edge plasma
rarefaction, in the absence of cooling !
Pedestal density and pressure reduced by
15-30 energy confinement (H98) const
Dpped 15-30
In ITER, magnetic pump-out must be compensated
by additional pellet fuelling to ensure fGW
0.85. Can ELMs still be suppressed in this case?
If so, what is the reduction of pped, H98 due
convective losses?
I-11 Moyer
P1-32 Fenstermacher
36
ELM control n1 to n16 TF
ELM frequency can also be increased by both low n
(1,2) and high n (16) toroidal field
perturbations, generated with external coils. The
former with error field correction coils (EFCCs),
the latter due to toroidal field (TF) ripple.
In both cases, the pedestal density reduced due
to magnetic pump-out
For the same collisionality, ELM size reduced by
a factor of 2, when TF ripple increased from 0.1
to 1
ITER
pi
Change related to smaller conductive losses, i.e.
mainly convective ELMs
pe
Reduction much less pronounced at higher density
(fGW 1).
ELM control by EFCC discussed in
Saibene EPS 2007
I-12, Liang
P1-1 Jachmich
37
ELM control summary
technique not optimized
Consider the best results achieved so far
38
Conclusions
Compatibility between plasma and PFCs is not a
binary signifier! Best measured as the impact on
reactor performance, e.g. fusion gain, Q.
On the basis of our present knowledge
(experiment) and understanding (theory), it
appears that this impact, DQ/Q0, is negligible in
existing tokamaks with C walls, but could be
significant ( 30-50 or more) for ITER and DEMO
with metal walls
The dominant contribution to DQ/Q0 is the
transient heat load limit and hence the
requirement of small ELMs (DW/Wped lt 1), which
entails a reduction of the pedestal pressure by
30-50 and H98 by 10-15
Although active ELM control by pellet injection
and magnetic perturbations hold much promise, it
remains to be seen whether these methods offer a
smaller DQ/Q0 then the more conventional method
of Type-III ELMy H-mode
Since high density (fGW 0.85) and high radiation
(frad 0.75) are necessary in ITER to ensure
detached divertor operation and reduce core
plasma dilution, and the exact criteria governing
the Type-I to Type-III transition are not fully
understood, one may yet find that the transition
to Type-III ELMs becomes unavoidable
39
Quo Vadis ?
time
D-phase
ITER H-phase
Teoria
Praxis
Use validated codes to predict impurity levels in
hydrogen phase in ITER
Integrated core-edge-SOL
Impurity seeding experiments in full metal
machines, e.g. JET ILW, AUG W
Edge-SOL turbulence modelling link to transport
models
Hydrogen experiments in existing devices
Predict hydrogen phase in ITER
Use hydrogen phase of ITER to validate edge-SOL
codes under Ohmic conditions
ELM control experiments
Use validated codes to predict ITER exhaust under
Ohmic and L-mode conditions in deuterium plasmas
Kinetic ELM simulations
40
FIN
41
Limiter vs divertor recycling
SOL
Limiter
C0, CxDy
D2, D0
D2, D0
Impurities, eg. C0, CxDy
lD0 few cm, lC0 1 cm, lCxDy few mm
Divertor target
Intimate contact with edge plasma
PFCs removed from edge plasma
Little recycling/cooling in the SOL results in a
hot, tenous SOL plasma
Colder, denser SOL plasma, due to local recycling
/ cooling
High erosion yields, poor pumping
Lower erosion yields, improved pumping
Strong influx of both fuel and impurity neutrals
into the edge
Fuel and impurity sources screened from the edge
by the divertor plasma
Improved plasma purity, i.e. lower Zeff
Impure edge core, i.e. high Zeff
42
Physical vs chemical erosion
Physical sputtering yield
Chemical erosion yield (D on C)
increases with projectile energy and mass, while
decreasing with target (PFC) material atomic mass
decreases with D ion flux and is sensitive to C
target temperature
43
