1. Overview on Plasma Rotation, MHD and Electrode Biasing Experiments in the TCABR Tokamak 2nd IAEA Research Coordinating Meeting (RCM) of the Coordinated Research Project (CRP) on "Joint Research Using Small Tokamaks I.C. Nascimento 1, Yu.K. Kuznetsov 1, R.M.O. Galvão 1, J.H.L. Severo 1, A.M.M. Fonseca 1,, I.B. Semenov 1, O.C. Usuriaga 1,, V.S. Tsypin 1,, Z. O. Guimarães-Filho 1, M.V.A.P. Heller 1 V. Bellintani Jr. 1, E. Sanada 1, J.L. Elizondo 1, I. El Chamaa-Neto 4 A.N. Fagundes 1, W.P. de Sá 1 1Universidade de São Paulo, São Paulo, SP, Brazil 2Kurchatrov Institute, Moscow, Russia 3Universidade de Campinas, Campinas, SP, Brasil 4Universidade Tuiuti do Parana, Curitiba, PR, Brasil

2. TOKAMAK TCABR Parameters: Major radius R = 0.615 m Minor radius r = 0.180 m Toroidal Magnetic Field B T = 1.07 T Line-averaged electron density n emax = 4.5x10 19 m -3 Plasma Current I pmax = 110 kA, duration 100 ms Peak electron temperature T e0 = 600 e V Peak ion temperature T i 0 = 200 e V Zeff ~ 2 - 4 Energy confinement time follows the Neo-Alcator Scaling law

3. PLASMA ROTATION

8. MAGNETIC ISLAND ROTATION AND PLASMA ROTATION Magnetic island rotation was measured for discharges in similar conditions as the ones used for the measurement of residual plasma rotation using a set of 22 Mirnov coils displaced along the poloidal direction. The results show that the magnetic island (3, 1) rotates together with the plasma body near r/a = 0.89, corresponding to q = 3, and 3.7 at the limiter [J. H. F. Severo, I. C. Nascimento et al. Phys. Plasmas 11 (2004) 846]

9. MHD Island Width Determined by ECE Measurement of Temperature

11. IbIb Limiter RPRP V0V0 R1R1 C0C0 VbVb Electrode D C1C1 r z Polarization set-up and electrode Graphite d=2.0 cm, l=0.80 cm, V b < 400 V, I b < 200 A Electrode position: 1-2 cm into the plasma ceramic

12. H mode with biased electrode H modeTime evolution of plasma current, electrode bias, loop voltage, plasma density, SOL density, H  intensity and poloidal  for a discharge with electrode biasing (full lines) and without biasing (dashed lines)

13. Analysis of the discharge #11925 with the transport code ASTRA. The ASTRA transport code was used to evaluate the influence of biasing on the energy confinement time τ. The figure shows the measured time evolution of the density for the central chord. A quasi parabolic radial profile was adjusted to reproduce the experimental data. The time evolution of the poloidal beta was obtained by a fit to the normalized experimental values using data from the ECE radiometer for T e without biasing. Figure shows the loop voltage V exp and the best fit V ASTRA, obtained using the plasma current, the poloidal β, while for Z eff a linear growth from 2.0 to 2.4 factor is τ e /τ ALC = 8.6 ms/4.4 ms = 1.95, decaying to 1.5 at the peak density (2.6 × 1013 cm−3).

14. Regime of electrode biasing showing strong MHD activity

15. Effect of electrode voltage on plasma turbulence and ion saturation suppression. Temporal behaviour of ion saturation current in SOL (r = 19 cm) and bias current. The minimum voltage necessary for producing decrease in ion saturation current and turbulence is 80- 100 V. Ion saturation current was averaged at 20 μs intervals.

16. Effect of bias voltage with fast (a) and slow (b) rise- times on electrode current and plasma density.

17. Data on bias current I b, ion saturation current Is and its RMS fluctuations I s,rms in SOL, at r = 19 cm, measured 5 ms after the bias triggering are presented. The ratio n emax /n e0 of maximum value of line-averaged plasma density during biasing to that before biasing is also shown. Effect of stationary bias voltage with fast rise-time.

18. Radial profiles of floating potential, local plasma density, and RMS amplitudes of their fluctuations before (dashed lines) and during (solid lines) biasing are shown for V b  300 V, below the voltage necessary for bifurcation.

