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Modeling of EAST DivertorModeling of EAST Divertor

S. Zhu  

Institute of Plasma Physics, Chinese Academy of Sciences

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1. Introduction1. IntroductionEAST is a superconducting tokamak under construction

and will be performed in1. shaped cross section2. with Double Null and Single Null divertor configurations

The scientific and engineering missions of EAST areto explore the reactor relevant regimes of: 1. long pulse lengths2. high plasma core confinementand to establish technology basis of full superconductingtokamak for future reactor

The object of its divertor isto develop and verify solutions for power exhaust and particle controlin steady operational state.

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Nominal Upgrade

BT0 (T) 3.5 4.0

Ip (MA) 1 1.5

ICRH (MW) 3 6

LHCD (MW) 3.5 8

ECRH (MW) 0.5 1.5

NBI (MW) 0 8

Main parameters in the different phases of the operation

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The EAST divertor should be designed to accommodate (in the I & II phase) :

1. total power load of 7.5MW 2. long duration discharges

τpulse= 60 - 1000s

During the last decade, some expected benefits of a closed divertor have been confirmed by experiments.

To increase the “closure”, the EAST divertor :

is deep and consists of• vertical target • tightly fitting baffle• dome in private flux region

Structures of Divertor Structures of Divertor and internal componentsand internal components

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2.1 SOLPS Modeling1. Couples a multi-fluid plasma code B2 with a Monte-Carlo neutral code Eirene ( imported from IPP-Garching, David Coster, Andrei Kukushkin )

2. Simulations done for H/D + C (physical+Chemical sputtering)

3. Simple Recycling model / neon puffing + pumping R = 1 at all surfaces4. The anomalous perpendicular transport model: constant in space, with the thermal diffusivities χi⊥= χe⊥

5. Pi,cb = Pe,cb

2.2 Computational Mesh 122 poloidal 24 radial

2. Simulation Model2. Simulation Model

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Toroidal field, BT (T) 3.5

Plasma current, Ip (MA) 1.0

Major radius, R0 (m) 1.94

Minor radius, a (m) 0.46

Elongation at separatrix,κx 1.69

Upper triangularity at separatrix,δux

0.32

Lower triangularity at separatrix,δlx

0.54

   

Safety factor, q95 4.1

Plasma internal inductance, li 0.95

Poloidal beta, βp 1.40

   

Plasma volume, VP (m3) ~11.9

2.3 Major parameters

of the EAST SN

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Toroidal field, BT (T) 3.5

Plasma current, Ip (MA) 1.0

Major radius, R0 (m) 1.94

Minor radius, a (m) 0.47

Elongation at separatrix,κx 1.76

Upper triangularity at separatrix,δux

0.56

Lower triangularity at separatrix,δlx

0.56

   

Safety factor, q95 4.5

Plasma internal inductance, li 1.32

Poloidal beta, βp 1.58

   

Plasma volume, VP (m3) ~12.5

2.3 Major parameters

of the EAST CDN

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Computational & Physical Domain

SOL

Core

UpperDivertor

LowerDivertor

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3.1 Effect of the vertical targets

(1) Neutrals produced at the target plates are preferentially reflected towards the separatrix.

(2) Hence ionization is enhanced near the vicinity of the separatrix.

(a) neutral density and (b) ionization source (H+ ions m-3s-1)

3. Results of the SOLPS 3. Results of the SOLPS PredictionPrediction

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(3) As the power is mainly conducted through theregion close to the separatrix:

• the peak heat flux is reduced and the profile is broader.• electron density peaks more towards sep.• temperature profiles looks “inverted”.

Comparison of profiles across the target(a) vertical target (b) target normal to flux surface

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3.2 Effect of PFR baffle

A PFR baffle structure (dome) is introduced • to prevent neutrals from escaping back into the bulk and SOL plasma through the PFR • to increase the PFR neutral pressure, which favors divertor pumping

The gaps between the vertical targets and dome should be optimized, so that they can allow neutral to reach the divertor pumping system but impede their escape back into the plasma.

The result of optimization calculations performed for the EAST divertor shows there is an optimum width of the pumping gap for this

dome and vertical target configuration.

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3.3 Effect of divertor topology

The poloidal field coil system of EAST allows us to run in SN or DN magnetic configurations for more

flexibility in experiments.

Connected Double Null

Disconnected Double Null

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The heat flux sharing by the divertors will be strongly affected by the variation in the magnetic topology of the divertor.

