H. D. Pacher1, A. S. Kukushkin2, G. W. Pacher3, V. Kotov4, G. Janeschitz5, D. Reiter4, D. Coster6

1INRS-EMT, Varennes, Canada; 2ITER Organization, Cadarache, France; 3Hydro-Québec, Varennes, Canada; 4FZ Jülich, Germany;

5Forschungszentrum Karlsruhe, Germany; 6Max-Planck IPP, Garching, Germany

presented at PSI200818th Int. Conf. on Plasma-Surface Interactions

Toledo, SpainMay 2008

Impurity seeding and scaling of edge parameters in ITER

Outline

1. Results for nonlinear neutral model with carbon

Update scaling (PSI2006, IAEA 2006)

2. Edge/divertor simulation: Impurity-seeded carbon-free divertor

3. Core simulations: Effect of impurity seeding on ITER operating diagram

Conclusions

1

Edge/Divertor ModelB2-Eirene (SOLPS4.3)

Now routinely nonlinear neutral modelneutral-neutral collisionsD2 molecular kineticsParallelized

2 domes

2

1. Full carbon wallC sputtering: phys. + const. Ych

2. Carbon-free wall with neon: (wall same as 1. but no C erosion)

3. Variant: Full Be wall with neon:(=> with Ne small difference from 2., Be concentration small, Be radiation small)

Edge density limit Density analogue of from n scaling

Scaling Update3

Scaling results PSI2006:Key parameter is normalised pressure

0.4

0.60.8

1

3

5

7

0.2 0.4 0.6 0.8 1

352_c4_z2_5

JETMk2 16JETMk2 24ITER 86

ITER 100ITER 130Demo 200

Demo 400Demo 500fit

qpk norm.

μ

=-1.17m€

μ ≡pDT # P #−0.87 f f

−0.8 fw−1 q95 #

−0.27 fnn−1R#

-1.21

is at detachment of either divertorSame curve for qpk from

JET at 16 MWto DEMO at 500 MW(2006)€

μ =1

€

fsat_n ≡ μ 0.43 : f sat_n = 0.9 same as μ = 0.78

Previous was linear neutral model except for some points=> Update required.

€

μ

PowerDT pressureat PFR(Throughput)

Factors Size

DT flux - Scaling with μ and S4

With nonlinear neutral model

Both domes, both S: DT neutral influx to core:

linear model was:

i.e. stronger variation

Value at is 2.4 times that previously

Total influx is still small:

i.e gas puffing provides little core fuelling (opacity to neutrals)

56789

10

20

30

0.4 0.6 0.8 1

409_iPubB_z1_2

Γ _ _DT n sep [ -Pa m3s-1]

=0.7 m μ

F12 Se=60

F47 Se=56

F47 Se=28

fitfitfit

€

ΓDT _ n _ sep ~ S#0.5μ 0.7

€

ΓDT _ n _ sep ~ S#0.3μ 0.36

€

μ =1

€

ΓDT _ n _ sep < 15 − 25 Pa - m3s−1 for μ <1

Value at is about 1/3 previous (linear)

Helium small, rises less strongly toward lower pressures

helium does not constrain operation unless pumping reduced strongly or dome removed

linear model was:

0.1

0.2

0.3

0.4

0.5

0.4 0.6 0.8 1

409_iPubD_z1_4

Γ _ _He n sep [ -Pa m3s-1]

= -0.86 m μ

Helium - Scaling with μ and S5

0.001

0.002

0.003

0.004

0.0050.006

0.4 0.6 0.8 1

409_iPubD_z1_2

nHe_sep [1020m-3]

m= -1 μ

F12 Se=60 F47 Se=56 F47 Se=28 fit fit fit

With nonlinear neutral model

Helium : slight difference between domes"bump" at related to detachmentScaling:

€

nHe _ sep ~ S#−0.7μ -1 for μ < 0.65

€

ΓHe _ n _ sep ~ S#−0.7μ

−0.86 for μ < 0.65

€

~ S#−1μ -2

€

μ =1

€

μ =1

But:

Impurity radiation in inner divertor volume is smaller than with C:

CarbonNe 3%

Ne 2%Ne 1.5%

Ne 1%Ne 0.5%

Ne 0.25%Ne 0.1%

0

20

40

60

80

100

0.2 0.4 0.6 1 3

409_iPubNe3_z02_1

Prad [MW]

μ

0

5

10

15

20

25

30

35

40

0.2 0.4 0.6 1 3

409_iPubNe3_z02_2

Pimp_rad_divin [MW]

μ

Neon seeding without carbon - power6

With neon seeding:

Impurity radiation for

similar to but smaller than for Cvaries little with

(self-consistent carbon is):

€

0.01 < cNe < 0.03

€

cC ~ 0.025 − 0.03

€

2 /3 at μ =1, 1/3 at μ = 0.4

€

cNe

Ne - T7

With neon:

Temperature at inner target higher ( C chemical erosion,low Ne radiation at < 10 eV)

No additional factor in for detachment

With carbon-free and neon at inner target compared to C:plasma power higherradiation lower,total power a bit higherpower load higher (peaking)

1

10

0.2 0.4 0.6 1 3

409_iPubNe3_z03_3

Te_div_inner_max [eV]

μ

CarbonNe 3%Ne 2%Ne 1.5%

Ne 1%Ne 0.5%Ne 0.25%Ne 0.1%

€

μ

=> peak power load shifts from outer to inner target (see next)

1

10

0.2 0.4 0.6 1 3

409_iPubNe3_z05_3

qpk_inner [MW/m2]

μ

1

10

0.2 0.4 0.6 1 3

409_iPubNe3_z05_4

qpk_outer [MW/m2]

