Tearing-mode natural frequency behaviour under mode-locking in T-10 tokamak

The results of the m = 2, n = 1 tearing mode natural frequency measurements in the course of the mode-locking by a controlled halo-current are presented. The halo-current in the rail limiter?plasma?vacuum vessel?external circuit with a controlled EMF source?rail limiter loop was used. The halo-current represented a superposition of a direct-current pulse for the mode-locking and an oscillating-current component for the natural frequency measurements by the resonant response of the mode. In the experiment, the mode signal oscillations at the frequency of the halo-current oscillating component were observed even if the mode rotation stopped under the pulse of the halo-current. The maximum amplitude of these mode oscillations was achieved when the frequency of the halo-current oscillating component was equal to the frequency of the mode before switching on the halo-current. It means that under the conditions of the T-10 experiment, there was no variation of the mode natural frequency upon termination of the mode rotation by the halo-current. This mode behaviour can be attributed to the well-known effect of frequency locking in an externally driven non-linear oscillator. The experiment was simulated with a model of a rotating non-linear tearing mode in the presence of the halo-current.