EMC3-EIRENE modelling

of toroidally-localized

divertor gas injection

experiments on Alcator

C-Mod

J.D. Lore1, M.L. Reinke2, B. LaBombard2, B. Lipschultz2, R.M Churchill3, R. Pitts4, Y. Feng5

1Oak Ridge National Laboratory, Oak Ridge TN, USA 2York Plasma Institute, University of York, UK

3Plasma Science and Fusion Center, MIT, Cambridge MA, USA 4ITER Organization, St Paul Lez Durance, France 5Max Planck Institute for Plasma Physics, Greifswald, Germany

21st PSI, Kanazawa Japan. May. 26-30, 2014

ITER requires partially detached divertor

plasmas

• Future reactors such as ITER must operate with partially detached divertors to reduce peak heat fluxes

• During burning plasma operation ITER will use extrinsic impurity seeding via a set of toroidally spaced divertor gas injectors (N2, Ne, Ar)

– Toroidally localized injection may lead to asymmetry in radiated power, detachment, heat flux

• Experiments were run on Alcator C-Mod at request of IO to investigate potential asymmetry

– Clear toroidal variation in radiated power, impurity line emission, divertor conditions measured with a single divertor puff

– Experiments led to increasing the number of injection locations from 3 to 6 in ITER

• 3D plasma/neutral transport code EMC3-EIRENE has been used to model these experiments

– Need to validate 3D models to perform predictive simulations for future experiments

2

Gas injection locations

ITER

21st PSI, Kanazawa Japan. May. 26-30, 2014

C-Mod Experiments were Performed with

Toroidally Localized Divertor Gas Injection

• Set of 10 reproducible discharges: 1090814(006-016)

– Ohmic L-Mode, Ip=1MA, Bt=5.4T, ne~1.1e20 m-3, q95~3.75

– Divertor is in the high recycling regime

• N2 injected into divertor at ~0.9s through a single valve each shot gas location shifts relative to diagnostics

– Gas location analogous to ITER conditions

3

1090814005 1090814006 1090814011

Puff Location

Details available in Reinke, et al.,

PSFC/RR-14-3

21st PSI, Kanazawa Japan. May. 26-30, 2014

Reproducible toroidal asymmetry is

measured in edge and divertor diagnostics

• Experiments are well diagnosed, with many divertor and SOL views to constrain and validate modeling

• Toroidal modulation measured in nitrogen line emission, Prad, and divertor electron pressure

4

Probes

Details available in Reinke, et al.,

PSFC/RR-14-3

21st PSI, Kanazawa Japan. May. 26-30, 2014

• The EMC3-Eirene code1

– 3D fluid plasma model (EMC3) coupled to kinetic neutral transport and PSI (EIRENE)

– Classical parallel transport with prescribed anomalous cross-field diffusivities

– Trace fluid impurity model (Ta=Ti,naZa<<ni) with feedback to main plasma through electron energy loss

– Outputs: 3D neutral and fluid plasma quantities, surface loads on to PFCs

– No cross-field drifts or kinetic corrections

• Simulation of N puff experiments

– High resolution, full toroidal grid with single N0 puff in divertor

– Inputs: Pcore=1.25MW, ncore=1e20m-3, constant cross-field diffusivities, N0 strength from puff calibration, Rimp=0.5

– PFR is largely transparent to N0, ionization occurs near separatrix

– Impurity radiation largest in flux tubes connecting to the divertor near the outer strike point

5 [1] Feng, J. Nucl. Mater. 266-269 (1999) 812

Puff Location

Te (eV)

Experiments are modeled using the

3D EMC3-Eirene code

21st PSI, Kanazawa Japan. May. 26-30, 2014

Matching upstream profiles results in colder,

denser divertor plasma than experiment

• Spatially constant cross-field diffusivities chosen to approximately match upstream profiles

– χ⊥= 1 m2/s, D⊥ =0.3 m2/s.

