  We simulated an axial vircator with a three-cavity resonator and expected
generation efficiency 6-7 percents. For adequate description of physical
processes taking place inside a vircator we used two independent PIC codes:
self-developed INPIC and free XOOPIC. Based on both the analysis of the
vircator proposed in [1] and consideration of the devices operating at
cathode-anode voltages under 450 kV we suggest 3 possible designs of a
three-cavity resonator such that enable one to produce High Power Microwave in
GW power range.