Impurity seeding confinement
Energy confinement (H98) decreases with density
(fGW) and radiation (frad)
JET
ITER
44
Parallel loss model of ELM exhaust
Low n plasma cools faster than it dilutes
mainly conductive losses.
Consider the radial motion of the pedestal plasma
subject to parallel losses.
High n cooling and rarefaction comparable
mainly convective losses.
Describe as a plasma filament, moving with some
effective radial velocity.
Explains why small ELMs (high n) are mainly
convective
Evolution of density and temperature of the
filament using a fluid model
n
Ti
DTe Dn
Te
DTe gtgt Dn
45
Fraction of ELM energy to the wall
Smaller ELM filaments must travel slower,
consistent with interchange dynamics
Consider a typical Type-I ELM on JET
Use mid-pedestal values of n, Te, Ti and
effective radial velocity of 600 m/s measured
using limiter probes
This yields an estimate of 10 of ELM energy to
wall, in agreement with the infra-red measured
value.
IR measurements indicate that smaller ELMs
deposit a smaller fraction of energy on the wall
How can this be understood ?!
Also observed as the energy missing from the
divertor
46
Impact of PFCs on fusion gain
QDT Pfus / Pheat pDT tE f(Zeff)
Critical temperature gradient
DT fuel dilution with Zeff
Profile stiffnessT(0) / T(a) const
Core plasma DT pressure
Impurity accumulation
Collisional transport
Turbulent transport
MHD equilibrium stability
need cool, dense edge plasma
or active ELM size control pellets, RMPs
radiative impurity seeding
need cool, dense edge plasma
Edge plasma DT pressure
Edge localised modes (ELMs)
Edge SOL transport collisional and turbulent
ELM size decreases with edge collisionality
partially detached divertor operation
Transient loads on PFCs
Steady loads on PFCs
must not exceed 0.5 MJ/m2 in 250 us
must not exceed 10 MW/m2 10 eV
Erosion, ablation, melting, cracking
47
Tokamak plasma scenarios
Baseline
Advanced
current drive
inductive (pulsed)
auxiliary bootstrap
transport barriers
edge (ETB) core (ITB)
edge (ETB)
dq/dr gt 0 q0 1 q95 3
dq/dr gtlt 0 q0 gt 1 q95 4 - 5
Safety factor, q
48
Ignition vs. Exhaust beta limits
49
Plasma turbulence in the Edge-SOL
Mostly drift-Alfven dynamics in the edge region
Intermittent transport implies strong
fluctuations in far-SOL quantities
Mostly interchange dynamics in the SOL region
Local flux not related to local gradient!
Turbulence driven by edge pressure gradients,
which build up together with poloidal flow shear,
Mean field approximation, used in most edge
transport codes, is not accurate
damped by parallel losses and sheath dissipation
ltnTgt ? ltngtltTgt, etc.
Hence, need global edge turbulence codes
Quiescent periods interrupted by intermittent
ejection of plasma filaments
R-4 by Boedo
These advect mass and energy into the far-SOL,
while draining to the divertor
50
Impurity seeding dilution
(Zeff 1) increases linearly with the radiative
heat flux and decreases as the square of line
average density
Scaling predicts higher value of Zeff in ITER
than assumed for the reference scenario (2.3 vs
1.7)
ITER
G.Matthews et al, NF ?? (1999) ???
51
Type-III ELMs controlled by resistive-MHD, i.e.
stable to ideal MHD (P-B) modes.
What is the condition for the I-III transition?
Critical pedestal temperature/resistivity ?
52
Expression for ELM energy to wall on JET
Interchange driven amplitude scaling with
convective ion losses combined with
moderate-ELM (DW/W 5, DW/Wped12) e-folding
length, yields so that fraction of ELM energy
to wall can be approximated as where Dped is
the pedestal width and DSOL is the
separatrix-wall gap. eg. when DW/W reduced by a
third, then (Wwall/W0) 10 for 3 cm gap, see
below.
W.Fundamenski et al, PSI 2006 subm. to
J.Nucl.Mater
53
Conclusions

