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It has been recently observed that a scalar field with Robin boundaryconditions (RBCs) can trigger both a superradiant and a bulk instability for aBTZ black hole (BH). To understand the generality and scrutinize the origin ofthis behavior, we consider here the superradiant instability of a Kerr BHconfined either in a mirror-like cavity or in AdS space, triggered also by ascalar field with RBCs. These boundary conditions are the most general onesthat ensure the cavity/AdS space is an isolated system, and include, as aparticular case, the commonly considered Dirichlet boundary conditions (DBCs).Whereas the superradiant modes for some RBCs differ only mildly from the oneswith DBCs, in both cases we find that as we vary the RBCs, the imaginary partof the frequency may attain arbitrarily large positive values. We interpretthis growth as being sourced by a bulk instability of both confined geometrieswhen certain RBCs are imposed to either the mirror-like cavity or the AdSboundary, rather than by energy extraction from the BH, in analogy with the BTZbehavior.

Superradiant instabilities in the Kerr-mirror and Kerr-AdS black holes with Robin boundary conditions