The electrostatic potential profile in the accelerating gap in a plasma‐prefilled magnetically insulated intense pulsed ion beam diode has been determined using laser‐induced fluorescence spectroscopy. Results for a 300–400 kV planar diode are presented, both with and without an electron emitting vane protruding from the cathode. In both cases, a 5–6 mm accelerating gap forms in the 1012–1013/cm3plasma in a few nanoseconds. The experimental potential profiles are not consistent with electrons confined to a sheath near the cathode. Rather, electrons are required throughout the gap to explain the observations.