Transcript Slide 1

Integrated power exhaust strategies for ITER:
a few remarks on the problem
W. Fundamenski (UKAEA, EFDA-JET)
with contributions from V.Philipps, G.Matthews, S.Brezinsek, A.Loarte, F.Sartori,
C.Lowry, P.Lang, Y.Liang, T.Eich
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W.Fundamenski
Avila, Spain, 7-10/01/08
Ignition vs. exhaust criteria for a reactor
In order for any exothermic reactor to operate in steady-state,
(i) fresh fuel must be added at the rate at which it is consumed,
(ii) this fuel must be heated, ideally by the reactions themselves,
(iii) fuel must be confined, by whatever means are available, for sufficiently long to allow the
exothermic processes to continue,
(iv) the energy and ash must be removed from the system at the rate at which they are
created,
(v) the impurities released from the reactor walls during this exhaust process must not inhibit
the ignition (burn) of fuel, and
(vi) the reactor itself, primarily its walls, must not be damaged by all the exhaust processes.
Conditions (i)-(iii) represent ignition criteria, conditions (iv)-(vi) as exhaust criteria.
Taken together these constitute the criteria of mutual compatibility between the
reaction processes and materials/components in an exothermic reactor
Since the latter provide the boundary conditions for the thermodynamic quantities,
they effectively determine the maximum achievable energy gain for a given
reactor design, i.e. a give combination of fuel and hardware.
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Ignition vs. exhaust criteria for ITER
In order for ITER to operate in steady-state,
(i) D & T must be added (NBI & pellet fuelling) at the rate at which they are consumed
(ii) D & T must be heated by RF, NBI and dominant a heating
(iii) D & T must be confined for sufficiently long to ensure Q = Pfus / Pheat = 10
(iv) The (neutron, photon and plasma) power and He ash must be removed from the
system at the rate at which they are created
(v) the Be, C & W impurities released from the reactor walls must not inhibit the continued
burn (Q=10) of D & T by either dilution and/or radiation,
(vi) The plasma facing components must not be damaged by the above processes.
e.g. joint requirement of partial detachment of both divertor legs (to reduce the inter-ELM
heat loads below ~10 MW/m2), and ELM-mitigation (to reduce the ELM transient energy
loads below ~1 MJ/m2).
(iv)-(vi) effectively impose the boundary conditions for the plasma density and
temperature, and hence determine the maximum achievable Q in ITER, for a
given set of internal and external hardware, i.e. TF, PF and control coils; PFCs;
heating, fuelling, pumping and cooling systems, etc.
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Ignition and MHD stability limits
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Power exhaust limits
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MHD stability, ignition & exhaust b limits
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1.0
W.Fundamenski
droplet ejection
and
bridging of tiles
after 50 shots
0.5
PAN fibre
erosion
after 10 shots
significant
PAN fibre
erosion
after 50 shots
PAN fibre
erosion of
flat surfaces
after 100 shot
erosion starts
at PFC corners
negligible
erosion
0.5
melting of the
full tile surface
(no droplet
ejection)
melting of
tile edges
negligible
erosion
Transient heat load limits for ITER
1.0
1.5
energy density / MJm-2
CFC
1.5
energy density / MJm-2
W
ITER adopted a value of 0.5 MJ/m2 for the maximum allowed ELM energy load
Avila, Spain, 7-10/01/08
Maximum permitted ELM size for ITER
Using best estimates for divertor wetted area and in-out asymmetry, one finds
DWELM = QELM x Sin x (1 + Pout/Pin) = 0.5 MJ/m2 x 1.3 m2 x 1.5 ~ 1 MJ
Assuming Wdia ~ 400 MJ and Wped / Wdia ~ 1/3, then DWELM/ Wped < 1%
This requires a decrease in the ‘natural’ ELM size by a factor of ~ 20
Some caveats, e.g. the above assumes that the simulated ‘ELM’ pulse shape in the
plasma gun is the same as the real ELM pulse in a tokamak
2%
1%
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Divertor heat loads due to ELMs
Determination of divertor ELM power flux time dependence
   i ,o  2    i ,o  2
   i ,o  2 
   
  exp  
 
Pi ,o (t )  Ai ,o 1  
  t    t 
  t  




T.Eich
W. Fundamenski
TRINITI plasma gun
(from A.Loarte)
AUG-Eich
JET-T. Eich
more than 60% of DWELM,div arrives after qELM,divmax  smaller DTsurfELM
This inherent skewness could allow for a moderately larger ELM load (~ 30% higher)
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Steady and transient wall loads on ITER
Type of Interaction
Parallel (‫)װ‬
Units
2001
PID
Latest
Proposal
Outer Midplane
~1°
Perpendicular (┴)
Radiation
┴
MWm-2
0.5
0.5
0.5
Power Conducted
between ELMs
‫װ‬
MWm-2
None
None
3
Power Conducted by
ELMs
‫װ‬
MWm-2
None
0.93
2.5 (3.4)
Energy Conducted by
ELMs
‫װ‬
MJm-2
None
0.19 (0.93)
0.06 (1.7)
Radiation
┴
MWm-2
0.5
0.5
0.5
Power Conducted
between ELMs
‫װ‬
MWm-2
None
None
5
Power Conducted by
ELMs
‫װ‬
MWm-2
None
3.8
33
Energy Conducted by
ELMs
‫װ‬
MJm-2
None
0.77 (3.8)
0.8 (17)
┴
MWm-2
MW
1.3
15
1.4
15
0.6
8
Near second X-point
5
ITER-FEAT
Sc enar io #2
4
3
2
Z, m
1
0
-1
q=1 .5
q=2 .0
-2
Baffle Region
q=2 .5
q=3 .0
-3
MARFE
Start-up & Ramp-down
-4
-5
3
4
5
6
7
8
9
‫װ‬
R, m
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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
– Quantitative prediction beyond our abilities in the near future
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ELM size reduction by pellet injection
Type-I ELM frequency can be increased by injection of small deuterium pellets,
provided that pellet freq. > 1.5 natural ELM freq. (results from AUG)
•
Can the effects of plasma fuelling and ELM pacing be decoupled?
