A simple MHD model of a plasma rotating through the periodic ripple of the toroidal magnetic field of a Tokamak is considered. It is shown that the plasma becomes unstable when the equilibrium toroidal rotation velocity exceeds the sound speed. The rippled field is equivalent to the magnetic 'wiggler' of a free electron laser. In the plasma case, long wavelength Alfven waves and short wavelength ion acoustic waves are excited.
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