Divertor Requirements and Performancein ITERM. Sugihara
ITER International Team
12 th International Toki Conference
Dec. 11-14, 2001

Contents
Overview of requirement and prediction for divertor
performance taking ITER case as an example(1) Required divertor performance(2) Predicted divertor performance(3) Further model development needed and remaining
uncertainty(4) ELM effects and mitigation(5) Summary_With contributions from :A. Kukushkin, A. Loarte and ITER IT Physics Unit

1. Required divertor performance(i) Heat removal(ii) Fuel density control(iii) Exhaust of helium ash and other impurities(iv) Providing proper magnetic configuration for enhanced
confinement (H-mode)
Requirements
1. Peak power load on the targetplates ( q pk )2. Helium concentration in thecore plasma ( CHe )3. Zeff in the core plasma
4. Upstream plasma density ( ns )5. D-T particle throughput( Γ DT )*core
6. Core fuelling ( Γ DT )*q pk ≤ 10 MW / m 2
CHe ≤ 0.06
Zeff ≤ 1.6
ns ≤ ne / 3
Γ DT ≤ 200 Pa ⋅ m 3 / score0 ≤ Γ DT
≤ 100 Pa ⋅ m 3 s−1• 6 requirements must be simultaneously satisfied* are also control actuators

Specific features for divertor control(i) Control actuators; not so many
- Divertor geometry- Gas-puffing (Throughput ; Γ DT ) *core- Core fuelling ( Γ DT )*- Pumping speed- Impurity seeding (Ne, Ar)

(ii) Divertor and Core Performance are closely linked ;SOL/Divertor ⇔ Pedestal ⇔ CoreCore
performanceoCore<T>, <n>SOLseparatrix

PedestalCore

Pedestal
Tped(Stiffness)

nped

ELMoTransport

Separatrix
ns e pDivertor
performance- Development of Modelling to include Pedestal continues,
but not yet complete.
- Presently CEI (5cm inside separatrix) is calculation
boundary and transport barrier is not yet properlymodelled.

2. Predicted divertor performance
Prediction by B2/Eirene divertor code
Basic models- D = 0.3 m 2 / s , χ = 1 m 2 / s
w/o parameter and spatial dependence- ELM effect is not included (time averaged)- Carbon sputtering (physical + chemical), but they areabsorbed at every surface encounterd- Partial detachment (only near separatrix is detached)

Optimization of divertor geometry
Key strategy to reduce peak power load:
Enhance neutral accumulation, in particular, near
the separatrix region for outer target plate• Vertical Target Plate + Divertor Dome
• V-shape Target geometry ;effective in accumulating
neutrals near separatrix(JET)⇒ ≈ 30% reduction of qpk in ITER
q2pk

[MW/m ]

86MW
straight

1686MW
old V

1286MW
new V
8100MWstraight
100MW
old V

4V-shape100MW
new V

00.24 0.26 0.28

0.3

0.32 0.34 0.36
20-3n [10 m ]sstraight(Kukushkin, EPS 2001)

• Gas flow between inner and outer divertor;⇒ increase neutral recirculation in the outer
divertor target region (higher power flux)⇒ reduce peak heat load (JET, JT-60U)

ΓinΓΓ

outpump- ≈ 20% reduction of qpk by gas flow between inner andouter divertor

Separatrix density
Dominant effect on divertor performance and can becontrolled by gas puffing (throughput; ΓDT ) to some extent
gascoreΓDT = ΓDT+ ΓDT• Saturation corresponds todetach. of inner divertor(increased neutral density)• Higher ns for higher
power to detachInductive operation• Peak power load and helium concentration
X

Reference operation
PSOL=86MW(Pf=410MW, Ptotal=123MW, Q=10, frad=0.3)
High fusion power with high Q
PSOL=100MW (Pf=600MW, Ptotal=145MW, Q=24, frad=0.4)
High fusion power with low Q
PSOL=130MW (Pf=600MW, Ptotal=187MW, Q=9, frad=0.3)• Peak power load and helium concentration for reference
operation mode is well within the requirement.
• Fusion power (helium source) and throughput dominate
helium concentration, while pumping speed is lessimportant• Reasonably wide operation window is available for thereference inductive operation mode, while density window
is not so wide (∆ns between qmax and complete detachment)

