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Abstract. Experiments on a non-disruptive halo-current influence on the m = 2 mode behaviour at the flat-top stage of a tokamak discharge are presented. The halo-current in the (Rail Limiter - Plasma - Vacuum Vessel - External Circuit - Rail Limiter) loop was used. An EMF source controlled with a preprogrammed signal or with a feedback m = 2 signal was introduced into the external part of the halo-current circuit. The EMF source generated oscillating halo-currents with up to 500 A amplitude in the frequency range 0-20 kHz. In the case of the preprogrammed control signal the switching on of the EMF source resulted in the shift of the m = 2 mode frequency to the frequency of the halo-current oscillations. In particular, the rotation of the m = 2 mode stopped under a pulse of zero-frequency halo-current. In the tokamak discharges when the mode rotation stopped by itself before the switching on of the oscillating halo-current, the mode rotation was restored at the halo-current frequency. In the case of the halo-current feedback control by the m = 2 mode signal, the effect depended on the choice of the phase shift in the feedback loop. Some increase or decrease of the m = 2 mode amplitude as well as some variations of the mode frequency were observed at different values of the phase shift. The halo-current effect on the m = 2 mode behaviour can be attributed to a coupling between the m/n = 2/1 magnetic islands and the halo-current magnetic field. The experiment was simulated on the assumption that the tearing mode is affected by the halo-current magnetic field helical component with the same space structure. The equation for the disturbed poloidal flux in the presence of the external helical surface current was used for the analysis. In the calculations for the T-10 conditions, the mode behaviour under the effect of the halo-current was similar to the experimental observations.
IAEA 2003
