Control- oriented SXR simulation on Tore Supra: present status and perspectives

Control-oriented SXR simulation on Tore Supra: present status

and perspectivesD. VEZINET, D. MAZON, R. GUIRLET, D. CLAYTON, M. O’MULLANE

7th Workshop on Fusion Data Processing Validation and AnalysisMarch 26 28, 2012‐

ENEA Frascati

Tuesday 27th, March 2012 Didier VEZINET

Means:

Objective :

Integrated SXR diagnostic


Give a fast estimation of the total density distribution of each impurity in the plasma

SXR diagnostics: (spatial resolution ≈ 5cm)• Poloidal tomography :

(82 lines of sight, currently available, absolutely calibrated)

• Toroidal pinhole camera:128 pixels, potentially energy-resolved (prototype under development, more pixels in near future)

Simplified SXR simulation tool (presented here)



• State of the art : Common issues on most Tokamaks regarding impurity studies

• Some preliminary observations:The influence of transport on SXR emissivity and impurity density

profiles

• General framework of a new approach for impurity density estimation:General framework => domain of validity ?Description of a simplified approach in which impurities are

known• Example of application :

Ni trace injection on Tore Supra => testing the domain of validity of the general

framework

• Atomic data validation / comparison

• Remaining issues and solutions envisaged:Several impurities => identification and density estimation using

the GEM ?• Conclusion ans perspectives



State of the art of some issues on most tokamaks:

• SXR data is integrated spectrally (but is usually spatially localized thanks to tomographic reconstructions)

• Spectrospic measurements are integrated spatially (but have good spectral resolution)

• Transport coefficients for simulations have to be fitted using minimisation algorithms that cannot always guarantee the solution is not a local minimum



Some observations:In some configurations, transport may not play a very important role as far the total impurity density and the total emissivity profiles are concerned…

Courtesy of Daniel Clayton*Simulation run on STRAHL, with CHIANTI atomic data, stationary state after Fe injection and no exchange at the boundary

* D. Clayton, Johns Hopkins University, Baltimore, MD, USA



Some observations:In some configurations, transport may not play a very important role as far the total impurity density and the total emissivity profiles are concerned…

* M. O’Mullane, University of Strathclyde, Glasgow UK

Courtesy of Martin O’Mullane*ITER-like Simulation run on SANCO, with ADAS atomic data, stationary state with Ni constant influx and a recycling coefficient, with flat ne profileSlide 13/22 of CDR, ITER, 21st, February 2012



General Framework of a new appraoch:These are preliminary but encouraging results !

Further studies are needed, with experimental data and realistic simulations, to get a more accurate idea of the overall influence of transport on the total density and emissivity profiles.

In the yet-to-be-determined domain of validity of this assumption (transport only has limited influence), a simplified approach, without transport (D = 0, V = 0) can be developped to get an estimation of the total density profile.

The idea is not to assume that there is no transport ! But to assume that transport only has a limited impact on the total density and emissivity (in some cases)!



Data verification : atomic dataFractional abundances vs Te for typical Tokamak ne (D = 0, V = 0)

Cumulated absolute differences for Fe

Region of worst agreement Region of best agreement

Interval of interest for SXR in Tokamaks

ADAS = atomic database for plasmas in general (with some work done on W)CHIANTI = atomic database for astrophysics (spectral analysis)



Simplified approach (impurities are known):ADAS allows for simple computation of the radiated SXR, all the complexity of atomic physic is contained in the coefficients:

Te and ne are locally measured (ECE and reflectometry for example)D and V are set to zero in our simplified approach

If we assume a pure hydrogenic plasma with only one known impurity :

Spe Zee

ZSpeee

ZSpeSpee nTKnTfnnXI ,,

eeZSpeSpeee

ZSpe nTfnnTn ,,,0,0

Spe Z

eeZSpe

eebbeefbeeffeeZSpee

nTK

ZElnTKZElnTKZElnTKnTVDnnXI

,

,,,,,,,,,,,,

XTXn ee

eeZHH

Zee

ZSpeee

ZSpeSpee nTKnnTKnTfnnXI ,,,

Quasi-neutrality of the plasma => Z

ZSpeSpeHe Zfnnn



ADAS allows for simple computation of the radiated SXR, all the complexity of atomic physic is contained in the coefficients : Requires an absolutely

calibrated SXR tomography diagnostic !

