J. Plasma Fusion Res. SERIES, Yol.5 (2002) 69J6

Divertor Requirements and Performance in ITERSUGIHARA Masayoshi
ITER International Team, Naka-machi, Naka-gun, Ibaraki, 311-0193, Japan(Received: 11 December 2001 / Accepted: 17 Mav 2002)

Abstract
A variety of functions required for the ITER divertor are (i) heat removal, (ii) fuel density control,(iii) exhaust of helium ash and other impurities, (iv) providing proper magnetic configuration forenhanced confinement (H-mode). These requirements are shown to be satisfied in ITER by thesophisticated numerical code for divertor (B2-EIRENE) validated with various experimental databases.
More work is necessary for the development of a pedestal model, transport in the SOL region andseparatrix density, which somewhat influence the divertor solutions. Analysis based on numerical code
calculation shows that core fuelling is necessary to form the density pedestal required in ITER, which isequipped with high field side pellet fuelling for this purpose. Transient large heat load during Type-I
ELM activity could cause large erosion of the divertor plate, though the proposed ELM models and the
database still need further improvement and development. Further inclination of the divertor target plate
can mitigate the effect of ELM energy load. The discharge regime with small ELMs and high pedestalpressure (Type II ELM) can be used for hybrid and steady-state modes of operation. The inductive highQ mode can also be operated with Type II ELMs with reduced plasma current and fusion power, though
its operation window is narrow. Further improvement of confinement with low density and,/or higher 4e5
can wiclen the window sisnificantly.

Keywords:
tokamak, divertor, SOL, pedestal, H-mode, ELM, ITER

1. Introduction
The major mission of ITER (International
Thermonuclear Experimental Reactor) is to achieve

fuelling capability.
The requirement of heat removal must be satisfied

both during steady and transient conditions. During

400MW of fusion power with Q=10 for several hundred
seconds by inductive current drive [1]. In addition,
steady state operation with Q=5 by non-inductive
current drive is to be aimed at. Reasonable operation
windows for these operation scenarios are indispensable
to accomplish the overall missions of ITER. To achieve

steady conditions, the peak heat flux density must be
below an engineering limit. Heat removal during Type I

ELMs is a main issue during transient conditions.
Energy loss during Type I ELMs is correlated with highpedestal pressure, which is required for good energy

these missions. relevant plasma performance is required

confinement, and consequently, the heat load could besevere. It is therefore of primary importance to predict

which is highly dependent on the divertor performance,

the energy loss during ELMs in ITER by predictive

e.g., radiative cooling, helium ash exhaust and impurity

models or scalings based on the ELM database.
Development of mitigation methods of the ELM effectcontrol. The divertor performance must also bemust also be carried out in parallel with thecompatible with the engineering requirements. e.g., heatremoval capability, DT particle throughput and core

@2OO2 by The Japan Society of Plasma

C o r re spondin g author' s e -mail : s u g iham @ ite rgp s.j a e ri. g o.j p

69

Science and Nuclear Fusion ResearchSugihara M., Divertor Requirements and Performance in ITER

Table 1 Divertor requirements in ITER.
Requirements
1. Peak power load on the target plates (%k)

9pr < 10 MWm2

2. Helium concentration in the core plasma (G")
3.

lnin the core plasmacH" < 0.06z"ft< 1.6

4. Upstream plasma densifl (n")

n. < 0.33x1020 m-3

5. D-T particle throughput (G')

Gr < 200 Pa.m3/s

6. Core fuelling (1-o'?'")

0 f l-d?* < 100 Pa.m3s-ldevelopment and improvement of these predictivemodels and database.

In this paper, the required divertor functions forcore plasma perfonnance and the divertor's engineering
constraints are surnmarised in Sec. 2. In Sec. 3, divertor
performances are examined using the sophisticatednumerical code B2lEirene. In Sec. 4, the remaining.31'?n,€]"'"(Stitl Profils)
P'duncertainties and further necessary development of themodels are briefly reviewed. In Sec. 5, the effects ofType I ELMs are examined and a prediction of ITER

ELM

'ranspoltcase is performed using proposed models. The
uncertainties of each models and database are discussed.
Possible mitigation methods and future R&D which isnecessary are also presented.
Fig. 1 Schematics of the links between core plasma anddivertor plasma through a pedestal.

