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Particle transport in TCV electron internal transport barriers (eITBs)

Particle transport in TCV electron internal transport barriers (eITBs). E.Fable, O . Sauter, and the TCV team CRPP – EPFL, Switzerland. Particle transport understanding important for a future reactor performance .

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Particle transport in TCV electron internal transport barriers (eITBs)

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  1. Particle transport in TCV electron internal transport barriers (eITBs) E.Fable, O . Sauter, and the TCV team CRPP – EPFL, Switzerland

  2. 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. Motivations

  3. 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. Particle transport in L-mode (rev.)

  4. 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 ITGTEM 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.

  5. 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. TCV eITBs

  6. TCV eITB typical current profile Typical current profile for a TCV fully non inductive eITB case

  7. 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 (?)

  8. Experimental observations (1) Time traces showing eITB formation and steady state sustainment in a fully non inductive scenario eITB density profile (red) more peaked than Ohmic (black) ! q profile is reverse inside rV~0.5, strong ECH power in center

  9. Experimental observations (2) - 30 mV +30 mV

  10. 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)

  11. 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)

  12. 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

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