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Searching for flux surface asymmetries in plasma potential in the TJ-II stellarator

Presented by Carlos Hidalgo

M. A. Pedrosa, C. Hidalgo, M.A. Ochando

and the TJ-II team

Laboratorio Nacional de Fusión, CIEMAT

Acknowledgments: P. Helander, A. Alonso, F. Castejón, J. M. García-Regaña, J. L. Velasco, J. Arévalo

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Motivation •  Power exhaust and impurity accumulation issues are

potentially show-stoppers for fusion both in tokamaks and stellarators.

•  Particularly crucial is the development and understanding of mitigation methods for core impurity accumulation –  i.e. Understanding the physics mechanisms of HDH (W7-AS) and

impurity holow (LHD) regimes

•  High charge of impurities leads to high sensitivity to poloidal variation of electrostatic potential. Then, clarifying its role on particle transport is an important open issue.

Searching for experimental evidence of flux surface

asymmetries in plasma potential in TJ-II

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Why poloidal / toroidal asymmetries in plasma potential?.

MAGNETIC TOPOLOGY EFFECTS:

asymmetries sustained by thermal particle losses

HEATING DRIVEN EFFECTS: asymmetries sustained by

magnetically trapped electrons (ECRH) or ions (ICRH).

Alcator Cmod / M. L. Reinke et al., PPCF 54 (2012) 045004 TJ-II / M A Ochando et al 2003 PPCF 45 (2003) 221

Three dimensional maps of collisional particle fluxes using Monte Carlo codes show strong poloidal /toroidal anisotropies in TJ-II F. Castejón et al., NF 49 (2009) 085019. Neoclassical impurity transport J. M. García-Regaña et al., Varenna 2012

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Searching for potential asymmetries: experimental set-up

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Edge plasma parameters were simultaneously characterized in two different poloidal / toroidal positions approximately using two similar multi-Langmuir probes: Probe 1 is located in a top window entering vertically through one of the “corners” of its beam-shaped plasma and at φ≈35º. Probe 2 is installed in a bottom window at φ≈195º and enters into the plasma through a region with a high density of flux surfaces (i.e. lower flux expansion) than Probe 1.

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Searching for potential asymmetries: How to label magnetic flux surfaces ?

0

0.2

0.4

0.6

0.8

0.3 0.4 0.5 0.6 0.7 0.8 0.9

max[γ

1,2(V

f)]

Line averaged density (1019

m-3

)

ECRH

Electron root

Ion root

Sheared flows

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Using the amplitude of Long Range Correlations (zonal flows).

Evidence of long-range correlations amplification during in the proximity of the Electron-Ion root transition [M. A. Pedrosa et al., PRL 2008 / J.L. Velasco et al., PRL-2012]

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-80

-40

0

40

80

-40 -30 -20 -10 0 10

Vfs Mean D-H (D) Vf D mean Max LRC timeVf D mean ne≈0.8 timeVf D mean ne≈0.4 time

Vf (

V)

z-zLCFS (mm)

-80

-40

0

40

80

-25 -20 -15 -10 -5 0 5

Vfs Mean D-H (B) Vf D mean Max LRC timeVf D mean ne≈0.8 timeVf D mean ne≈0.4 time

Vf (

V)

z-zLCFS (mm)

Deuterio  D/H≥2  

Probe  B  

Max  LRC  

region  

Max  LRC  

region  

Probe D

First observation of flux surface potential variation in TJ-II /ECRH plasmas

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-80

-40

0

40

80

-40 -30 -20 -10 0 10

Vfs Mean D-H (D) Vf D mean Max LRC timeVf D mean ne≈0.8 timeVf D mean ne≈0.4 time

Vf (

V)

z-zLCFS (mm)

-80

-40

0

40

80

-25 -20 -15 -10 -5 0 5

Vfs Mean D-H (B) Vf D mean Max LRC timeVf D mean ne≈0.8 timeVf D mean ne≈0.4 time

Vf (

V)

z-zLCFS (mm)

Deuterio  D/H≥2  

Probe  B  

Max  LRC  

region  

Max  LRC  

region  

Probe D

First observation of flux surface potential variation in TJ-II /ECRH plasmas

•  Poloidal/ toroidal potential variation in the order of 10 – 30 V in floating potential.

•  Assuming that electron temperatures have zero parallel gradients, these results are reflecting asymmetries (poloidal / toroidal) in plasma potential.

•  Poloidal / toroidal asymmetries depend on plasma conditions

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Plasma density (8 x 1018 m-3)

Floating potential (V)

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Plasma density (6 x 1018 m-3) near the electron-ion root transition

Floating potential (V)

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CONCLUSIONS

EVIDENCE OF POLOIDAL / TOROIDAL ASYMMETRIES IN EDGE PLASMA POTENTIAL?

YES

(particularly in some plasma conditions)

NO

Are they relevant for particle / impurity

transport?

YES

(Epoloidal x B drifts in the range of 100 m / s)

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CONCLUSIONS / Questions

EVIDENCE OF POLOIDAL / TOROIDAL ASYMMETRIES IN EDGE PLASMA POTENTIAL?

YES

(particularly in some plasma conditions)

NO

Are they relevant for particle / impurity

transport?

Impact of EpoloidalxB drift on impurity transport

A missing ingredient for understanding HDH W7-

AS and LHD impurity hole regimes?.

A missing ingredient for understanding impurity

control by ECRH / ICRG in tokamaks?

W7-X ?

YES

(Epoloidal x B drifts in the range of 100 m / s)