μ

Ne - peak power8

With neon:

Peak power shifts to inner target

1

10

0.2 0.4 0.6 1 3

409_iPubNe3_z05_6

qpk [MW/m2]

μ

€

inner larger μ <1

€

outer larger μ <1

Only points for which larger load is at divertor plotted below

€

for 0.01 < cNe < 0.03

€

qpk |Ne = 0.7qpk |C ~ μ−1.2

Peak powerhas same scaling but is 30% lower

than with C(but flux expansion and angle are

not same)

Ne - nDT and ne 9

The strongest effect of neon is:

As neon density increases, => DT density decreases strongly

CarbonNe 3%Ne 2%Ne 1.5%

Ne 1%Ne 0.5%Ne 0.25%Ne 0.1%

0.1

0.2

0.3

0.4

0.50.6

0.2 0.4 0.6 1 3

409_iPubNe3_z03_4

nDT_sep [1020m-3]

μ

0.1

0.2

0.3

0.4

0.50.6

0.2 0.4 0.6 1 3

409_iPubNe3_z03_6

nDT_sep+40*nNe_sep [1020m-3]

μ

0.001

0.01

0.2 0.4 0.6 1 3

409_iPubNe3D_z1_5

nNe_sep [1020m-3]

μ

=> ne decrease ~80% of nDT decrease over range

varies little=> explains why neon radiation varies little with concentration

€

nDT_sep |Ne + 40 • nNe_sep |Ne =1.45nDT_sep |C μ−0.34 for cNe ≤ 0.01

Factor 40 broadly consistent with ratio of ionisation energy ~100 for 8<Z<9 -

Less power available for DT recycling

€

ne • nNe

Ne - helium10

As neon increases to

relative to carbon

helium density at separatrix progressively decreases by 2.2

helium neutral influx to core progressively decreases by 7.5

CarbonNe 3%

Ne 2%Ne 1.5%

Ne 1%Ne 0.5%

Ne 0.25%Ne 0.1%

fit Cfit high cNe

€

cNe = 0.03

0.1

1

10

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409_iPubNe3_z13_2

nHe_sep norm.

m=-1 μ

0.1

1

0.2 0.4 0.6 1 3

409_iPubNe3_z13_3

Γ _ _He n sep . norm

=-0.86 m μ

Tentatively attribute to:lower DT and electron densities in divertor plasma

=> lower opacity of inner divertor plasma to neutrals=> more efficient pumping=> lower He densities and fluxes upstream

Details to be worked out

(He reduction stronger in inner divertor)

Core/edge model11

Core transport in Astra MMM, stabilised by ExB and magnetic shear,time-average ELMs, sawteeth, fitted to JET and AUG, also fits Sugihara pedestal scaling (EPS2008)

Because of the opacity of the ITER SOL, core fuelling controls mostly the core density, gas puffing controls mostly the edge (including peak power load via μ)

profiles, self-consistent pedestal width and height operating window

cf e.g IAEA2006, also paper submitted to Nucl. Fusion

linked to edge via scaling relations from B2-EIRENE

At constant alpha particle power has maximum as n increases

=>Low temperature limit of alphapower from fusion cross-section

Operating diagram12

€

Plane Paux (inverted) − ne

€

Paux

Peak power load set given value across window by varying gas puffing (throughput and

€

Plane Q − Palpha

€

≤

Operating diagram limits13

For qpk always within limit, a point lies within the operational window if for that point:

Max. attainable alpha power (roll-over of P with <ne>

min Q for ITER mission(5 for ITER)

Edge density limit (full detachment)

H-L transition

Available heating power (73MW)

€

fsat _ n = μ 0.43 < 0.9

Operation at qpk= 5 MW/m2; C vs. C-free+Ne14

If very low peak power load required:

With carbon:Excessive gas puff (up to 300 Pa-m3/s)

would be requiredbut density limit dominates,

=>limiting the actual throughput used=>limiting alpha power

With carbon-free and neon:Q is lower

less gas puff needed=> alpha power is higher

Operation at qpk= 10 MW/m2; C vs. C-free+Ne15

The operating window is smaller in both Q and alpha power with neon Core fuelling:a bit higher at the same alpha power with neon because of increased core radiation (lower T)Gas puffing:much lower with neon (60 vs 120 Pa-m3/s) since favourable scaling of peak power in previous

section demands less gas puffing in addition.

If moderate peak power load required:

Operating diagram summary16

Superposition shows:

For Ne relative to C:

low peak power:advantage with neon in alpha power,

disadvantage in Qsame throughput at limit

moderate peak power: disadvantage both in Q and alpha power throughput lower by a factor 2

(but lower S would do the samething)

edge density limit plays strong role with C, less so with neon

Conclusions17

1. Scaling from edge modelling with nonlinear neutral model updated:- DT neutral influx to core higher than previous but remains small

-> gas puffing ineffective for core fuelling- more benign helium scaling toward lower pressure

-> helium is not strong constraint for ITER unless pumping strongly reduced

2. Edge: carbon-free+neon relative to carbon:- radiated power similar, peak power load 30% lower, but with same scaling- peak power load shifted to inner divertor- helium density and helium influx to core even lower- DT density decreases as neon density increases because less power is available

for DT3. Operating window: carbon-free+neon relative to carbon:

- Operating window increases relative to carbon only when stringent low peak power is specified, which would require excessive gas puffing

- For moderate specified peak power loading:- neon reduces operating window- but also reduces throughput

(as does reducing pumping speed, with less impact)

=>Mutually consistent modelling of edge and core shows that for ITER operation carbon-free operation with neon seeding offers only limited advantage over carbon operation

Carbon-free operation at low seeded impurity levels needs to be examined further, as does W