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The dynamics of expanding cathode and anode plasmas has long been recognized as a limiting factor of high-power vacuum diodes. An investigation of the characteristics of plasmas in a planar explosive emission diode powered by a 275-kV, 110-ns pulse is presented. In addition to current and voltage measurements, an ultra fast framing camera was employed to study transient characteristics of the plasma instabilities. It was shown by optical diagnostics that the anode plasma was clearly visible for the velvet cathode. Moreover, the anode plasma expanded rapidly across the anode-cathode gap, and the light emitted from it was much more intensive than that from the cathode plasma. In particular, we find that plasma transit times from anode to cathode are hundreds of nanoseconds, a very distinct and experimentally observable phenomena described in this article.