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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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!≈%&"#$
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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)
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