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The double-gap S-band vircator operation has been investigated at sub-microsecond duration of the high-current electron beam generated in a planar diode. This version of vircator (see Fig. 1), although driven by a high-current (ges10 kA) relativistic electron beam, nevertheless allows both relatively efficient microwave generation and also radiation frequency stability, due to the use of a single-mode two-section rectangular RF cavity. The double-gap vircator powered by the accelerator SINUS-7 (1 MV, 20 kA, 50 ns) demonstrated ~1GW peak power of microwave radiation in the S-band at ~25 ns pulse length with ~5% efficiency . Earlier experiments with longer driving beam pulses  at the voltages close to those of  confirmed the constancy of the radiation frequency, as determined by the cavity, during the pulse. However, the microwave pulse was significantly shortened (< 100 ns) as compared to the accelerating voltage pulse that was explained by electron and ion emission from the plasma formed on the cavity walls leading to absorption of the microwave energy. In this work, we present the results of the experiments performed at moderate accelerating voltages, <550 kV, and longer pulse durations, ~500 ns.