An apparatus for controlling flow of coolant into an inter-stage cavity of a turbomachine is described. The cavity is bounded by a first turbine stage, a second turbine stage axially displaced along a common axis of rotation with the first turbine stage, and an annular platform bridging a space between the axially displaced first and second turbine stages. An annular plenum chamber is arranged inboard of the annular platform, the annular plenum chamber having one or more inlets for receiving coolant and one or more outlets exiting into the cavity, whereby, in use, coolant is delivered into the cavity at an increased pressure compared to coolant entering the plenum chamber at the inlet. The apparatus is beneficially arranged immediately upstream (with respect to the flow of a working fluid through the turbomachine) of an inter-stage seal assembly.