Particle transport in TCV electron internal transport barriers (eITBs)
description
Transcript of Particle transport in TCV electron internal transport barriers (eITBs)
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Particle transport in TCV electron internal transport
barriers (eITBs)
E.Fable, O . Sauter, and the TCV team
CRPP – EPFL, Switzerland
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Motivations
• Particle transport understanding important for a future reactor performance.
• Study scenarios assessing usage of fully non inductive current source, eventually characterized by internal transport barrier (ITB) formation.
• Importance of elucidating the physics of particle transport in ITBs.
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Particle transport in L-mode (rev.)
• Ohmic L-mode plasma density profile is ‘entangled’ with (temperature) and current.
• Neoclassical effects (Ware pinch, off-diagonal terms) usually negligible.
• Mechanisms of density peaking identified in Turbulent EquiPartition (TEP ~q or s) and anomalous THermoDiffusion (THD ~Te), arising from TEM/ITG activity.
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Density behavior with ECH (rev.)• Density profiles in (Ohmic) L-mode
plasmas ,and many H-modes, show flattening after injection of on/off-axis ECH, due to a decrease in inward thermodiffusion pinch after ITGTEM transition [Angioni, Zabolotsky] (assuming q profile not changed).
• Off-axis ECH can largely change the current profile leading to a TEP-effect which accounts for the flattening.
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TCV eITBs
• In TCV a powerful ECRF system used to heat electrons: 6 gyrotrons X2 at 82.7 GHz, total power 3 MW for 2s.
• Possibility of driving ECCD up to ~250 kA.
• eITBs created routinely in fully non inductive scenario: Co-CD off-axis, IOH = 0, current sustained by ECCD and bootstrap (up to ~80%), reverse q profile up to ρV ~ 0.6, ρ*
> 0.1 for strong eITBs.
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TCV eITB typical current profile
Typical current profile for a TCV fully non inductive
eITB case
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Density behavior in eITBs• eITBs, are characterized by completely different
turbulence regimes, turbulence may not be at all present in barrier region.
• Experimental observations during fully developed eITBs show particle transport is linked to Te via a relation of the type: R/Ln ~ 0.5 R/LTe suggests a thermodiffusion-type pinch.
• Current profile in eITB expected to be hollow with q profile reversal role of TEP is not clear near s=0 surface TEP should not play a role anyway (?)
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Experimental observations (1)
eITB density profile (red) more peaked than Ohmic (black) ! q profile is reverse inside V~0.5, strong ECH power in center
Time traces showing eITB formation and steady state sustainment in a fully non inductive scenario
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Experimental observations (2)
+30 mV
- 30 mV
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Experimental observations (3)
σe = Ln/LT at high R/LT
(strong eITB) approaches 0.5, independently of other parameters
off-axis Co-CD allows to obtain reversed q-profile eITB ne follows Te peaking increases (blue). Monotonic q profile with ECH normal flattening (black)
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Experimental observations (4)
Cnt(Co)-Ohmic perturbation enhance(degrade) the eITB Modulation of local magnetic shear influences particle transport
s tailoring influences link between particle and heat transport and the THD effect (j0,ohm>0, reintroduce TEMs, removing eITB with peaked j)
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Neoclassical back ? Neoclassical thermodiffusion predicts values of σe consistent with experiment, yellow box indicates our regime of operations (~0.45)
Neoclassical diffusion predicted inside the eITB for a test case. Very low! Slow time scales for density evolution expected