1. M. Hron ETFP workshop Kraków 11/09/2006 Presented by M Hron J Stockel, J Brotankova, I Duran, J Adamek, M Stepan, O Bilykova M Spolaore, E Martines, P Devynck, P Peleman, G Van Oost, L van de Peppel Institute of Plasma Physics, Academy of Sciences of the Czech Republic EURATOM Association IPP.CR, Prague, Czech Republic In collaboration with EURATOM Associations: ENEA Padova (Padua, Italy), CEA (Cadarache, France), Etat Belge (Ghent University, Belgium), Edge plasma studies on the CASTOR tokamak

2. M. Hron ETFP workshop Kraków 11/09/2006 CASTOR tokamak 1958 built in Kurchatov Institute, Moscow 1977 put in operation in IPP Prague 1985 reconstructed (new vessel)

3. M. Hron ETFP workshop Kraków 11/09/2006 CASTOR tokamak MAIN PARAMETERS major radius0.4 m minor radius85 mm plasma volume0.1 m 3 plasma current10 kA toroidal magnetic field1.3 Tesla pulse length30 ms plasma density 1-2*10 19 m -3 plasma temperature150 eV edge plasma density2*10 18 m -3 edge plasma temperature15 eV Manpower20 My MAIN PHYSICS TOPICS Edge plasma physics fluctuation measurements, biasing Wave plasma interaction fast particle generation, wave propagation Diagnostics development SXR spectroscopy advanced probes

4. M. Hron ETFP workshop Kraków 11/09/2006 Diagnostics

5. M. Hron ETFP workshop Kraków 11/09/2006 Edge plasma diagnostics Electric probes Classical Langmuir probes IV characteristics, local Te, ne, Ufl at the plasma edge, routine measurements Radial & Poloidal & 2D arrays of Langmuir probes for spatially / temporaly resolved measurements of plasma fluctuations Oriented probes Rotating Mach probe, Gundestrup probe for flow measurements during biasing experiments Advanced probes Tunnel probe - a quite novel concept for fast T e measurements

6. M. Hron ETFP workshop Kraków 11/09/2006 60 mm Poloidal array of 124 probes Poloidal resolution  = 2.9 deg (3 mm) 64 fast channels available - signals of one half of the ring can be monitored simultaneously. Rake probe Distance between the tips 2.5 mm Total length 35 mm Movable on the shot to shot basis U float or I sat mode of operation Probe arrays

7. M. Hron ETFP workshop Kraków 11/09/2006 Poloidal distribution Radial distribution at the top of the torus Measured by the rake probe in a single shot Measured by the poloidal ring in four shots Floating potential profiles

8. M. Hron ETFP workshop Kraków 11/09/2006 Ring represents the poloidal limiter Plasma is not centered, but downshifted Separatrix is not defined by the limiter Tips at the top – localized in the SOL Connection length >> 2  R to a material surface (shield) depends on the local helicity of magnetic field lines - q(a) Tips at the bottom - Closed Magnetic Field Lines Respective position of separatrix and probes

9. M. Hron ETFP workshop Kraków 11/09/2006 Turbulence in the SOL

10. M. Hron ETFP workshop Kraków 11/09/2006 Poloidally periodic patterns (bipolar) propagating poloidally are evident. Poloidal direction LFS TOP HFS Bottom Time 0.5 ms Potential “valley”Potential “hill” U fl ( , t) – raw data

11. M. Hron ETFP workshop Kraków 11/09/2006 Poloidal direction Time lag [ms] Poloidal periodicity as confirmed by cross-correlation analysis The reference probe is located at the top of the torus Poloidal periodicity

12. M. Hron ETFP workshop Kraków 11/09/2006 Dominant poloidal mode number is found to be m = 6-7 (standard discharge conditions on CASTOR) Poloidal mode analysis

13. M. Hron ETFP workshop Kraków 11/09/2006 The safety factor q(a) was increased in time by ramping down the plasma current. q(a) Time [ms] Dominant mode number m clearly follows the evolution of the edge safety factor q(a) m 8765487654 8765487654 Poloidal mode analysis

14. M. Hron ETFP workshop Kraków 11/09/2006 Conclusion - Turbulence in SOL Flute-like structure elongated along the magnetic field lines Radial dimension~ 1 cm Poloidal dimension~ 1 cm Lifetime~ 1-40  s Poloidal wavelength~ 5-15 cm Only a single (bipolar) turbulent structure exists in the SOL. Snakes q-times around the torus m=q, n=1 mode Starts (and ends) on the Ion (and Electron) side of the poloidal limiter Propagates poloidally due to the local ExB drift experimental data folded on the toroidal surface (toroidal angle = time)

15. M. Hron ETFP workshop Kraków 11/09/2006 Biasing

16. M. Hron ETFP workshop Kraków 11/09/2006 Motivation  Generate electric fields in the edge plasma  manipulate with ion flows via ExB drift  reduce plasma fluctuations  improve particle&heat confinement Massive electrode is inserted in the edge plasma and biased with respect to the vessel Biasing experiments density H_alpha U_bias I_bias biasing phase 1050152025 t [ms]

17. M. Hron ETFP workshop Kraków 11/09/2006 Biased flux tube - originates at the electrode and extends upstream and downstream Peaks - Intersection of the biased flux tube with the poloidal ring Electrode is localized within the SOL and biased with respect to the vessel Poloidal distribution of floating potential SOL biasing

18. M. Hron ETFP workshop Kraków 11/09/2006 Poloidal position of the biased flux tube versus the local safety factor (#14076 &14077) PROJECTION OF THE ELECTRODE 4 3 2 1 5 Position of peaks 2 and 3 is independent on q- value, but determined by the poloidal extent of the biasing electrode. Peaks 1,4,5 vary with the edge safety factor as expected SOL biasing