(done on TS)

Z

eeZSpeee

ZH

Zee

ZSpeee

ZSpe

eeZHe

eSpe

nTZfnTKnTKnTf

nTKnnXI

Xn,,,,

,(*)

Case of trace impurity injection : * the background emissivity can be subtracted* the assumption that there is only one species becomes valid

Z

eeZSpeee

ZSpe

eSpe

nTKnTf

nXI

n,,

Simplified approach (impurities are known):



Application: Ni trace injection in Tore Supra (TS # 40801)

• D and V assumed to be stationary, equal for all ionisation stages• D and V profiles were found iteratively (genetic algorithm) to best fit experimental data (assuming

poloidal symetry). • Experimental inputs : Te (ECE) & ne (reflectometry)• Experimental constraints : SXR (line-integrated, relative constraint), Prad (bolometry, absolute

constraint), spectroscopy, ...

Impurity transport studied in detail (*)

Solution found for (D,V)

(*) D. Villegas, R. Guirlet et al., Physical Review Letters, vol. 105, no. 3, Jul. 2010.




Application: Ni trace injection

(*) D. Villegas, R. Guirlet et al., Physical Review Letters, vol. 105, no. 3, Jul. 2010.

Simplified approachSimulation / 2,5

(calibration bolometry ≠ SXR)

Normalised brightness

Normalised error estimation (nNi)

Same order of magnitude (ratio 2,5)Similar total density profile (shape)

Scattering = uncertainty of the tomographic reconstruction

MeasurementsSimplified approach

(D,V) = (0,0)Simulation / 2,5

General framework: more relevant some time after the injection (estimated error decreases with t)


N

rho

NiVD

Ni nnN

Err1

20,0,1



Application: Ni trace injection (*)

XI

Xne XTe

(*) XnNi

With poloidal symetry assumption

(*) D. Villegas et al., Physical Review Letters, vol. 105, no. 3, Jul. 2010.

In this simplified case, a SXR tomographic reconstruction yields a poloidal mapping of the total density of the impurity considered




Remaining issues to be adressed

• With n impurities:

If several impurities have to be considered, they have to be identified (VUV spectroscopy, ...)

eeZHe

eeeEEeeElEl nTKn

nXInTAnnTAn ,,..., lnln11

1 equationn unknowns

Can be solved with a means of determining additional constrains / equations (ratio between each impurity…)

Limits of the simplified approach :

• (Domain of validity has be determined)

GEM ?



Solution envisaged : The GEM-based pinhole cameraA pinhole SXR camera with 128 pixels viewing the plasma in a toroidal direction.

Advantages:• Toroidal view

=> additional geometrical constraint: better tomographic inversion

=> Prospective 3D tomographic inversions

=> Object of significant research efforts in Europe: evolutions to come (more pixels, …)• Energy resolution

=> electronic discrimination threshold (only 1 on our prototype)

=> Impurity identification ?



Use GEM’s energy resolution to identify impurities, by concentrating on expected line emissions, in each pixel of the tomographic inversion

El Zebbefbeff

ZEle ZXTKZXTKZXTKnnI ,,,,,,

On every expected line : fbffbbZXe IIELiKnnI ELi

ELi

ELiKnIII

nbbe

fbffZXELi

ELi

No assumption on

transport !

E (eV)

I

Expected line 1

Ifb

Expected line 2

Requires sufficient energy resolution* and absolute calibration W = 8 %Ni = 4 %

* Depends on plasma composition < 10 % would be a good compromise≈ 15 % for today’s prototype => evolutions to come

SXR lines in a plasma with Fe, Ni, Cu and W

Solution envisaged : The GEM-based pinhole camera



Conclusion and perspectivesThe domain of validity of the assumption that transport has little influence on the total SXR emissivity and on the total impurity density still has to be investigated. Preliminary but encouraging results have been obtained. ADAS was chosen as the reference database for atomic data

Additional information is necessary when several impurities are present, the GEM’s energy resolution could be an answer (+ VUV spectroscopy ?). The GEM’s energy resolution may have to be refined.The GEM is evolutive: finer spatial and temporal resolution is achievable.

The simultaneous use of GEM + DTOMOX will allow either for finer tomographic reconstructions (with knowledge of the toroidal symetry) or for prospective 3D reconstructions.

Documents

Control- oriented SXR simulation on Tore Supra: present status and perspectives