Stability of parametrically excited drift waves

The propagation and stability of drift waves is examined, by means of a perturbation technique, in geometries where the magnitude of the magnetic field varies periodically along the field lines. Two sets of modes are found which can be driven unstable by resonant particles trapped in the magnetic field non-uniformity. One set of modes consists of standing waves with wavelength twice that of the magnetic field non-uniformity. The other set of modes consists of standing waves composed essentially of the sum of a component which does not vary along the line and a component which is a standing wave with wavelength equal to the wavelength of the magnetic field non-uniformity. The advantage of this method is that it predicts simply and pictorially the real frequency of oscillation for these modes.