This paper reports on an experimental investigation of the spatial variation in electron density in a cylindrical inertial‐electrostatic plasma confinement device. A microwave cavity operating at 4 and 8 mm employing both longitudinal and transverse modes and a laser heterodyne system operating at 3.39&mgr; were used to investigate the charge structure. The microwave data showed a nonlinear increase in electron density as a function of center electrode current, implying trapping for ion injection. The laser heterodyne data showed a distinct oscillatory form of electron density as a function of radius for both electron and ion injection, with densities of 8×1011electrons/cm3and variations in maxima to minima exceeding 30% of the average density.