Theoretical and experimental behavior of a compact magnetically insulated line oscillator electromagnetic structure

The magnetically insulated line oscillator (MILO) is a high power microwave source capable of delivering output powers greater than 1GW at a frequency of several gigahertz. The device is a crossed electric and magnetic field oscillator without any external structure to establish the static magnetic field, required to guide the electron beam, thus making it compact. We report on a compact MILO device based on the U.S. Air-Force design where all dimensions are reduced by a factor of 2. All the key points are reviewed using the Electromagnetic-PIC (Particle-In-Cell) code MAGIC in two-dimensional geometry. The operating frequency of such a device is 2.44GHz, leading to an output power slightly above 1GW for an operating voltage of 500kV and a maximum current of 45kA. The output power can be increased up to 2GW by optimizing the output coupling and reducing the beam loading effect. The analysis of the compact electromagnetic structure is based on the calculation of the external Q factor (Qext). The Qext coefficient is calculated for each resonant frequency of the four-cell coupled periodic resonator which accounts for the main part of the MILO slow wave structure. The optimized geometrical parameters such as the extractor vane radius (Rext=47mm) and the collector length (Lcol=92mm), which constitutes the output coupling section, lead to a maximum of simulated output power. A prototype has been built according to this geometry. The compact MILO structure is characterized by some cold tests (without electron beam) injecting a low rf power by a network analyzer; experimental results are presented and discussed in comparison with the simulation.