2. Required divertor performances
A variety of functions are required for the divertor
in ITER. Major functions required are (l) heat removal,(2) fuel density control, (3) exhaust of helium ash andimpurity control, (4) provision of a proper magnetic
configuration for enhanced confinement (H-mode).

governed by the characteristics of the critical pressure
gradient due to the ideal ballooning mode for the Type-I
ELMy discharge regime (ITER's reference operation
regime). High pedestal pressure required for goodMore specifically, the requirements can be summarisedin Table 1. Those six requirements must be

energy confinement of the core plasma can result inhigh energy deposition during Type-I ELMs on thedivertor plate l2l, which can have a large impact on thedivertor life time as well as on the impurity generationfrom the divertor plate.
Other constraints are the engineering limits, such as

simultaneously satisfied. On the other hand, since thecore plasma and SOl/divertor plasma are closely linked
through the H-mode edge transport barrier region(pedestal region), the required divertor performances

must be satisfied consistently with the core plasma

depends on the pedestal temperature due to the profile

peak heat flux on the divertor plate, DT particle
throughput due to the tritium inventory limitation, core
fuelling due to the limitation of the particle injection

stiffness and the average density is very close to thepedestal density due to the expected flat density profile.
The pedestal density is closely linked with the separatrix
density through the fuelling scheme and the transporttechnique, and pumping speed. The upper limit of theDT particle throughput and the core fuelling rate are
requirements, but at the same time, these are control
actuators. The numbers of actuators are not so many andparameters. Schematics of these links are shown in Fig.

l. The average temperature in the core plasma strongly

performance strongly depends on the separatrix density

they are; (i) Divertor geometry, (ii) DT particle
throughput, (iii) Core fuelling, (iv) Pumping speed andand transport in the SOL region, which is smoothly
connected to the pedestal region where transport isgeometry is not a real time actuator but is fairly

characteristics of the barrier region. Divertor(v) Impurity seeding (Neon, Argon and others). Divertor

70Sugihara M., Divertor Requirements and Performance in ITER

important to supplement the controllability of thedivertor with small numbers of actuators. Actuators (ii),(iii) and (iv) can control the separatrix density z, tosome extent, and the helium concentration as well as the(a):at- '

peak heat flux can be controlled through n, as shown
later.

3. Predicted divertor performances
The divertor performance of ITER has been
examined and optimised using a sophisticated numericalcode B2lEirene, which has been validated against
various experirnental results in ASDEX-U, JET and JT-

60U. Details of the code and model validation are
published elsewhere [3,4]. Major assumptions for theexaminations and optimisations of ITER are summarised
as follows;r Transport coefficients; D=O.3 mzls, y=l m2ls, noparameter dependence and spatially constant both in
SOL and pedestal regions.

r The effect of ELMs is time averaged (not included
explicitly),r Carbon sputtering (physical and chemical) is properly

Fig. 2 (a) V-shape and straight bottom plate of divertor.
(b) Connection of inner and outer divertor region

to allow neutral flow from inner to outer.
recognised experimentally and numerically that particle

flux is higher at the inner divertor target than the outerand the other way around for the heat flux. For thisreason, detachment of the inner is easier. By connecting
the inner and the outer divertor region, neutrals can flowfrom the inner to the outer and neutral accumulation

included, while they are assumed to be absorbed atnear the separatrix of the outer divertor can beevery surface encountered.

increased. This feature has been confirmed by JET [6]and JT-60U experiments [7]. An ITER calculation has
been done by changing the probability for neutrals toflow from the inner to the outer. The actual ITER design
corresponds to a probability of 0.56 (l7O-220 Pa.m3/s)
and the peak heat load can be reduced by abott 20 7ocompared with the case of no gas flow between the
inner and the outer divertor regions.
The upstream separatrix density n, has a dominant
effect on divertor performance. With increasing 2", thepeak heat flux can be substantially reduced. However, n"
must be within the range consistent with high energy
confinement during H-mode. On the other hand, n, cano The divertor plasma is optimised in a partiallydetached condition.

