  Orbital angular momentum of light is a core feature in photonics. Its
confinement to surfaces using plasmonics has unlocked many phenomena and
potential applications. Here we introduce the reflection from structural
boundaries as a new degree of freedom to generate and control plasmonic orbital
angular momentum. We experimentally demonstrate plasmonic vortex cavities,
generating a succession of vortex pulses with increasing topological charge as
a function of time. We track the spatio-temporal dynamics of these angularly
decelerating plasmon pulse train within the cavities for over 300 femtoseconds
using time-resolved Photoemission Electron Microscopy, showing that the angular
momentum grows by multiples of the chiral order of the cavity. The introduction
of this degree of freedom to tame orbital angular momentum delivered by
plasmonic vortices, could miniaturize pump-probe-like quantum initialization
schemes, increase the torque exerted by plasmonic tweezers and potentially
achieve vortex lattice cavities with dynamically evolving topology.
