PLASMA RESPONSE TO EXTERNAL ROTATING MAGNETIC FIELDS IN THE JFT-2M TOKAMAK

摘要

Experimental results from the JFT-2M tokamak show that an external rotating resonant magnetic field transfers toroidal momentum to a plasma and modifies the radial profile of plasma rotation in the toroidal direction. The rotating m = 2/n = 1 resonant magnetic field created by the external coils transfers the toroidal momentum to an m = 2/n = 1 magnetic island through j x B torque. There exists a limit on the attainable frequency shift at which the external rotating field can sustain forced synchronous rotation of the magnetic island. There is a phase lag between the external rotating field and the motion of the magnetic island under the synchronization. When the frequency shift reaches the limit and the phase lag exceeds a critical value, the external field no longer sustains the synchronous motion and the rotation frequency of the magnetic island returns to its natural frequency. Two characteristic time constants are found in the process whereby the magnetic island returns from forced rotation to its free rotating motion when the external field is turned off. These experimental results are consistent with a model proposed earlier in which the plasma inertia in the island region is balanced against the sum of the local electromagnetic torque and viscous torque. However, the change of toroidal plasma rotation velocity measured by charge exchange recombination spectroscopy is less than half of that estimated from the frequency change of m = 2/n = 1 magnetic island rotation observed by the magnetic probes under the assumption that the change of magnetic island motion caused by the external field is entirely in the toroidal direction.