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Locked mode rotation

Time evolution of the helical deformation: the radial displacement of the plasma surface in the poloidal plane as a function of the toroidal angle is shown during the current flat-top. The change of the deformation shape when toroidal rotation starts at 20 ms is clearly visibleThe resort to the dynamo implies a breaking of the toroidal symmetry with a consequent plasma distortion and localization of heat flux to the wall. For high current long pulses, a system for active control of the position of the helical deformation is required, able to move the perturbation all around the first wall surface to uniformly distribute the flux load.
Induced rotation of the plasma distortion has been obtained by generating with the toroidal field coils an m=0 n=1 rotating external perturbation.

Mode rotation can set in when the torque applied by external field overcomes that due to static error fields. The mode accelerates until the drag torque due to plasma viscosity and eddy currents in the vacuum vessel balances the applied torque.

Both drag torques are proportional to the mode rotation frequency for slow rotation.
Since the applied torque is proportional to sin of the phase difference between the external field and the plasma mode, the m=0, n=1 mode can hook up to the applied rotating perturbation so that this phase difference remains constant.

The system behaves like a synchronous electrical motor with viscous damping and can have a stable operating point when the perturbation leads the mode by less than 90ยบ. As long as the perturbation amplitude is sufficient, the phase relation between mode and perturbation is such that the induced rotation frequency matches the perturbation frequency and continuous mode rotation is observed.

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