Planar explosive emission diodes operating at moderate pulse energy densities of a few joules per square centimeter conform normally with the Child-Langmuir law in its basic unipolar form. Several recent publications [D. A. Shiffler , IEEE Trans. Plasma Sci., 30, 1232, (2002)] suggest, however, that the bipolar flow develops in the diode at comparatively low electron current densities of similar to10 A/cm2 and pulse durations of similar to50 ns. Understanding the nature of a charged particle flow (unipolar or bipolar) in explosive emission diodes is of paramount importance for interpretation of the experimental results. We have therefore investigated the operation of a diode having velvet cathodes at voltages in the 40-170 kV range, electron current densities of 4-16 A/cm2, beam power densities of 0.3-2.2 MW/cm2, and pulse energy densities of 0.5-3.3 J/cm2. Within these latitudes of operating parameters, unipolar flow was always observed and bipolar flow was achieved only when specific measures for generation of anode plasma were introduced. Prolonged operation of the CsI coated cathodes may lead to a deposition of thin CsI films on the anode and, potentially, other elements of the high voltage structure. In interpreting the plasma images, care must therefore be taken to distinguish between the light coming from the anode plasma and the luminescence from CsI films. (C) 2003 American Institute of Physics.