19. M. Hron ETFP workshop Kraków 11/09/2006 Terminates on the electron and ion side of the poloidal limiter at the bottom part of the torus. Intersects q-times a poloidal cross section Originates at the electrode Extends upstream and downstream along the magnetic field lines Unfolded torusPoloidal cross section SOL biasing

20. M. Hron ETFP workshop Kraków 11/09/2006 E pol xB tor drift in radial direction I sat Bias /I sat OH E rad Convective cells BIAS ohmic Electrode A significant modification of density profile is observed during the SOL biasing

21. M. Hron ETFP workshop Kraków 11/09/2006 E r (r) during V fl peaks 10  s Sudden rise of oscillating behaviour during the biasing phase The effect involves a wide radial region Edge plasma biasing

22. M. Hron ETFP workshop Kraków 11/09/2006 Creation and collapse of sheared region U fl E rad Huge radial electric fields characterize the edge region in between Ufl oscillations High sheared region (transport barrier) propagates towards the wall at about 200 m/s becoming progressively thinner before being destroyed

23. M. Hron ETFP workshop Kraków 11/09/2006 More clear evidence of a periodic radial propagation of high density structures is provided by the fluctuating part of I sat signal U fl I sat Ejection of particles

24. M. Hron ETFP workshop Kraków 11/09/2006 Highly time resolved (1 MHz) measurement of H  allowed a detailed investigation of it behavior during bias: a quite clear periodic oscillation is observed also on H  The H  periodic increase exhibit a good correlation with the radial propagations of density streams H  response

25. M. Hron ETFP workshop Kraków 11/09/2006 Mach numbers show an equivalent behaviour with the 10 kHz poloidal and toroidal flows swap during the relaxations. M II MM ~100  s 0.1 0.2 0.3 0.4 0.5 11.611.812.0 time [ms] 0 Modification of flows

26. M. Hron ETFP workshop Kraków 11/09/2006 Summary - Biasing Biasing experiments resulted in effective inducing of an improved plasma confinement, characterized by steeper gradients of density and radial electric field. SOL biasing creation of a bised flux tube in the SOL radial drift of particles (E pol x B tor ) modification of the density profile Edge plasma biasing periodic creation and collapse of a transport barrier (high shear region) at  10 kHz critical gradients achieved both on floating potential and plasma density radial propagation of high density structures response of the neutral particle influx from the wall

27. M. Hron ETFP workshop Kraków 11/09/2006 SKring: Is*Epol poloidal profiles

28. M. Hron ETFP workshop Kraków 11/09/2006 SOL biasing - zoom of U float (# 12009) TIME [  s] Time averaged Poloidal direction PEAK 1 PEAK 2 PEAK 3 Instantaneous

29. M. Hron ETFP workshop Kraków 11/09/2006 Poloidal angle Time [ms] U fl (q,t) fluctuations – b iasing Propagation velocity increases, as expected Poloidal mode number seems to be reduced

30. M. Hron ETFP workshop Kraków 11/09/2006 Flow measurement – Gundestrup probe Instantaneous polar diagram Flows I sat ratio: III (5/1)(7/3) Quiet phase1.45.2 Spike3.52.1 _ M Hron et al.

31. M. Hron ETFP workshop Kraków 11/09/2006 D-rake insertion r=55 mm Confirmed by insertion of the d-rake probe at different radial positions

32. M. Hron ETFP workshop Kraków 11/09/2006 d-rake: Epol profiles A periodical behavior is observed also on E pol E r >3E pol Also highest E pol is concentrated in a specific radial region of about 1 cm width within the LCFS  65 mm LCFS

33. M. Hron ETFP workshop Kraków 11/09/2006 d-rake: Vf and Is measurements A periodical behavior is observed also Is, with a clear correlation with Vf periodic oscillations A radial propagation of high density blobs can be guessed at each cicle animazione

34. M. Hron ETFP workshop Kraków 11/09/2006 SKring: Vf, Is, Epol poloidal profiles Simultaneous polodal measurements of Is and Vf shows that Higher density fluctuations are detected periodically Poloidal sector “top”

35. M. Hron ETFP workshop Kraków 11/09/2006 d-rake: Vf(r) and Is(r) single event A build up of a strong gradient is periodically observed both in Er and density, followed by an abrupt relaxation The radial propagation of density streams can be estimated v r  0.7 km/s

36. M. Hron ETFP workshop Kraków 11/09/2006 Dynamic behaviour: V f and v ExB VfVf V ExB  -dV f /dr during a single relaxation with duration of ~0.1ms, the plasma rotates several times poloidally around the entire circumference a change of rotation direction is observed during relaxations the strong sheared region is confined within LCFS

37. M. Hron ETFP workshop Kraków 11/09/2006 d-rake: Vf and Is radial gradients Strong gradient relaxation Also a strong density (Is) gradient is build up and crashes periodically during biasing

38. M. Hron ETFP workshop Kraków 11/09/2006 Sk-ring all Vf signals (  =63°-177°) Slight non uniformity on poloidal distribution of V f is observed so that a periodic behavior results also on E pol High correlation with periodic relaxations of radial edge profiles Poloidal profiles

39. M. Hron ETFP workshop Kraków 11/09/2006 Sk-ring all Vf signals (  =63°-177°) The poloidally distributed measurements of Vf at r=87mm shows the fingerprint of relaxations during bias bias Poloidal profile LFS HFS Radial profile SK ring r 0

40. M. Hron ETFP workshop Kraków 11/09/2006 SKring: polodal distribution of Epol Vf measurements distributed on four poloidal sectors: LFS, top, HFS,bottom The non uniform poloidal distribution of Epol is confirmed The stronger periodic behavior related to oscillations is observed mainly on the top region top bottom