A new type of charged black hole bomb

Black hole bombs are usually constructed by surrounding an ergoregion by a mirror. The fields propagating between the event horizon and the mirror are prevented from escaping to infinity and reflected back to the ergoregion, thus undergoing repeated superradiant scattering which leads to a linear instability. We introduce a new construction in which the field is outside the mirror and is therefore prevented from falling into the black hole but is free to escape to infinity. Provided the mirror is inside the ergoregion, it turns out that this still causes linear instabilities. This behaviour is observed on Reissner-Nordstrom and de Sitter-Reissner-Nordstrom backgrounds using numerical simulations, based on a semi-implicit discretisation on a first-order system formulation of the partial differential equations governing the evolution of the scalar field. We also perform simulations for a standard black hole bomb and for another type of contraption: a sandwich black hole bomb.