First, optimisation of divertor geometry has been
extensively performed. It is found that the enhancementof neutral accumulation in the divertor region, inparticular, neal the separatrix region of the outer target
plate, is essential for the reduction of peak heat load.
Long divertor length, vertically inclined divertor plateand installation of the divertor dome are effective forthis purpose and have been employed from the initial
phase of ITER design as a basic divertor structure.
Later, more sophisticated refinements of the geometry
are performed after detailed numerical investigations.
The first refinement is a modification of the bottom partof the divertor near the strike point of the outer platefrom a straight-shape to a V-shape configuration asshown in Fig. 2. With this refinement, it is expected thatneutral particles accumulate more near the separatrix
and accelerate the partial detachment of plasma in thisregion, where the heat load is highest. JET demonstrated
this effect experimentally [5]. In fact, the divertor code
calculation for ITER shows that peak heat load can be
reduced by about 30 Vo with this refinement. The second
reflnement is a modiflcation of the support structure of

the divertor dome to permit neutral gas circulation
between the inner and outer divertor. It is widely

be controlled by gas-puffing and/or core fuelling(throughput l-or) to some extent. The controllable rangeof n, is actually rather narrow (0.27 < n.(1020) < 0.32 for

70 ( lp1(Pa.m3/s; < 200 and Ps6r(power acrossseParatrix)=86 MW). With increasing Psor, ,. increases
while the controllable range is relatively unchanged.

Based on the detailed examinations andoptimisations for divertor performance described above,
representative scenarios for inductive and steady state
operation of ITER are investigated [8]. Major

parameters for the reference operation modes are
summarised in Table 2. For the reference inductive
operation mode, the peak heat load 401 decreases froml0 MW/m2 to 4 MW/m2 when the DT particle

Sugihara M., Divertor Requirements and Performance in ITER

Table 2 Major parameters for inductive and steady-state operation scenarios in ITER.
Steady-state operation

Inductive operation
Plasma major /minor radius (m)6.2 t2

6.212

Elongation rg, / triangularity 4,

1.7 I 0.33

1.7 I 0.33

Plasma current (MA) / Toroidal field (T)15 i 5.3

10 / 5.3

Safety factor q"u3

4.5

Fusion power (MW)

410

340

Total heating power Pt",", (MW)

123

128

Power across separatrix PsoL (MW)

86

100

Q value10

5.7

Average density fr" (1920.-s1

1.01

= 0.7

throughput I-p1 is changed from 70 to 180 Pa.m3/s

10(correspondingly n, is changed from O.27 to 0.32 x 1020
m-3, which is within the requirement) for fixed pumpingcpedestal I :9'9=28Pam3/s(56)
density in ITER--"r".or;'ttP^ tlt
Z

E

Required-oB

speed (20 m3/s1. The helium concentration Cg" also
decreases with this increase of .|-p1 from 5 to 2 Ea dlue totthe increased helium exhaust efficiency with increased
neutrai compression. Other series of calculation resultswith increased fusion power and pumping speed showthat the fusion power (helium source) and 1-p1 dominate
the helium concentration, while the pumping speed is

//It

P6
o

i'"'

rc =14Pam3/s

ht

t

o

o

"ot" (28)D=0.06 m2 /s

34
o
|l,

tto

less important.

o-

For the reference steady state operation mode,

o=o.u
2

operation density must be reduced, which increases thepeak heat load. On the other hand, connection length
becomes longer due to decreased plasma current orincreased 4e5, which compensates the reduction ofdensity to some extent. Calculation results show that
4pr=10 MWm2 is reached at ns= 0.26 x 1020 m-3, which^'{( ):pedesrat fueilang

2345
Separatrix density n{10le m'3 )""0

Fig. 3 Pedestal density evaluated for diffusion coefficients in the pedestal region E0.3 and 0.06m2/s
and for different core fuelling f3"*=14 and28Pa.m3/s. The corresponding required pedestal
fuellings are shown in parentheses.

is slightly higher than the requirement of nt3=0.23x1020
m-3. In spite of relatively low n,, helium concentration
stays low due to lower fusion power. To further reduceof neutral particles can penetrate inside the pedestal
region due to narrow SOL thickness. However, if only

the peak heat load within the requirement of n,, initialstudy of impurity seeding is performed. With 0.4 Vo ofneon seeding, =30 Vo reduction of qo1 is achieved while
increase of effective charge AZ"1s=0.4 (total Zerf1.6) is

gas-puffing is used, high pedestal density (zo"a=1020m-3)
cannot be achieved even if the diffusion coefficient is

neo-classical level (0.06 m2ls;, since only a small
fraction of gas-puffed neutrals can penetrate across theseparatrix due to thick SOL in ITER. This indicates amarginally acceptable.

