Tunable vircators with E-beam premodulation

Summary form only given, as follows. Presents recent results of theoretical and experimental studies of HPM production in virtual cathode systems with electron beam premodulation. The phase separation of electron current in these devices is due to strong energy dependence of electron current passage just above the critical current level and develops at distances on the range of the wave length or shorter. Thus, the generator is efficient even with its self length is not over the wavelength where the kinematic bunching is inefficient. This provides working capacity of the device without external magnetic field. In theory, conditions for electromagnetic wave excitation in single- or double-gap vircator systems are anal, zed. Optimum amplitudes and phasing of RF fields in the gaps and the corresponding maximum microwave efficiencies are estimated in theory, supported with 1D-3D PIC simulations. The efficiency of double-gap system was found to multiply exceed the efficiency of single-gap system. The maximum efficiency for a double-gap system demonstrated in 3D electromagnetic simulation with KARAT code approached 20% at gigawatt power level. Experiments were performed using high-current periodically-pulsed electron accelerator SINUS-7 (pulse width 50 ns, maximum 2 MeV, 20 kA). L-band and S-band vircator systems were produced with output power of up to 1 GW and efficiency of up to 6% at 20-30 ns microwave pulse pulse width. The range for continuous frequency tuning reached 15% at half power level. Batch regime of periodically-pulsed operation of the vircator was realized (batch length 1 sec at 50 p.p.s. or 20 sec at 20 p.p.s.).