The tapered MILO

Abstract

New magnetically insulated line oscillator (MILO) designs using tapered slow-wave structures and axial power extraction are presented and analyzed. Simulations show them to substantially outperform previous MILO variants, predicting efficiencies up to 20%. Experiments are in good agreement with simulations. Two gain mechanisms are shown to contribute to this high efficiency. The first part of the paper shows how the new designs and their two amplification mechanisms overcome limitations of earlier designs. Linear dispersive properties are examined, and results from particle-in-cell (PIC) simulations are summarized. The tapered MILO gives amplification without the mode competition seen in earlier /spl pi//2-amplifier and double MILO designs. Simulation of tuning the tapered MILO shows a 30% frequency bandwidth to 3 dB of peak power. Tapered MILO simulations and experiments are shown to be in good agreement; both show that the output radio frequency (RF) power is a simple quadratic function of the supply voltage over the range of voltages accessible to the experiment. All of the major features of the results from the laboratory experiment are well described by the idealized electrodynamical model implemented in the simulation program, indicating that plasma formation problems are not important for the voltages used. Experiments both with velvet and with carbon-felt-coated cathodes give RF pulses whose duration is limited solely by the power supply; velvet agreed most closely with simulation, whereas carbon-felt cathodes showed slower RF growth but agreed well in steady state.