19. TCABR regimes with electrode biasing REGIMES I AND II (a) – Regime I: Typical discharges with strong MHD activity during electrode biasing; (b) – Regime II: Typical discharges with partial or total suppression of high amplitude MHD activity

20. MHD modes in two regimes of TCABR operation with biasing Time behaviour of MHD modes m = 2, 3, 4-6, n=1. (a) Shot #17993, Regime I, excitation with biasing; (b) Shot #18145, Regime II, without bias, with MHD; (c) Shot #18144 Regime II, partial suppression with bias

21. Effect of electrode voltage on edge plasma parameters. Plasma in regime I with electrode at r E = 16.5 cm, for shot #17994. Temporal behaviour at r = 17.0 cm of: (a) transport, floating potential, root mean square of the poloidal electric and magnetic field fluctuations E θ rms, Bθ,rms, respectively; (b) voltage and electrode current, Mirnov oscillations, dB/dt, and central chord line-averaged current.

22. Effect of electrode voltage on edge plasma parameters. Plasma in regime I with electrode at r E = 16.5 cm, for shot #17994. Temporal behaviour at r = 17.0 cm of: (c) electric field at r = 17.5 cm. (d) Radial profile of the H α emission at 25 ms (before biasing), 41 ms and 42.5 ms with biasing and without and with MHD activity, respectively.

23. Effect of electrode voltage on edge plasma parameters. Plasma in regime II with electrode at r E = 17.0 cm, shot #18696. Temporal behaviour at r = 17.0 cm of: (a) transport, floating potential, root mean square of the poloidal electric and magnetic field fluctuations E θ rms, B θ,rms, respectively; (b) voltage and electrode current, Mirnov oscillations, dB/dt, and central chord line-averaged current

24. Effect of electrode voltage on edge plasma parameters. Plasma in regime II with electrode at r E = 17.0 cm, shot #18696. Temporal behaviour at r = 17.0 cm of (c) electric field at r = 18.0 cm. (d) Radial profile of the H α emission at 25 ms (before biasing), 40 ms and 50 ms with biasing and without and with MHD activity, respectively.

26. Frequency power spectra of floating potential flutuations. Shots #17994, #18696 and #18699, top to bottom, respectively

27. Contour plot of S(k,f) for potential fluctuations in r/a=0.95 for: shot #17994 Regime I, shot # 18696, Regime II, shot #18699, Regime II, (top to bottom, respectively)

28. Temporal evolution of impurities in biasing experiments in TCABR

29. Effect of impurity radiation on H mode

30. Plasma rotation in the TCABR tokamak Results Poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, except near the plasma edge. Plasma rotation in the core is opposite to the direction of plasma current. Toroidal velocity agrees with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/a ~0.56 and r/a ~0.89 (the minor radius of TCABR tokamak a=18 cm). The radial electric field calculated using the measured velocities is negative. Toroidal rotation changes sign near the plasma edge. Until now there is no satisfactory theory to explain experimental results for toroidal rotation applicable to TCABR. Present results are in general agreement with corresponding results of similar tokamaks.

31. MAGNETIC ISLANDS AND PLASMA ROTATION The comparison of the measured velocities of the magnetic island (3,1) with the poloidal velocity of the background plasma at r/a=0.89 show good agreement. For the island (2,1) the poloidal and toroidal velocities are higher than the predictions of the neoclassical theory by 30% and 10%, respectively. We do not have yet measurements for this island in comparable plasma conditions.

32. The results show that the radial profile of the electron temperature in TCABR is not completely flattened in the vicinity of the magnetic island and follows the Fitzpatrick model [Phys. Plasmas, 2, 825 (1995)] and not the Rutherford model [Phys. Fluids 16, 1903 (1973)]. This confirms that the mechanism responsible for the perturbed temperature profile is the competition between the strong anomalous perpendicular transport and convective parallel heat transport. The measurement of the radial profile of T e provided data to calculate the width of 2.4 cm of the island (2, 1) as shown in the figure. RADIAL PROFILE OF ELECTRON TEMPERATURE IN THE VICINITY OF MAGNETIC ISLANDS AND ISLAND WIDTH

33. Discharges with strong, (excitation) and weak (suppression) MHD activity, and with and without bifurcation; Without bifurcation the confinement improves gradually above 80-100 Volts, and above this voltage the confinement increases gradually reaching plasma quality comparable to H-mode; Near the onset of the MHD instability the radial electric field in our measurements show strong decrease; The radiation losses increase with MHD activity and change the radiation profile shifting the higher radiation towards the centre of the plasma; Strong negative effect of MHD instability (Mirnov modes) on the edge transport barrier; In discharges with weak MHD instabilities we observe saw tooth oscillations of bias current similar to that observed on the TEXTOR (without active power supply in our experiment); The effect of MHD activity in some discharges is compatible with Stringer´s theory on neoclassical transport in the presence of fluctuations (NF, 32,1992, 1421. The analysis of some discharges show indication of formation of zonal flows. ELECTRODE BIASING ON TCABR Main results