Variational method for drift waves in a sheared magnetic field

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Abstract
A variational method is developed, which permits systematic improvement of the test function by increasing the number of variational parameters. The method is applied to the case of collisionless drift waves in a plasma slab with cold fluid ions. Using first the simplest test function, the authors obtain analytical expressions for the conditions under which this approximation yields marginal stability, and they compare these expressions with numerical results obtained by other authors. They then show numerically that the mode frequency converges as the number of variational parameters is increased. In the parameter range where the simplest test function yields marginal stability, they find weak damping when a sufficiently large number of variational parameters is included.