4. Further model development needed andremaining uncertainty

need for core fuelling. Relation between the separatrix

Although extensive model validations have been
done for the B2lEirene code, there still remains some

expressed asdensity and the pedestal density can be approximately

hped= ns + r:or"Awd / SD

uncertainty, and further model development is necessary(1)

in some areas.
For present devices, the appropriate density
pedestal can be reproduced by the code with thewhere f$.", /p"a and S are core fuelling rate, pedestal
width and plasma surface area, respectively. Here, theassumed diffusion coefficient, since significant amountshows the achievable pedestal density for the present

particle pinch term is omitted for simplicity. Fig. 3

72

Sugihara M., Divertor Requirements and Performance in ITER

Table 3 Allowable energy density during ELMs and the energy loss fraction withrespect to pedestal stored energy for CFC and tungsten divertor plate in
ITER. 106 ELM events and 100MJ of pedestal stored energy are assumed.

cFc

W

0.55

0.72

3.4 - 8.6

4.4 - 11

Allowable AWetul&r* (MJ / m'z)

for 106 ELM events* (=1000 shots)
Allowable AWEalllWp.a (%) for 106 ELM events withdeposition area QLr = 2 x S"" = 6 - 15m2
W""a = 100MJ

divertor model calculation (solid line), reduced diffusion

uncertainty in Sp1y. First, .ls1y is widened during ELMs

coefficient with core fuelling rate of 14 Pa.m3ls (dashedline) and 28 Pa.m3ls (dotted line), respectively. /0"6 isassumed to be lOcm [9]. Here, core fuelling denotes theparticle injection inside the transport barrier region. Injection of particles within the pedestal region (pedestal

from its width in-between ELMs S,,, roughly by a factorof 2. When S,, is estimated from the divertor code results, the area is rather wide due to the partial detachedcondition and is evaluated as =7.5 m2. On the other
hand, if we assume the width of heat flow becomesmuch narrower during ELMs due to burn-through, thewidth can be estimated as =5 mm mapped on the outer
midplane, which provides S""=l m2. From these assessments, the range of S61y is expected to be 6-15 m2.
Consequently, criteria for the allowable energy density
AW1yylSs,ya and allowable energy loss fraction to thepedestal stored energy AWil-MlWr"6 for CFC (2 cm
thick) and a tungsten (1cm thick) divertor plate, which

fuelling) can also achieve the pedestal density required
for ITER, while, in this case, the efficiency of density
increase is lower compared with core fuelling. A roughestimation of the fuelling for this case is shown in thebracket. High field side pellet injection (50-100 Pa.m3/s
and deposition depth=0.15a with 500 m/s) is foreseen
for ITER so that these core fuelling requirements can be
met. Divertor performance is dominantly determined byz. and power flow across the separatrix. Therefore, thedivertor solutions presented in this paper will be un-

can withstand 106 ELM events (=1000 shots). aresummarised in Table 3.

So far, several models have been proposed forchanged, even when the proper pedestal model is introduced, as long as n, is kept in a similar range.

summarising the experimental data AWs,ylWr"a. Hete

we will briefly review three representative models (i)collisionality y* model t101, (ii) parallel transport time
q1 model [1] and sheath model [2] as well as theiruncertainties. Common feature of all of the models is

Other uncertainties remaining in this divertor model
are the assumed transport coefficients in the SOL region

and an acceptable range of separatrix density for good
H-mode confinement. Further database archive and data
examinations in these issues are being carried out as partthat the energy loss is closely correlated with theof the ITPA (International Tokamak Physics Activity)by the Divertor/SOl physics Topical Group.

pedestal pressure (stored energy).

