Edge plasma physics and relevant diagnostics development on the CASTOR tokamak

1. Edge plasma physics and relevant diagnostics development on the CASTOR tokamak Presented by M Hron for the CASTOR team Institute of Plasma Physics, Academy of Sciences of the Czech Republic EURATOM Association IPP.CR, Prague, Czech Republic and collaborators EURATOM Associations: ENEA Padova (Padua, Italy), CEA (Cadarache, France), Etat Belge (Ghent University, Belgium),

2. CASTOR tokamak 1960 built in Kurchatov Institute, Moscow 1977 put in operation in IPP Prague 1985 reconstructed (new vessel) 31.12.2006 shutdown

3. CASTOR tokamak MAIN PHYSICS TOPICS Edge plasma physics fluctuation measurements, biasing Wave plasma interaction fast particle generation, wave propagation Diagnostics development SXR spectroscopy advanced probes MAIN PARAMETERS major radius 0.4 m minor radius 85 mm plasma volume 0.1 m3 plasma current 10 kA toroidal magnetic field 1.3 Tesla pulse length 30 ms plasma density 1-2*1019 m-3 plasma temperature 150 eV edge plasma density 2*1018 m-3 edge plasma temperature 15eV Manpower 20 My

4. Diagnostics

5. Probe arrays • Rake probe • Distance between the tips 2.5 mm • Total length 35 mm • Movable on the shot to shot basis • Ufloat or Isat mode of operation 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. 60 mm

6. Floating potential profiles Poloidal distribution Radial distributionat the top of the torus Measured by the rake probe in a single shot Measured by the poloidal ring in four shots

7. Respective position of separatrix and probes 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 >> 2pR to a material surface (shield) depends on the local helicity of magnetic field lines - q(a) Tips at the bottom - Closed Magnetic Field Lines

8. Turbulence in the SOL

9. Ufl(q, t) – raw data Time 0.5 ms Bottom Poloidally periodic patterns (bipolar) propagating poloidally are evident. Poloidal direction HFS TOP LFS Potential “valley” Potential “hill”

10. Poloidal periodicity Poloidal periodicity as confirmed by cross-correlation analysis The reference probe is located at the top of the torus Poloidal direction Time lag [ms]

11. Poloidal mode analysis Dominant poloidal mode number is found to be m = 6-7 (standard discharge conditions on CASTOR)

12. Poloidal mode analysis 8 7 6 5 4 The safety factor q(a) was increased in time by ramping down the plasma current. q(a) 8 7 6 5 4 Dominant mode number m clearly follows the evolution of the edge safety factor q(a) m Time [ms]

13. 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)

14. Biasing

15. biasing phase density H_alpha U_bias I_bias 0 5 10 15 20 25 t [ms] Biasing experiments 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

16. SOL biasing Poloidal distribution of floating potential Electrode is localized within the SOL and biased with respect to the vessel Biased flux tube - originates at the electrode and extends upstream and downstream Peaks - Intersection of the biased flux tube with the poloidal ring

17. SOL biasing Poloidal cross section Unfolded torus • 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

18. Convective cells EpolxBtor drift in radial direction Epol ohmic IsatBias/IsatOH Electrode BIAS A significant modification of density profile is observed during the SOL biasing

19. Edge plasma biasing Er(r) during Vfl peaks • Sudden rise of oscillating behaviour during the biasing phase • The effect involves a wide radial region 10 s

20. Ejection of particles Ufl Isat • More clear evidence of a periodic radial propagation of high density structures is provided by the fluctuating part of Isat signal

21. ~100s 0.5 0.4 0.3 M 0.2 0.1 MII 0 11.6 11.8 12.0 time [ms] Modification of flows • Mach numbers show an equivalent behaviour with the 10 kHz • poloidal and toroidal flows swap during the relaxations.

22. 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 (Epol x Btor) 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

23. Summary and future plans

24. Summary • EDGE PHYSICS • Edge plasma polarization • Convective cells • Relaxation phenomena • Emissive electrode – late 2006 • M. Hron et al: Detailed measurements of momentum balance during the periodic collapse of a transport barrier, 33rd EPS Plasma Physics Conference, Roma, Italy, 19-23 June, 2006 • P.Devynck et al: Plasma Phys. Control. Fusion 47 (2005) 269-280 • J.Stockel et al.: Plasma Phys. Control. Fusion 47 (2005) 635-643 • Electro-magnetic properties of the turbulence • A Bencze et al: Observation of zonal flow-like structures using autocorrelation-width technique, Plasma Phys. Control. Fusion 48 (2006) S137-S153  • P. Devynck et al: Dynamics of turbulent transport in the Scrape-off-Layer of the CASTOR tokamak, accepted for publication in Physics of Plasmas, in October 2006 

25. Summary • Density fluctuations • Fluctuations of density and turbulent particle flux • P. Peleman et al: Highly resolved measurements of periodic radial electric field and associated relaxations in edge biasing experiments, PSI Conf., Hefei China, 2006, P3-23, accepted for publication in Journal of Nuclear Materials

26. Summary • DIAGNOSTICS DEVELOPMENT • Electric probes • Further experiments and modelling: • Tunnel probe for Te measurements • Ball pen probe • R. Dejarnac et al.: Study of SOL plasma by advanced oriented Langmuir probes on the CASTOR tokamak, to be submitted to PPCF  • J. Stöckel et al: Advanced probes for edge plasma diagnostics on the CASTOR tokamak, submitted to Journal of Physics, Conference Series.  • Hydrogen absorption in metallic membranes • Experiments performed in late 2005 • prepared for publication • M.E. Notkin et al: Measurements of the suprathermal hydrogen flux on the CASTOR tokamak, to be published in Nuclear Instruments and Methods in Physics Research Section B 2006 

27. Summary • CORE TRANSPORT AND TURBULENCE • Transport of non-intrinsic impurities • Experiments performed on CASTOR • Participation on T-10 experiments • V.Piffl et al: Measurements of line radiation power in the CASTOR tokamak, 33nd EPS Conference on Plasma Physics, Roma, 19/6-23/6/2006, P-2.126 • V.Weinzettl et al: Snake-like structures after pellet injection in the T-10 tokamak, 33nd EPS Conference on Plasma Physics, Roma, 19/6-23/6/2006, P-4.080  • EDUCATION • Experimental training course on tokamak physics • July 2006, 16 participants from 9 countries

28. Summary EXPERTISE EXCHANGE Turbulence and biasing experiments – a) Ghent University, Ghent, Belgium b) RFX, ENEA Padova, Italy c) CEA Cadarache, France d) LPMI, Nancy University, France e) IST Lisbon, Portugal f) Nuclear Fusion Institute, Kurchatov Institute, Moscow, Russia g) ERM/KMS Brussels, Belgium Diagnostics development and improvement – a) CEA Cadarache, France b) Innsbruck University, Austria Core transport and and turbulence – a) Budker Institute of Nuclear Physics, Novosibirsk, Russia

29. Future plans CASTOR shut down at the end of 2006 negative biasing using emissive electrode Magnetic properties of turbulence probe head prepared for TJ-II Simulations of plasma deposition in tile gaps modelling of plasma penetration into castellated tile gaps Educational activities Education of stuents of Czech Universities Experimental training course on tokamak physics organized by IPP Prague and KFKI Budapest