• Downstream pressure match within ~2x, however simulated divertor conditions have low Te, high ne

– Similar result found in 2D simulations, attributed to neglected kinetic effects

– EMC3-EIRENE has no kinetic corrections, improvements needed in code

– Future modeling will attempt to match downstream conditions by reducing upstream density

6

upstream

Toroidally averaged profiles

21st PSI, Kanazawa Japan. May. 26-30, 2014

Impurity puff results toroidal modulation in divertor

pressure near strike point

• Repeatable modulation in electron pressure near outer strike point measured in experiment

• Toroidal trend qualitatively reproduced by simulation

– Pressure changes caused by toroidal variation in momentum sinks (ionization and charge exchange) upstream caused by local reduction in electron temperature

– Localized electron cooling due to localized nitrogen radiation in flux tubes connected to target near strike point

Toroidally averaged

profiles Pressure modulation with

puff location Toroidal modulation

qualitatively matched by

EMC3

21st PSI, Kanazawa Japan. May. 26-30, 2014

Parallel impurity flows dominated by

main ion friction Ti gradient

8

iZieZez

Zi

zizz

z

TTEeZVV

mpn

||||||||

)(10

Impurity

pressure

gradient

Main ion

friction

Electro-

static Electron

Temp.

Gradient

Ion

Temp.

Gradient

i

e

iiz T

n

TVV ||

2/3

~

• Dominant forces are friction and ion temperature gradient, and independent of Z

– Divertor temperature is low, friction dominates in PFR

– Non-recycling neutrals are trapped in PFR

• Experiments have shown evidence of drifts in SOL and PFR, not accounted for in this model

21st PSI, Kanazawa Japan. May. 26-30, 2014

Trends in N line emission captured near x-point, PFR

may require cross-field drifts

9 [1] Smick et al., Nucl. Fusion 53 (2013) 023001.

[2] Boswell et al., J. Nucl. Mater. 290-293 (2001) 556

• Clear toroidal asymmetry in NV emission in chords viewing near x-point

• Above x-point temperature gradient force pushes impurities upstream into view, results in inverted profile

• View through x-point peaked at puff location, main ion friction dominates, pulls impurities out of view

• Toroidal behavior seems to be well described by parallel impurity forces in SOL near x-point

• Below x-point: Friction dominates in model, results in in downstream peaking. Experiment peaked at puff loc.

– Cross-field drifts may be required to capture impurity behavior in PFR, experiments have demonstrated importance [1,2]

21st PSI, Kanazawa Japan. May. 26-30, 2014

Toroidally asymmetry in target heat flux

predicted near outer strike point

• Impurity radiation results in net reduction in power carried by plasma to targets

– reduced from 930kW to 730kW (Pin=1.25MW)

• Toroidal asymmetry in Prad results in toroidal asymmetry in heat flux near outer strike point

– Toroidal extent will depend on machine size, divertor geometry

10

No puff With puff

plasmaPtarg

Q (MW/m2) Q (MW/m2)

ρ (c

m)

ρ (c

m)

21st PSI, Kanazawa Japan. May. 26-30, 2014

Summary

• C-Mod experiments were performed to assess toroidal asymmetry caused by local divertor impurity injection for ITER

• The 3D edge transport code EMC3-Eirene has been applied to model these experiments

• Measured net reduction and toroidal variation in divertor pressure at OSP are qualitatively reproduced

• Modeled toroidal asymmetry in NV emission near x-point have similar trends as experiment, however cross-field drifts are likely required to match behavior in PFR

• Toroidal asymmetry in target heat flux is predicted with a single divertor injection location

• Experiments proposed to investigate effect in H-Mode with IR camera data

11

21st PSI, Kanazawa Japan. May. 26-30, 2014

Impurity ionization in PFR

• Gas injection is deep in divertor

• Plasma is nearly transparent to neutrals, ionization occurs near separatrix where Te>10eV

12

21st PSI, Kanazawa Japan. May. 26-30, 2014

Comparison to ledge bolometers

• Similar trends in LBOLO, large discrepancies in DBOLO

– Strike point position is critical for DBOLO

13

21st PSI, Kanazawa Japan. May. 26-30, 2014

The EMC3-Eirene1 Code

14

piii SnDVn bII

Sources from plasma-neutral interactions provided by Eirene

Trace impurity model

miiiiiiiii SpVnDVmVVVnm IIIIIIIIIIIIIIII b

eiieiiiiiiiiii

impeeieeeeeeeeiee

STTkTnnDTTVTn

SSTTkTnnDTTVTn

25

25

25

25

IIII

IIII

b

b

• Monte-Carlo fluid plasma model (EMC3)

• Fully 3D geometry for plasma, divertor, PFCs

• Arbitrary 3D B field model can be implemented

• Self-consistent coupling of fluid ions and electrons, kinetic neutral transport and PSI (EIRENE)

• Classical parallel transport (η||, κe, κi)

• Prescribed anomalous cross-field diffusivities: Da, χe, χi, η⊥= miD

• Trace impurity model

• No drifts or kinetic corrections

[1] Feng, J. Nucl. Mater. (1999)