Electron densities between the electrodes of a flashover anode intense ion beam diode are studied with laser interferometry. Visible and infrared data show no measurable density near the anode and at mid‐gap during ion beam emission. Hence, anode or cathode plasma closures do not cause impedance collapse. However, a two‐phase density build‐up observed near the cathode during beam emission appears associated with impedance collapse and with the termination of the ion beam. Substantial density is measured after the end of the ion beam, consistent with ionization of the neutrals produced by the flashover process. ©1995 American Institute of Physics.