The y* model is based on the fact that AWs-ylWp"ais simply correlated with y* measured at the pedestal
density and temperature over wide range of vx (twoorders of magnitude). The fitting expression is written as

5. ELM effects and mitigation
High pedestal pressure required for good H-mode
confinement can result in large divertor erosion due toAWur*lwwd= 0.064(vx)-03'

Type-I ELMs, which is the reference ITER operation
mode for the Q>lO inductive scenario. Criteria for the

Although correlation is fairly good, the underlying physics basis must be clarified to extrapolate to ITER. One
possible mechanism is that the MHD amplitude associated with ELMs is reduced with increasing density (orallowable energy loss due to ELMs can be expressed as

AW..u/(S"rr,/Gi" )

(:)

(2)

which is a measure for surface temperature rise of thetarget. Here AWur*, Ssu'r, ?rlrra are energy loss, deposition area and duration time for ELMs, respectively. In
JET, t"t =290 ps has been observed and it is rather independent of density and triangularity. There is large

collisionality), which is actually observed in JET [13].
However, it is still unclear whether this reduction is attributed to the reduction of MHD amplitude itself or an
increase of mode number (increased decay of MHD signal at pick-up coils). Also the mechanism as to how this73

Sugihara M., Divertor Requirements and Performance in ITER

MHD amplitude relates to the actual kinetic energy loss

From the examinations described above, it isfrom the pedestal must be identified. A significant loss
fraction AWs-ylWp"F(15-20) 7o is predicted for ITER
(v*=0.04) when this model is applied.
The q; model is based on the assumed physics
mechanism in that the pedestal region is connected tothe divertor due to ergodization and the parallel energy

concluded that all models are not complete and stillneed much more work. Although the range of model

predictions and uncertainties are very large, ITER
predictions are summarised in Fig. 4 when these models
are simply applied to ITER. Solid lines show theallowable energy loss fraction for CFC and tungsten
plate (Sur"=lQ m2 is assumed) and dotted lines show thepredicted loss fraction for each ofthese models. Dashedlines show the allowable energy loss fraction for thecase of further inclined divertor plate (poloidal angle istransport along field line (characterised by !1) together
with the ELM duration time ?8u,,{ determine the eventual
energy loss from the pedestal. Experimental data seemto be well summarised by the q; model for fixed

halved from the present valr'te 22.8" to 11.4").
Preliminary studies for engineering feasibility(alignment and assembly) show that this further
inclination would be possible. Possible physicaldisadvantageous effects on the increased particle
recycling on the upper part of the divertor plate, and?su'a(=200ps). The fitting expression is written as

AW.r*lW*= | H-

(4)

, 9ELM

Actually, however, ?s1M is different for individual machines, i.e., =20O ps in JET, =300 ps in DIII-D, =400 Fs
in ASDEX-U. When these values of ?Bp1,a are used formodification of plasma profile around the X-point are

preliminarily examined by the B2lEirene code.
Calculation results show no significant increase offitting the experimental data, the correlation becomesmuch worse (RMSE=0.041 compared with 0.03 with
fixed z"rr=290 ps). A simple application of this model

particle recycling and no noticeable change of plasma
profile. Another possible issue of this further inclinationis that the flexibility of position control of the separatrix
will be somewhat limited. This will be acceptable oncethe operation mode is fixed for engineering testing
purposes. In fact, the life time becomes a more
important issue for this testing phase. Use of a tungsten
divertor plate will also be possible during this phase,since the disruption probability should be low and themelting of tungsten plate due to the disruption heat load

to ITER predicts a(12-15) Vo energy loss fraction.

The sheath model is based on the energy flux
through the sheath of the divertor plate assuming that
the SOL is filled with pedestal plasma parameters. The
expression is written as

AW"r*lWFo* yn*c"(T*)Tr6(5)

where Tand cs are the heat transmission coefficient andsound velocity, respectively. By nature, this model will
provide the maximum available (upper limit) heat flux

through the sheath when a normal sheath is formed.
0.3

When compared with experimental results, the generaltrend is well reproduced, while the model provides
higher values for ASDEX-U and DIII-D, while fairly

Allowable- -close values to the JET data. Application of this model
to ITER predicts =5 Vo for the energy loss fraction. An
unresolved issue of this model is that the physics picture^si;;-irnd'---- pffi,'."

