High power microwave generation from KALI 5000 pulse power system

Experiments were carried out to generate High Power Microwaves (HPM) from KALI 5000 pulse power system using an axial and a coaxial virtual cathode oscillator (vircator). The KALI 5000 pulse power system is a Marx generator and Blumlein line based system capable of operation at 1 MV, 50 kA, 100 ns. The typical electron beam parameters were 400 kV, 20 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. It was found that the generated HPM power is almost negligible in an axial vircator configuration due to severe impedance collapse problem. Impedance collapse in a planar diode due to fast cathode and anode plasma expansion and bipolar flow across the diode gap. However, HPM could be generated in a coaxial vircator configuration because of the fact that the electrode plasma expansion velocity of the cylindrical diode is much smaller as compared with the planar diode for the same accelerating gap and diode voltage. Therefore, much higher voltage can be obtained for the cylindrical diodes as compared with the planar diodes for the same accelerating gap. The measured HPM power density and the electric field strength at antenna mouth are (4.3 m distance from the Vircator window) 104.5 kW/m² and 6.27 kV/m respectively. Frequency of the HPM signal was 2.9 GHz and a time-dependent frequency analysis shows that the frequency remains constant in time for almost entire pulse duration.