(EX/P8-9) Suppression and Excitation of MHD Activity with Electrically Polarized Electrode at the TCABR Plasma Edge

I.C. Nascimento1), Yu. K. Kuznetsov1), Z. O. Guimarães Filho1), I. El Chamaa Neto2), O. Usuriaga1), A. M. M. Fonseca1), R. M. O. Galvão1)4), I. L. Caldas1), J. H. F. Severo1), I. B. Semenov3), M. V. P. Heller1), V. Bellintani1), J. I. Elizondo1), E. Sanada1)
1) Laboratório de Física de Plasmas, Instituto de Física, Universidade de São Paulo, São Paulo, Brasil
2)Universidade Tuiuti do Paraná, Curitiba, Brasil
3) Kurchatov Institute, Moscow, Russia
4) Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brasil

Abstract.  The purpose of this experimental work performed in the TCABR tokamak, using electrically polarized electrode, was to clarify why in some discharges excitation of strong MHD activity was observed while in others this instability was suppressed. To do this, the plasma current was varied to cover a range of safety factor from 2.8 up to 3.6, allowing the H-mode barrier to partially scan the spectrum of MHD modes and interact with the magnetic islands. A set of 22 Mirnov coils was used to detect the magnetic oscillations and a triple Langmuir probe to measure plasma fluctuating potentials and ion saturation currents. The investigation of MHD excitation with biased electrode was performed with discharges with low amplitude MHD activity. After applying the bias the amplitude of the MHD activity strongly increased. The analysis of the data indicated that in this case the dominant mode was m=2, n=1 and the m=3, n=1, the passive mode. The positive electric field showed strong and fast decrease near the onset of the MHD activity. On the other hand, to identify the mechanism of MHD suppression, the tokamak parameters were adjusted to obtain reproducible discharges with strong MHD activity without electrode biasing. For these discharges the dominant mode was m=3, n=1. After applying the bias to the electrode the MHD activity was strongly decreased. The proposed explanation for the suppression is that the m=3 mode, located near the biased electrode at the plasma boundary, is suppressed together with the passive mode m=2. Strong quenching of the turbulence was also detected.

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