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Overview of TJ-II experiments

Overview of TJ-II experiments. Presented by Joaquín Sánchez Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Spain Institute of Plasma Physics, NSC KIPT, Kharkov, Ukraine Institute of Nuclear Fusion, RRC Kurchatov Institute, Moscow, Russia

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Overview of TJ-II experiments

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  1. Overview of TJ-II experiments Presented by Joaquín Sánchez Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Spain Institute of Plasma Physics, NSC KIPT, Kharkov, Ukraine Institute of Nuclear Fusion, RRC Kurchatov Institute, Moscow, Russia Associação EURATOM/IST, Centro de Fusão Nuclear, Lisboa, Portugal General Physics Institute, Russian Academy of Sciences, Moscow, Russia A.F. Ioffe Physical-Technical Institute, St.Petersburg, Russia ORNL, US PPPL, US Carlos III University, Spain Univ Politecnica de Cayalunya, Spain

  2. TJ-II Flexible Heliac 4.6 m VF coils Vacuum vessel Plasma TF coils Central conductor

  3. TJ-II High Resolution Scattering Thomson Heavy Ion Beam Neutral Beam Line Neutral Beam Line B (0) ≤ 1.2 T, R (0) = 1.5 m, <a> ≤ 0.22 m 0.9 ≤  ≤ 2.2 ECR and NBI heating

  4. TJ-II goals • Stellarator physics • progress in the development of a disruption free, high density, steady state reactor based on the stellarator concept. • Basic fusion physics, relevant also to tokamaks & ITER.

  5. Overview • Confinement, electric fields and transport • Momentum transport • Plasma – wall • Conclusions

  6. Confinement, electric fields and transport • Momentum transport • Plasma – wall • Conclusions

  7. Global confinement and heat diffusion ne Ti Te Ne profiles show a gradual evolution from the hollow shape typical of ECH plasmas (on-axis) to bell-shaped profiles at the NBI phase (400 kW). E. Ascasíbar et al., Nucl. Fusion 45, 276 (2005) Diffusivities range from 1 to 10 m2/s.cedecreases with line density and iota in agreement with global confinement studies. (V. I. Vargas et al., EPS-2006) International Stellarator Confinement Data Base A. Dinklage et al., IAEA EX/P7-1 (Friday)

  8. Plasma potential profiles ne Te Off-axis ECH + NBI • Measurements of plasma potential show the evolution of the electric field from positive at ECH plasmas to negative at the NBI regime. The smooth change from positive to negative electric field observed in the core region as density is raised is correlated with global and local transport results, showing a confinement time improvement and a reduction of electron transport. A. Melnikov et al., EX/P7-3 Friday fp

  9. Plasma potential profiles In addition to edge shear layer (measured by probes) Doppler Reflectometry sees a second, deeper, shear layer which moves inwards as central density rises T. Estrada et al. Nuclear Fusion 46 (2006) S792

  10. Probabilistic Transport Models • Based on Continuous Time Random Walk / Master Equation • Avoids assumption of locality • Mathematically sound approach to modelling of critical gradient • Earlier work showed: • Power degradation • Stiffness / anomalous scaling with system size • Fast transport • etc. • Now, studied effect of perturbations • Pulse propagation: • Sign reversal (due to flux accumulation) • “Ballistic” and “instantaneous” propagation time time Ballistic Instantaneous B. van Milligen et al., TH/P2-17 (today)

  11. Confinement and magnetic topology Why do rationals trigger ITBs?: an open issue Due to a rarefaction of resonant surfaces in the proximity of low order rationals which is expected to decrease turbulent transport Due to the triggering of ExB sheared flows in the proximity of rationals TJ-II role of different low order rationals (3/2 vs 4/2, 5/3…)

  12. Core Transitions : role of low order rationals 5/33/24/3 • Positioning a low order rational (e.g. 3/2, 4/2, 4/3) near the core triggers controllably CERC (use, e. g., induced OH current or ECCD). But so far no CERC triggered by 5/3. • T. Estrada et al., PPCF 2005 / FST 2006 Te ne Ip International Stellarator Profile Data Base Yokoyama et al , EX5-3 Thursday

  13. Core transitions triggered by 4/2 rational ne Difference between the SXR reconstructions before and during CERC Ti SXR profiles Te Ip • CERC triggered by the n=4/m=2 rational has been studied in TJ-II ECH plasmas. • Changes in both Te and Ti. • The SXR tomography diagnostic shows a flattening of the profiles localized around  ≈ 0.4 with a m=2 poloidal structure. • The rational must be inside the plasma to trigger the transition. • T. Estrada et al.EX/P7-6 Friday

  14. Confinement, electric fields and transport • Momentum transport • Plasma – wall • Conclusions

  15. Momentum transport: plasma core 6 4 2 0 -2 -4 -6 0.2 0.4 0.6 0.8 1 1.2 1.4 • change of sign of the poloidal rotation direction I • depends abruptly on plasma density. • In low-density plasmas the poloidal direction corresponds to a positive radial electric field, at higher densities negative radial electric fields are deduced from the measured poloidal rotation. • Results consistent with HIBP measurements • qualitative agreement with neoclassical theory calculations that predict that the change of sign of the radial electric field is mainly due to a change in the ratio of the electron to ion temperature • B. Zurro et al., FST-2006 E < 0 (km/s) r pol V E > 0 r 19 -3 n [10 m ] e