• JET data indicates that bigger (more intense)
  ELMs deposit a larger fraction of their energy on
  the main chamber wall, which suggests that the
  radial Mach number increases with ELM size
• Two-field interchange model used to study size
  amplitude scaling
• It was found that over a wide range of
  conditions, the radial Mach number is expected to
  increase as the square root of both ELM size and
  amplitude,
• This implies that radial e-folding length of ELM
  filament energy also increases
• Model predictions in fair agreement with JET data
  
• Preliminary predictions for ITER indicate the
  added benefit of reducing the ELM size for small
  ELMs, DW/Wped lt 5, less than 2 of ELM energy
  deposited on the wall (near 2nd separatrix at
  upper baffle) contact with limiters is
  negligible.

54
ELM-wall limiter interaction on ITER
Same prescription as used to match JET data
(Type-I ELMs, DW/W 5 ) 8 of ELM energy
onto main wall at 5 cm (omp) 1.5 of ELM
energy onto limiter at 15 cm (omp)
ITER 2nd separatrix movable
limiter
Normalised ELM filament quantities
Normalised time since start of parallel losses
W.Fundamenski et al., Plasma Phys.
Control..Fusion, 48 (2006) 109
R.Aymar et al., PPCF 44 (2002) 519
55
ITER-like ELMy H-mode equilibria

• Type-I ELM-filaments clearly observed with Dr 2
  cm-omp (top)
• No Type-III ELM-filaments, despite proximity to
  upper dump plate (1.5 cm)

Type-III ELMs
Type-I ELMs
ITER
Dr 5 cm
ITER Dr /a 2.5
JET Dr /a 2
56
Ion impact energies on JET and ITER

• Predicted peak ELM filament quantities on JET and
  ITER (moderate Type-I ELMs)
• JET Ti,max(rlim) 185 eV (ion impact energy
  0.6 keV) at 4 cm
• ITER Ti,max(rlim) 350 eV (ion impact energy gt
  1 keV) at 5 cm 100 eV at 15 cm
• Lower bound estimates for moderate (DW/W 5 )
  Type-I ELMs

Peak ion impact energy
Radial distance from mid-pedestal location
W.Fundamenski et al., Plasma Phys.
Control..Fusion, 48 (2006) 109
57
Limiter SOL profiles
As expected, limiter SOL width decreases with
increasing plasma current
Physical mechanism not understood at the time !
1 MA
J.A.Tagle et al, 14th EPS 11C (1987) 662
2 MA
S.K.Erents et al, NF 28 (1988) 1209
JET
2 MA
3 MA
2 MA
3 MA
4 MA
Ip
3 MA
4 MA
5 MA
4 MA
Ip
5 MA
Ip
5 MA
D_perp Ip-2 Check in original paper
58
Limiter heat loads in ITER
JET diffusivities combined with a 3D
fluid-neutral code (EMC3/EIRENE)
qmax(MW/m2)
Calculated limiter heat loads of several MW/m2,
increasing with Ip
Ip (MA)
MC simulation of impurity transport (DIVIMP)
suggest W-limiter a problem
M.Kobayashi et al, NF 47 (2007) 61
ITER
59
ELM control and effect on inter-ELM loads

• On ITER, ELM size can be reduced by a combination
  of
• pellet pacing
• Necessary tool for deep (pedestal) plasma
  fuelling
• external magnetic field perturbation (EFCC coils,
  RMP coils?, TF ripple)
• 6x3 EFCC coils envision for error field
  correction
• TF ripple in the range of 0.2-0.5
• magnetic pacing (vertical kicks)
• Not clear if suitable to ITER at present
• impurity seeding (Type-III ELMs)
• In the absence of C as radiator with Be/W mix,
  impurity seedind necessary for partial detachment
• For all the above techniques it is imperative to
  determine
• The maximum reduction in ELM size (increase of
  ELM frequency)
• Associated reduction in confinement (H98) and
  fusion gain (Q)
• Associated increase in inter-ELM heat loads
  (q_div, Te_div, Ti_div, q_lim) and any
  detrimental effects on divertor plasma detachment
• Associated increase in core plasma pollution
  (Z_eff)
• Synergistic effects
• all the above processes (pellets/EFCCs/TF
  ripple/seeding) concurrent on ITER

60
Effective radial velocity const
Describing SOL transport by standard
parallel-perpendicular transport competition
relations, reveals a roughly constant radial Mach
number
Effective radial velocity
Effective radial diffusivity
Physical mechanism not understood for a long time
but great progress made in the last few years !!!