Global structure and stability of Kerr-Bertotti-Robinson spacetime

The surface $r=\mathrm{\infty }$ of the Kerr-Bertotti-Robinson spacetime is not a genuine boundary. We construct its natural analytic extension and show that $r=+\mathrm{\infty }$ of one KBR region is smoothly connected to $r=-\mathrm{\infty }$ of a neighboring one. Repeated continuation produces an infinite chain of regions connected by wormhole-like bridges and exposes the neighboring ring singularity without an intervening horizon, which violates the weak cosmic censorship conjecture. We then study a test massless scalar field on a two-universe scattering segment to probe the stability of the spacetime. For axisymmetric perturbations, we analytically establish purely imaginary growing quasinormal modes for every $\mathrm{\ell }$ and trace their origin to the chronology-violating region. In the $(\mathrm{\ell },m)=(2,2)$ sector, unstable branches occur for sufficiently large rotation and sufficiently small magnetic field. Their marginal real modes obey an exact horizon-flux balance, supporting a black-hole-bomb interpretation in which superradiant extraction is amplified by trapping within the double-barrier potential. The same cavity also supports families of weakly damped modes and may produce echo-like responses.