  16. Poloidal limiter Field lines View plane View cone Momentum transport: plasma edge • The development of the naturally occurring velocity shear layer requires a minimum plasma density.There is a coupling between the onset of sheared flow development and the level of turbulence (M.A. Pedrosa et al., PPCF-2005). • Sheared flows can be developed in a time scale of tens of microseconds (A. Alonso et al., PPCF-2006) M2 M3 M1 M.A. Pedrosa et al., EX/P4-40 Thursday C. Hidalgo et al., EX/P7-2 Friday

  17. Phase transition model and edge transitions Coupled nonlinear envelope equations for the fluctuation level and shear flow Nonlinear damping Shear flow stabilization Instability drive TJ-II data Viscous damping Reynolds stress • The emergence in TJ-II of the plasma edge shear flow layer as density increases can be described by a simple transition model. • The mechanism used in the model is the resistive pressure driven turbulence. • This model gives power dependence on density gradients before and after the transition consistent with experiment. B.A. Carreras et al., Phys of Plasmas (2006)

  18. Energy transfer between global (parallel) flows and turbulence ENERGY turbulence ENERGY (DC flows) Energy Transfer First measurements of the production term (P) in the TJ-II stellarator show the importance of 3-D physics in the development of perpendicular sheared flows and the development of significant parallel turbulent forces. B. Gonçalves et al., Phys. Rev. Lett. 96 (2006) 145001. C. Hidalgo et al., EX/P7-2 Friday

  19. Biasing experiments: electric field damping and transport • The ratio ne/ H (which is roughly proportional to the particle confinement time p) increases substantially (~100%) during biasing. • Flows decay after biasing in about 30 s: similar results have been found in other stellarator (HSX) and tokamaks (CASTOR) M.A. Pedrosa et al., EX/P4-40 Thursday

  20. Confinement, electric fields and transport • Momentum transport • Plasma – wall • Conclusions

  21. Plasma Wall Interaction Studies in TJ-II 13 1.2 10 100 T e 13 1 10 80 n e 12 8 10 60 12 6 10 40 12 4 10 20 12 2 10 0 0 0.75 0.8 0.85 0.9 0.95 1 Thomson reflectómetro Haz de He I667exp I667simulated Haz He ECE I706exp I728simulated 1.2 10 13 800 I728exp I706simulated Thomson #13567 3.5 #6296 #6296 700 1 10 13 3 600 12 8 10 2.5 500 ) -3 2 6 10 12 400 (cm e n 1.5 300 4 10 12 200 1 12 2 10 100 0.5 0 0 0 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 0.75 0.8 0.85 0.9 0.95 1 r Validation of the C-R model for He in a supersonic He beam • Comparison with reflectrometry, Li beam and TS--> density profile • Comparison with ECE Langmuir probes and TS--> temperture profile • Self consistency: reproduction of full emission radial profile #13567 emiss.(nm) 667 706 728 ) -3 (cm Te(eV) e n r ne Relative line intensity (A.U.) ne Te (eV) I(a.u.) e T r r r r

  22. Plasma Wall Interaction Studies in TJ-II 3 H C3/ne Isat /ne (Lim.A) H A3/ne 2 1 0 CV/ne Te (eV) 60 30 0 13560 13565 13570 13575 13580 13585 • Erosion / transport of C • Source of C: bulk graphite or pre-deposited CxHy films ? • Experiment: • limiter insertion (shot by shot) • Limiter C: graphite contaminated with ethylene (regenerated every shot) • Limiter A: clean graphite • Recycling (Ha) similar • C influx, mainly from contaminated limiter DZ< 2.5 cm DZ< 2.5 cm Limiter C in (A Out) Limiter A in (C out) a a Contaminated Clean 0.8 • Effects of limiter insertion in the plasmas depending on hydrocarbon deposition. Limiter C, contaminated. Limiter A, clean 0.4 shot number

  23. Tritium Inventory Control Through Chemical Reactions C deposition can be inhibited by N2 injection Demonstrated Asdex Up + Laboratory experimentsMechanism? Film precursor recombination prevents deposition -> C2Hx (would work at room temp energies ) Chemical sputtering of deposited film -> HCN (doubts for ITER, needs energetic ions , E>50 ev) Cold plasma experiment + new diagnostic (Cryo-trapping assisted mass spectroscopy) C2H2 Pre-deposited film+ N2 Plasma HCN C2H2 Pre-deposited film+ N2 /Methane Plasma HCN H2/N2 plasmas. Chem. SputteringHCN/C2 Hc’s=3 H2/N2/CH4 plasmas. ScavengersHCN/C2 Hc’s=0.1 Tabarés et al. EX/P4-26 Thursday

  24. Near term plans for TJ-II Lithium deposition evaporation +Ne GD Plasma 4 ovens, 1g/each. Symmetric Route to high ne high b operation > 1.6 MW NBI by early 2007

  25. Confinement, electric fields and transport • Momentum transport • Plasma – wall • Conclusions

  26. Conclusions • The investigation of plasma potential profiles reveals a direct link between electric fields, density and plasma confinement. Statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence. • TJ-II experiments clearly show that the location of rational surfaces inside the plasma can provide a trigger for core transitions. These findings provide critical test for different models proposed to explain the appearance of CERCs linked to magnetic topology. • In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. Contrarily, spontaneous sheared flows appear to be strongly coupled to plasma turbulence in the plasma edge, consistent with theoretical models for turbulence-driven flows. • Carbon erosion & redeposition studies: - carbon influx from film dominant over bulk graphite release - scavenger effect dominant over chemical sputtering in N2 puffing experiments .

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