The term trapped mode is usually referred to a mode that can
not propagate in the beam pipe, but is localized in a particular region inside the device, producing narrow resonances peaks in the coupling impedance. They can be
excited by the presence of discontinuities inside different devices of an accelerator, producing unwanted beam instabilities. It is therefore important to identify trapped
modes, especially for new elements to be installed in a high-intensity accelerator. We present a recent study of the coupling impedance due to trapped modes in a new extraction septum that will be installed in the CERN Proton Synchrotron in the framework of PS Multi-turn extraction (MTE) commissioning. Simulation and theoretical calculations were performed in order to understand performance limitations of the machine, to find cures to reduce the instabilities, and to evaluate beam-induced heating
