PLASMA ROTATION AND TOROIDAL DRIFT MODES

This paper discusses the structure of drift waves in a rotating toroid al plasma The rotation destroys an underlying symmetry that is the bas is for the conventional ballooning representations of perturbations in a torus and alternative descriptions are needed. One such description exploits the residual symmetry that persists despite rotation. It sho ws that sheared rotation annuls the toroidal coupling between perturba tions associated with different magnetic surfaces, so that cylinder cr iteria rather than toroidal 'ballooning' criteria again become relevan t. As expected, sheared rotation reduces the radial mode width, and pr esumably, therefore, the anomalous transport. It can also alter the sc aling of anomalous transport with magnetic field from Bohm to gyro-Boh m. Another description of perturbations leads, as is well known, not t o eigenmodes but to perturbations with a Floquet-like time dependence on a magnetic surface. We show that this Floquet solution actually con ceals an arbitrary time dependence of the perturbation! At the usual l eading order in a high mode number expansion, the Floquet form and the eigenmode form are equivalent and are equally valid descriptions. How ever, in a more accurate theory only the eigenmode form persists. The Floquet form, and its short-term growth rate, should be regarded as tr ansients associated with particular starting conditions and with the u se of an idealized (linear) velocity profile.