Steady state operation
Steady state
PSOL=100MW
Pf=340MW, Q=5.7,
Ptotal=128MW, frad=0.2
Longer connection lengthwith q95=4.5=> same qpk withlower nscf.
Inductive operation
PSOL=100MW
q95=3.0(Kukushkin)• ns(at qpk=10MW/m2)=0.26=> somewhat higher than ns ≈ ne / 3 ≈ 0.23=> Impurity seeding will be needed• Initial calculations with neon seeding (0.4%) ;⇒ ≈ 30% reduction of qpk (radiation region is getting
far from target plate compared with carbon)
qpk=10MW/m2 at ns ≈ 0.23 − 0.24
∆Zeff ≈ 0.4 (total Zeff ≈ 1.6)

3. Further model development needed
and remaining uncertainty(1) Transport in SOL region(2) Separatrix density under good H-mode confinement(3) Consistent pedestal model is not yet developed;- D = 0.3 m 2 / s , χ = 1 m 2 / s are too large in the pedestal(transport barrier) region=> low pedestal density ( nped ≈ ( 3.5 − 4.5 ) × 10 19 m −3 )=> e.g., neoclassical level D ≈ 0.06 m 2 / s and proper widthmodel for pedestal must be implemented=> Consistent boundary condition for core plasmatransport (to be developed)=> By proper pedestal model, core fuelling requirements
can be properly specified, which is consistent withthe expected density pedestal in ITER
Core fuelling is needed
because;• Gas-puffing is very
inefficient due to thick
SOL in ITER
• Only small fractionof gas-puffed neutrals
can penetrate acrossseparatrix

Specification of required core fuelling for expected density
pedestal in ITERn(r)C;• Core fuelling Γcore
Fuelling insidepedestalΓ i s e p~ Γ 0 s e p+ Γ c o r ens e p
Γ0 s e prp e d ∆ped

Γ 0 puff~10cm a∆=10cmped

Expected Pedestal
density in ITER

8

Γ Cc o r e=28Pam3 /s(56)Γ Cc o r e=14Pam3 / s(28)ped(101 9m- 3)

10Pedestal Density n• Fuelling in thepedestal region is also
possible but factor of
two larger fuelling isneededSOLnp e dP;• Pedestal fuelling Γcore
Fuelling between
separatrix-pedestal• With proper transportmodel in the pedestal
andcore or pedestal
fuelling can achieve
the expected pedestal
density

separatrix

Particle balance acrossseparatrix and pedestal

Pedestal fuelingΓ Pc o r epedestal

Core fuelingΓ Cc o r e62

D=0.06 m / s

Γ Cc o r e=14Pam3 / s
(28)

4
2

D=0.3 m / s223( ):Pedestal fuelling
4

Separatrix density ns e p (101 9 m- 3)• High field side pellet is prepared for ITER
⇒ required core fuelling is possible
50-100 Pa ⋅ m 3 / s
500 m/s (deposition depth ≈ 0.15 a ; inside pedestal)5

4. ELM effects and mitigation
High pedestal pressure required for good
confinement can result in large divertor erosion
due to Type-I ELMs
• Limit for divertor erosion due to ELMs
∆WELM /( SELM τELM ); surface temperature rise(Federici; SOFE, 2002)• Specification for τELM ≈ 200 µs• τELM in JET for various
density and triangularity

τELM ≈ 200µs(JET, Becoulet)• Specification for SELM ≈ 2 × Sssλq ; Power deposition width mapped on midplane has large
uncertainty
Power to the target - total, with plasma, and• Experimental datafor Sss are mostly
taken from attachedcondition=> λq ≈ 5mm
2=> SELM ≈ 6 m• From power load
profile in ITER ;λq =(10-13) mm
due to detachment
2=> SELM ≈ 15 m

radiation - mapped to the mid-plane [MW/m2 ]

10

total
plasmaradiation

7ITER-FE
Pin = 86
ns = 3 10
Sp = 40106

λ =5.6 mm

A. S. KukJun 2000

q00.010.02

xMP [m]• Criteria of ∆WELM / Wped for surface temperaturerise up to critical one
τ ELM = 200 µs
SELM ≈ 6 − 15 m 2