• Can ELM pacing be demonstrated at N_GW ~ 0.75?
fPel > 1.5 f0ELM
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ELM size reduction by EFCC coils with n=2
JET #70472 Bt = 1.85 T / Ip = 1.6 MA/ q95=4.0
PNBI
Type-I ELM frequency can also be
increased by introducing steady
state n=1 or n=2 fields
(x10 MW)
IEFCC
(x16 kAt)
 fELM ↑ ~ 15 Hz  ~ 40 Hz
~ 650 eV  ~ 250 eV
ne,l
(x1020m-2)
Te (keV)
 DTe ↓
24 kAt
core
edge
 T_e ↑(not fully understood)
 H98 ↓ (~ 0-20 %)
core
edge
Da (a.u)
12
 n_e ↓(pump-out)
14
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16
Time (s)
18
What is the change in confinement
when the magnetic pump out is
compensated by external fuelling?
What is the effect in impurity seeded,
20
highly radiative plasmas?
W.Fundamenski
Avila, Spain, 7-10/01/08
ELM size reduction with vertical kicks
Type-I ELM frequency can likewise be increased by fast changes (vertical
kicks) to magnetic equilibrium
Natural ELM frequency (~ 5 Hz)
increased to ~ 10-25 Hz
At freq. > ~35 Hz kicks do
not always trigger an ELM
Small reduction of Wdia and
pedestal quantities: ne , Te
Promising technique for ILW, in
which case the ELM size need
only be reduced by ~ 2-3 times
70426, 2MA 2.35T
Da
kick
FRFA current
Wdia
ne core (LID3)
ne edge (LID4)
Te edge
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Impurity seeded, highly radiative discharges
Finally, ELM frequency can increased substantially (factor of ~ 10) by affecting
a I-III transition, that is, by replacing Type-I by Type-III ELMs.
Best pulse at 2.75MA/2.2T, high d
• frad=0.75, Zeff ~ 1.5 – 2, N-seeded
• H98(y,2) ~ 0.83 (~ 17% degradation)
• bN ~ 1.9, n* reduced by ~ 2.5
H 89  b N
 0.45
2
q95
• Both divertors detached !
At present the only scenario compatible
with ITER requirements of 1% ELM
energy loss + partial detachment,
although at the penalty of
confinement degradation of 15-20%,
which yields Q ~ 4-5 @ 15 MA.
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Zeff=1.7
Q=10
domain
Old results
W.Fundamenski
Avila, Spain, 7-10/01/08
Effect of TF ripple on ELMs
ELM size, as well as H98, decrease with increasing TF ripple
– For the same n*ped, DWELM/Wped decreases by factor of two when TF ripple increases from 0.1% to 1%
– Change related to smaller conductive loss (DTe/Teped), rather than convective loss (Dn/nped)
– The reduction must less pronounced at higher density, i.e. close to N_GW ~ 1.
