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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