Plasma rotation and toroidal drift modes

This paper discusses the structure of drift waves in a rotating toroidal plasma. The rotation destroys an underlying symmetry that is the basis 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 shows that sheared rotation annuls the toroidal coupling between perturbations associated with different magnetic surfaces, so that cylinder criteria rather than toroidal 'ballooning' criteria again become relevant. As expected, sheared rotation reduces the radial mode width, and presumably, therefore, the anomalous transport. It can also alter the scaling of anomalous transport with magnetic field from Bohm to gyro-Bohm. Another description of perturbations leads, as is well known, not to eigenmodes but to perturbations with a Floquet-like time dependence on a magnetic surface. We show that this Floquet solution actually conceals an arbitrary time dependence of the perturbation! At the usual leading order in a high mode number expansion, the Floquet form and the eigenmode form are equivalent and are equally valid descriptions. However, in a more accurate theory only the eigenmode form persists. The Floquet form, and its short-term growth rate, should be regarded as transients associated with particular starting conditions and with the use of an idealized (linear) velocity profile.