We demonstrate that in a nonuniform plasma, the parametric decay instabilities of kinetic Alfvén waves could be quantitatively and qualitatively different from that in a uniform plasma. Specifically, the decay rate via ion Compton scattering is found to be enhanced, typically, by an order of magnitude. Furthermore, the parity of the decay kinetic Alfvén wave spectrum is broken, leading to finite net wave momentum transfer and, consequently, significant convective plasma transport with implications for confinement improvement. That localized absolutely unstable eigenmodes may be excited by a mode-converted pump wave is also presented.

Using field-theoretic formalism, we derive Ward identities for the diffusive transport of classical waves in disordered media of a quite general nature. We consider three cases: the scalar wave, the elastic wave, and the electromagnetic wave. For classical waves, it is the energy conservation that underlies the Ward identities, in contrast to the case of electronic systems, for which the Ward identity is a mathematical statement of charge conservation. For the three cases of classical waves, the Ward identities are of the same general structure, which differs from the electronic Ward identity and reflects the fact that energy transport, instead of charge transport, is accounted for in classical wave systems. The general structure of the Ward identities is seen to be independent of the details of the wave equations. ͓S1063-651X͑97͒06611-7͔

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