Optimization of excitation point in a coaxial TE11 vircator

The vircator is a compact and robust narrow band High-Power Microwave (HPM) source with a simple geometry. In the most basic version of the vircator, the operating frequency depends only on the distance between the anode and cathode and on the applied voltage. In a compact system the vircator is often powered by a Marx generator resulting in a frequency chirp caused by the double exponential voltage pulse generated by the Marx generator. For a coaxial vircator, the anode/cathode structure can be utilized as a resonant cavity to achieve higher output power and monochromatic radiation from the vircator.An experimental campaign utilizing a coaxial TE11 vircator has been performed, in which the excitation point and the applied voltage have been varied. Experimental evidence is presented showing that the frequency shift during the pulse can be avoided by choosing the proper excitation point. It was found that the excitation point has more influence on the operating frequency than the applied voltage. When the highest radiated fields were recorded the microwave radiation was monochromatic and only the fundamental mode was present. Both far field and waveguide measurements were in agreement with the TE11 mode. At the optimum positions the impedance of the vircator was lower, indicating improved coupling between electron beam and microwave field. To verify the experimental findings on optimum excitation point, electromagnetic simulations were performed. Even though the simulation model was greatly simplified by replacing the electron beams with current sheaths at the position of the emitters, the results were in good agreement with experiments.