Topological issues and relationships with turbulence in the edge region

1. Topological issues and relationships with turbulence in the edge region Presented by Matteo Agostini

2. Outline • Edge region: magnetic and kinetic point of view • Magnetic boundary, edge profiles and confinement properties • Interplay between magnetic boundary and turbulence in the density limit • Link between magnetic modes and turbulence

3. Introduction • In the Reversed Field Pinch configuration q exhibits a decreasing profile • In the edge region q = 0 (reversal surface) RFX-mod R = 2 m a = 0.46m Ip = 0.3 – 1.5 MA • In the reversal surface all the m = 0 modes resonate

4. Edge: Gradients • The edge region is characterised by strong radial gradients • They are source of free energy for the development of instabilities which could degrade the confinement

5. The edge region plays a fundamental role in the plasma confinement Edge: Gradients • The edge region is characterised by strong radial gradients • They are source of free energy for the development of instabilities which could degrade the confinement

6. Dt=0.4 ms Dt=1.2 ms Dt=0.8 ms Dt=0 f [mm] f [mm] f [mm] f [mm] r [mm] r [mm] r [mm] r [mm] Edge: Turbulence • In the region outside the reversal surface turbulence is responsible for the greatest part of the particle transport • It is caused mainly by coherent structures: density blobs • They are filaments elongated in the direction of the magnetic field • Blobs can interact with the magnetic islands

7. Edge: Magnetic Topology Magnetic topology of the edge is dominated by m = 0 islands due to modes resonating on the reversal surface Multiple Helicity Quasi Single Helicity Large spectrum of m=1 modes is destabilised Mode m=1 n = -7 dominates Do the magnetic islands influence the edge properties and the transport?

8. Magnetic Field & Edge Properties • HeI light emission in the plasma edge is modulated by the radial magnetic field • The maximum of emission is correlated with Brm=1

9. ISIS Magnetic Field & Edge Properties • HeI light emission in the plasma edge is modulated by the radial magnetic field • The maximum of emission is correlated with Brm=1 • Floating potential measured by probes array shows the same spatial periodicity of the radial magnetic field Indication of the interaction between plasma edge and local magnetic field

10. Islands & Profiles F= -0.2 #20367 Temperature profile flattens in the region inside the m = 0 island Temperature is constant on conserved magnetic lines Similar behaviour observed in tokamaks during NTM

11. Islands & Profiles • Flow velocity and radial shear vary during the discharge evolution

12. Islands & Profiles • Flow velocity and radial shear vary during the discharge evolution Higher velocity on conserved surface around the island

13. Islands & Profiles • Flow velocity and radial shear vary during the discharge evolution Lower velocity on X-point where magnetic field lines intercept the wall

14. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology

15. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology • The wall locking is the local source of density

16. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology • The wall locking is the local source of density • m = 0 island charges the wall with electrons

17. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology • The wall locking is the local source of density • m = 0 island charges the wall with electrons • Accumulation point where vf= 0

18. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology • The wall locking is the local source of density • m = 0 island charges the wall with electrons • Accumulation point where vf= 0 • Formation of high density toroidally localised region

19. n/nG > 0.4 Br > 0 Br < 0 Density Limit Density limit is due to the interplay between edge toroidal flow and magnetic topology • The wall locking is the local source of density • m = 0 island charges the wall with electrons • Accumulation point where vf= 0 • Formation of high density toroidally localised region • Edge cooling and soft landing of the plasma current

20. TS 0.78 < r/a < 0.87 TS 0.78 < r/a < 0.87 Temperature gradient at the edge become steeper Electron thermal conductivity decreases Scaling With m = 1 Modes Confinement improves with the improvement of the magnetic boundary control

21. THB 0.9 < r/a < 0.98 m = 1 & Turbulence The decrease of the secondary modes is correlated with an improvement of the pressure gradient at the far edge Pressure gradient is commonly recognised as a source of free energy for the edge turbulence Lp = pressure radial scale length

22. THB 0.9 < r/a < 0.98 THB 0.9 < r/a < 0.98 m = 1 & Turbulence The decrease of the secondary modes is correlated with an improvement of the pressure gradient at the far edge Lp = pressure radial scale length Lb = toroidal scale length of edge blobs

23. THB 0.9 < r/a < 0.98 Link With Other Devices • Correlation between edge blobs and pressure gradient is not a peculiar characteristic of RFX-mod Alcator C-mod and NSTX points reflect the same scaling

24. Conclusions Magnetic islands are found to exist around the reversal surface interacting with the edge plasma • Modify the edge profiles • Play a role in the density limit • Interact with the edge blobs • Modify the edge transport and confinement

25. Conclusions Magnetic islands are found to exist around the reversal surface interacting with the edge plasma • Modify the edge profiles • Play a role in the density limit • Interact with the edge blobs • Modify the edge transport and confinement Open issues: Study the mechanism of interaction between islands and edge turbulence Clarify the role of islands and turbulence in the density limit

27. Edge Parameters THB 0.9 < r/a < 0.98 THB 0.9 < r/a < 0.98

28. Turbulence Scaling

29. Common Properties NSTX Alcator C-mod

30. Density Limit Radiation pattern in the poloidal plane

31. Density Limit

32. ~ 4% of the total B(a) SHAx