CFC

W

Allowable ∆WELM / Wped (%) for 10
3.4- 8.3
4.4- 11
ELM events with deposition area
SELM ≈ 2 × SSS ≈ 6 − 15 m 2
Wped ≈ 100 MJ* This is also necessary to maintain plasma purity(≈1022 carbon/ELM event is produced)
6

Proposed models for experimental data summary
0.25

Collisionality ( ν )*0.2∆W ELM / Wp e d(Loarte, IAEA 2000)(∆WELM ν *) ∝ ( ν* ) P
WpedP ≈ -0.33

DIII-DJETASDEX-U

0.150.1
ν* I T E R0.05(15-20) % for ITER
00.01

τ// = 2 L C ( 1 + 3 / 2ν* )∆W ELM / Wp e dτELM

DIII-DJETASDEX-U

0.2(Janeschitz, PSI 2000)∆WELM tau //∆WELM1)
=()0τ
Wped
Wped
1 + //

0.15
0.1
0.05s

τELM ≈ 200 µs

0(12-15) % for ITER

τ / /I T E R
0200

400

600 800 1000 1200
τ // (µs )∆om=a/100

0.3× ( Bp / BT )om 4 πRom ∆om ∆t∆W E L M / Wp e d(Shimada, 2001)∆WELM = γkΓTped∆t=0.2ms

DIII-D_expDIII-D_model
JET_exp
JET_model
AUG_exp
AUG_model

Sheath model

Upper limit(≈ 5) % for ITER

100.25

Parallel transport ( τ// )(0.11
ν* (collisionality)

0.2

0.1

00500

1000

T p e d (eV)All models still need much more work for
ITER extrapolation

1500

ITER Prediction
0.3(Further inclined)

Allowable∆W/WELM

ped∆W E L M / W
ped

Model
prediction
ν* model0.2(inclined)τ//modelW0.1(inclined)

CFC

0100

sheath model

200

300

400

500

Fusion power (MW)• Very severe predictions based on ν and τ// models.
• Range of uncertainty and difference between models are
significantly large.
*

Possible mitigation methods(1) Further inclination of divertor plate• Poloidal projected angle 22.8° (2.1° real)⇒

11.4°• Possible disadvantage can be acceptable- Particle recycling on the upper part of divertor plate ;⇒ not significantly increased (B2/Eirene)- Separatrix line position control;⇒ may be acceptable once operation mode is fixed forengineering testing (life time becomes a more
important issue for this phase).
⇒ use of W divertor plate may also be possible during
this phase due to low disruption probability.

(2) Discharge regime of high pedestal pressure withsmall ELMs (Type II)• Most of the present machines show that( ≥ 3.5-4)• high q95• high δX(≥ 0.4-0.5)
are needed to obtain this alternative ELM regime.
• δX for ITER (=0.5) satisfies the required condition.
• Q=10 and Pfusion ≈250MW operation with q95 =3.5 (Ip=13MA)
is possible, though window is narrow.

Further increase of HH-factor- with lower density (many machines)- with higher q95 (HH=1.3 with q95 =3.6, n / nG ≈ 1 andvery small ELMs in AUG)
window becomes much wider.
• This small ELM regime will be accessible for Hybrid andsteady-state scenario ( q95 >3.5).
• Further R&D is needed to extend this small ELM regime tothe reference high Q inductive operation mode.
- Type II ELMs in-between Type I ( q95 =3, δX =0.5; JET)
could be a clue for R&D

5. Summary• Divertor requirements for ITER are summarised.
• B2/Eirene code calculations show that these requirements
will be satisfied for inductive operation mode.
• For non-inductive operation mode, impurity seeding
will reduce the peak heat load to meet the requirement.
• Further model development is necessary for B2/Eirene
to include proper pedestal model. It is indicated thatgas-puffing cannot fuel across the separatrix to form
proper density pedestal. High field side pellet fuellingis prepared in ITER to fuel inside the pedestal.
• Effect of Type-I ELMs on divertor plate could be severe
for high pedestal pressure required for good confinement,
while present prediction by proposed models are stillprimitive, and thus further development/improvement ofthe models as well as the database are necessary.
• Possible mitigation methods for Type-I ELM effect aresummarised; inclination of target plate and Type-II ELMs.
Hybrid and steady-state scenarios can be operated with
Type-II ELM regime. Further exploration to extend thisregime to high Q inductive operation mode should bepromoted.