Turbulent transport at the boundary of fusion plasmas

1. Turbulent transport at theboundary of fusion plasmas O E Garcia, A H Nielsen, V Naulin, J Juul Rasmussen Association EURATOM, Risø National Laboratory, Denmark R A Pitts, J Horacek, J Graves Association EURATOM, CRPP, EPFL, Switzerland W Fundamenski UKAEA/EURATOM, Culham Science Center, United Kingdom

2. Tokamak geometry with magnetic separatrix

3. Magnetically confined fusion plasma in the JET device

4. The magnetic divertor: scraping off the plasma Divertor operation: No plasma-wall interaction Ash (He) removal This assumes: Acoustic streaming║B Slow plasma drifts ┴B Experiments show: ┴ and ║ motion comparable plasma-wall interactions turbulent radial transport

5. TCV tokamak experiment Reciprocating probe measures: electric potential plasma density temperature transport Typical separatrix values: particle density n=2·1019 m-3 particle flux Γ=3·1021 m-2s-1 temperature T=20 eV Minor radius r=0.25 m Major radius R=0.89 m Magnetic field B=1.4 T Density ramp discharges

6. Interchange turbulence simulation domain All model parameters given by experiment Parallel transport modelled as linear damping Damping rate given by ║ magnetic field length Simulation domain: edge, SOL, wall shadow

7. ρ=0 ρ=0 ρ=1 ρ=1 0.001 0.01 0.1 1.0 -0.01 -0.005 0.0 0.005 0.01 Edge-SOL electrostatic (ESEL) turbulence simulations Simulations reveal intermittent eruptions of plasma into the SOL Transport by radial motion of plasma filaments aligned with B Blob like-structure in plasma density and dipole structure in vorticity

8. Time-averaged particle density profiles 4cm 5mm 2cm Profile broadens with increasing plasma density (length scale and extent) Nearly flat profile at large density, implying strong plasma-wall interactions Turbulence simulations in quantitative agreement with high density case

9. Particle density time series at wall radius Highly irregular oscillations with pronounced bursts in the plasma density Large fluctuations occur frequently, up to five times the rms values Fluctuations are skewed, with a fast rise and slow decay

10. Relative particle density fluctuation level Fluctuation level increases radially outwards Large relative fluctuation throughout the scrape-off layer The same statistics for both cases in the region of broad profiles

11. Skewness of particle density fluctuations Particle density fluctuations are strongly asymmetric in the SOL This indicates an abundance of positive bursts in the time series The same statistics for both cases in the region of broad profiles

12. Flatness of particle density fluctuations Time series dominated by strong particle density fluctuations The probability distributions are positively skewed and flattened The same statistics for both cases in the region of broad profiles

13. Conditional waveform at the wall radius Time series are dominated by large-amplitude bursts The conditional waveform has a steep front and a trailing wake The typical amplitude is four times the rms fluctuation level

14. Radial motion of an isolated plasma filament Initially no flow! Initially there is no flow, which arise by vertical charge polarization An isolated blob accelerate and propagate a large radial distance Secondary instabilities (R-T and K-H) can take place Eventually the filament decelerates and disperses

15. Radial velocity of isolated blob structures • Blob velocity scaling in the ideal limit: • structure size ┴B • curvature radius of B • acoustic speed

16. Summary and conclusions Transport in the plasma boundary region is due to turbulent fluctuations Radial motion of magnetic field-aligned filaments erupted from the edge Fluctuations display universality in the region of broad plasma profiles Turbulence simulations are in quantitative agreement with experiments Flux-gradient paradigm is surpassed bynon-locality and intermittency See poster presentation for more details