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THE END
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Some lessons from Be/W wall on JET
The presence of Be on main wall (limiters, dump plate) and W in the divertor
(especially W-coated CFC tiles) imposes new limits on plasma scenarios
Energy and power limits
Plasma compatibility issues
1. Main chamber PSI, mainly during
transients
1. The risk of W contamination of
(fuel dilution in) core plasma
2. Divertor steady state & transient loads
2. Reduced edge and divertor
radiation (in the absence of Carbon)
3. NBI shine-through
4. Special effects associated with RF
power (ICRH and LH)
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3. Hence, the need for impurity
seeding to replace Carbon as the
main radiating species
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Transient heat load limits for W-coatings on JET
Maximum coating test temperature in cyclic loading (200 pulses): 1600C
• W-C carbide formation starts at about 1000C (exponential increase),
“carbidisation” of the W layer should be avoided
• W-C carbides have lower melting point, less ductility, and release C
W-carbide
W
C
To have some margin for ELMs, Tmax should
be below 1600C ( 1200C)
 Surface temperature is limiting in most
cases (presently set by energy limits given
by metallic base structures)
Preliminary heat load tests in Judith simulator on 200m VPS (2000 Elms, 1 ms) found
power limit of ~ 0.3 GW/m2 (T. Hirai): ongoing analysis about failure mode at higher loads
Recommend maximum transient heat load ~ 0.2 GW/m2
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Maximum ELM size for W-coatings on JET
Using best estimates for divertor wetted area and in-out asymmetry, one finds
DWELM = QELM x Sin x (1 + Pout/Pin) ~ 0.2 GW/m2 x 500 s x 1 m2 x 1.5 ~ 150 kJ
For typical JET stored energies of Wdia ~ 5 MJ and Wped / Wdia ~ 1/3,
which translates into DWELM/ Wped < 9 % (less for larger Wdia).
This requires a decrease in the ‘natural’ ELM size by a factor of ~ 2
9%
DSOL ITPA meeting
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Avila, Spain, 7-10/01/08
Maximum ELM size for W-coatings on JET
Another possible methods of estimating maximum ELM size is based on
maximum tile temperature rise, and is thus dependent on tile temperature.
This gives somewhat higher limit for cold tiles (~ 250 kJ with safety margin).
DSOL ITPA meeting
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Maximum T_e_div for W-coatings on JET
W erosion proceeds by physical sputtering with an ion energy threshold (for
deuterium ions) of ~ 150 eV, or T_e_div ~ 150 / 5 eV ~ 30 eV.
• Erosion of bulk W plate is not a critical issue for its life time
• Erosion of W coatings, might be an issue, especially for the thinner coatings,
which pose a high risk of gradually revealing the C substrate
Need to find an optimum between cooling the divertor plasma (< 30 eV) to reduce
the erosion of W and introduction of seeding impurities which can themselves
increase erosion (lower energy threshold for higher Z)
Need to aim for partial detachment (T_e_div < 5 eV) at both divertor legs !
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ITER-like wall (ILW): Be wall
Restraint ring
protection
Be limiter Surfaces moved
forward 3cm (W- coated CFC
recessed)
Shinethrough Area
10μm W - coated
CFC recessed
Bulk Be ribbed Dump
Plate
Be coating of inconel
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ITER-like wall (ILW): W/W-CFC divertor
ZrO2
TZM spacers (coated with Al2O3)
Tungsten (>99.95%)
W coatings
Inconel 625
Bulk W tile
optional
copper inserts
Inconel 706
 8000 W- lamellas
W coatings:
• 200 m VPS (Plansee) selected at first but more R&D
show that thick VPS on CFC may not be reliable enough
• change to 14 m thin Re-W multilayer coating is very
likely (to be decided January 2008)
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Power and energy load limits for ILW
2
2100 7.5 MW/m on JET CFC
1800
Temp (K)
1500
Surface Temperature limits:
Elms
1200
• Steady state
900
300
• Transient (ELMs)
Steady state
600
0
2
4
6
8
10 12
Time (s )
ZrO2
TZM spacers (coated with Al2O3)
Tungsten (>99.95%)
Inconel 625
optional
copper inserts
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Energy limit for metallic for
substructures
Inconel 706
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Steady state heat load limits for ILW
The limit on the steady state heat load determined by the maximum allowed
coating temperature, heating power, radiation, SOL width and shot duration
Maximum coating temperature 1600C  1200C for ELM window
+INF
40
30
Start T = 220C
1800
25
15
forbidden
10
7.5 MW/m2
1500
Tmax<1200C
20
1200
900
Similar thermal response
for W, C and Be
600
5
0
Tungsten
Graphite
Beryllium
2100
Temp (K)
Power (MW/m2)
35
0
2
4
6
8
10
12
300
0
2
6
8
10
12
Time (s )
Shot length (s)
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ELM induced material loss on JET
JET experiments at high Ip with ITER-like values of ELM size up to 1 MJ
A. Huber/R. Pitts
ELM affected area on JET ~ 1 m2. Hence, above increase occurs at ~ 0.7 MJ/m2, although
the increase of radiation associated with ablation of surface layers, rather than bulk CFC
DSOL ITPA meeting
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Main chamber heat loads due to ELMs
ELM energy deposition at main chamber given by competition of parallel and
perpendicular transport and filament size + detachment dynamics
T. Eich/W. Fundamenski/R. Pitts
Do larger ELM filaments travel faster? What is their spatial structure?
DSOL ITPA meeting
W.Fundamenski
Avila, Spain, 7-10/01/08