Furtherinclined

o o'2
'\

=E

.---------q-.--\___ \

J

is somewhat contradicting the following experimentalU=<

observations on tB1ylt11. When this ratio is larger thanunity, the SOL is filled fairly quickly and the sheath expression is expected to be a good approximation, whilein the opposite ease (tsyylcl1<l), the model should overestimate the heat flux. When rslla=300 ps (DIII-D) andr,, model"

0.1\
sheath model

0L

r00
r00

rwu=4}D ps (ASDEX-U) are assumed, ts,ylt11>l fot
these machines, and, thus, the model is expected to be a

200

300

-Fusion Power (ltlW)

400

500

Fig. 4 Alfowable energy loss fraction AWsou,tlWe.a for CFC
and tungsten (solid lines). Allowable values when
the divertor plate is further inclined are also plot-

good approximation, while actually, it over-estimates
the data. In the JET case, where TELMltl<l for cutr=2gg
ps, the model is expected to over-estimate, while, actually, it predicts values close to the experimental data.

ted (dashed lines). Values predicted by various
models are plotted by dotted lines.
74

Sugihara M., Divertor Requirements and Performance in ITER

can be limited.

which is required for good confinement, while present
predictions by proposed models are still primitive andthe uncertainty of the database is large. Thus, further
development and improvement of the models as well as

Another possible method to avoid the Type I ELM
heat load is to employ the discharge regime.of small

ELMs with high pedestal pressure. This regime isobserved in nlost divertor machines [14-18] and is

the database are necessary.

conveniently categorised as Type II ELM, though thebasic physical features may not be the same. Dominantparameters to achieve this Type II ELM regime are 4e5
and d,, and their ranges are typically 4s5>3.5 - 0.4 and
4>0.4 - 0.5. The present ITER configuration (4=0.5)
satisfies the requirement for 6,. Although the plasma
current is 15 MA (qgs=3) in the reference ITER

Possible mitigation methods for Type-I ELM
effects are ; further inclination of the divertor target
plate and Type-II ELMs discharge regime. Hybrid andsteady-state scenarios can be operated with the Type-II
ELM regime with 4e5>3.5. Further exploration to extend
this regime to high Q inductive operation mode with
egs=3 or further improved confinement associated withinductive operation mode to achieve O=10, theoperation scenario with reduced plasma current at 13
MA (4e5-3.5) is also possible to achieve Q=10 bylower density or qs5>3.5 should be promoted.

Acknowledgements

reducing density (n/no=Q.71 and fusion power (=250MW). In this case, however, the operation window issignificantly reduced. Ifhigher confinement is achieved

The author is grateful to Drs. A. Kukushkin and A.
Loarte for valuable comments and discussions. The
author also expresses his thanks to the members ofPhysics Unit of ITER International Team. This report
was prepared as an account of work undertaken withinfor higher qss andlor lower density, the window
becomes substantially wider. Actually, a higher HH
factor has been observed with lower density for many
machines and rvith higher 4nr(=3.5) with high density (Zlno=1) in some machines [19]. Hybrid and steady state
scenarios in ITER can be operated with higher qss(>3.5),
which facilitates the access to this small ELM Type II
regime. Further R&D is needed to extend this small

the framework of ITER Co-ordinated Technical
Activities (CTA). These are conducted by theParticipants: Canada, the European Atomic Energy
Community, Japan, and the Russian Federation, under
the auspices of the International Atomic Energy Agency.

The views and opinions expressed herein do not

ELM regime rvith high confinement to the referencehigh Q inductive operation mode of ITER. Type II

necessarily reflect those of the Participants to the CTA,
the IAEA or any agency thereof. Dissemination of theinformation in this paper is governed by the applicableterms of the former ITER-EDA Asreement.

ELMs in-between Type I ELMs observed in JET (qrr=l
and 8=9.5; could be a clue for the R&D [l3].

6. Gonclusions

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76