Electron temperature (eV) and total parallel energy flux (W) contours in (a) SN and (b) CDN configurations

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0

1

1

2

2

3

3

4

4

5

1.0 1.5 2.0 2.5

ns (1019 m-3)

qpk

o(M

W m

-2)

Zs

3.0

hg20

05.a

sipp

.6b

SNCDN

The figure shows peak heat flux at the outer divertor plates and the Zeff at the separatrix as a function of the separatrix density for the SN and CDN divertor configuration.

As can be seen, both qpk0 and Z

s are reduced for the CDN configurations, as would be expected.

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As the configuration transitions from SN to CDN divertor, there exists configurations of disconnected double null DDN . In DDN, if the distance Δsep between both separatrices at the outer midplane is comparable to the SOL width of the parallel heat flux, a significant part of the heat flux can still flow along the outer separatrix to the second divertor.

(a) Electron temperature (eV) and (b) total parallel energy flux (W) contours in DDN configuration.

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Our modeling indicates that, for EAST, CDN has easier access to the detachment regime than SN.

This might be partialy due to the fact that in the CDN configuration, the separatrix strike points are closer to the target corners than in the SN, so that an effective "V-shaped target" is formed.

Such a configuration reduces the target loads because it helps to confine neutrals around the strike point and this facilitates partial plasma detachment. Particle neutral losses (ionization) contour at

CDN (upper) and SN (lower)

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3.3 Divertor Operational Windows of EAST

•Greenwald limit allows HT-7U to run safely with the line average densities up to 1.0×1020 m-3 in Ohmic discharges.•But LHCD efficiency requires much lower density.

3.3.1 Low recycling regime

•ne ， sep ~ 0.7×1019 m-3.

•The profiles show little drops along field lines.

•Temperature at target is high up to ~120 eV.

•The peak heat flux exceeds engineering constrain >5 MW/m2.

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3.3.2 High recycling regime

•Midplane separatrix density is ~1.4×1019 m-3.

•Significant gradients along field lines.

•High density and low temperature at target.

•Zeff has the ideal value of 1.4.

•But operational window is narrow.

Comparison of profiles at midplane and target(a) low recycling (b) high recycling

A high density and low temperature plasma exists close to the target

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3.3.3 Detachment regime• Transition to power detachment occurs at line average densities ~ 7.8×1019 m-3.• It is about 80% of the Greenwald limit and is much higher than the density limit required by the LHCD efficiency. • Consequently, additional approach such as gas puffing or impurity seeding should be adopted to attempt detachment.

Inner target•Te < 4 eV throughout most of the plate.•Te even < 2 eV at the separatrix.

Outer target•Te > 10 eV in the outer SOL.•Te already < 2 eV at the separatrix.

Shows:Complete detachment is attained in inner divertor; partial detachment attained in outer divertor; Detachment starts from the separatrix.

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• Low recycling regime: no flow reverse observed due to the lack of strong ionization there.

• High recycling regime: flow reverse occurs in both divertor. The reverse region is close to the separatrix.

• Detachment:flow reverse disappears at the inner target. Reverse region shrinks but not disappears at the outer target

Flow reverse v.s. Divertor Operational regimes

Mach number

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3.4 Extension of the operational space

• Consistency between the edge plasma and the core plasma operation is an important issue for the design of fusion devices. A simple Core-SOL-Divertor (C-S-D) model was developed to investigate qualitatively the overall features of the operational space for the integrated core and edge plasma.

( Cooperated with R. Hiwatari, A. Hatayama, S.Zhu, T.Takizuka and Y. Tomitaunder the JSPS-CAS Core University Program )

• This model was applied to assess the possibility of extending plasma operational space of LHCD experiments for EAST.

• By using the C-S-D model, it is revealed that gas puffing is an effective method to extend the operational space toward both lower Φ p and higher Qin region. • On the other hand, the upper boundary of Qin can be extended by the impurity seeding in the SOL-divertor region.

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4. Summary and Conclusions4. Summary and Conclusions

• In order to increase the degree of closure, the EAST divertor is designed to be deep and well baffled. Its vertical target plates preferentially reflect neutrals towards the separatrix and hence are beneficial to improve the power exhaust. The vertical divertor geometry also has effects on the detachment behavior.

• The heat flux sharing by the divertors will be strongly affected by the variation in the magnetic topology of the divertor. In DDN, if the distance Δsep between both separ

atrices at the outer midplane is comparable to the SOL width of the parallel heat flux, a significant part of the heat flux can still flow along the outer separatrix to the second divertor.

• Performing in the high recycling or detached divertor operating regimes is of particular importance for heat and particle control in steady state. To extend plasma operational space of EAST with LHCD or to attempt to produce detachment for the divertor plasma, additional approach such as gas puffing or impurity seeding should be adopted.