Effects of diode gap closure and bipolar flow on vircator microwave generation
Young, Douglas T.
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High power microwaves are useful in a wide variety of applications in science, industry and in the military. The virtual cathode oscillator, or vircator, has been seen to produce very high output power over a widely tunable range compared with other high power microwave sources. Many experiments and numerical simulations have been done to study various aspects of vircators and the radiation produced by them. Two effects which have not been previously studied are the effects of diode gap closure and bipolar flow on the microwave radiation produced in vircators. In this work, these effects are studied using the PIC code MAGIC. One- and two-dimensional diode simulations agree well with known analytical results formulated for these diode cases. In both one- and two-dimensional vircator simulations, diode gap closure and bipolar flow were studied in detail. The geometry used in the two-dimensional vircator was made to be similar to that of an experiment done at the Himeji Institute of Technology in Japan. This was done so that comparisons between the simulations and an actual experiment could be made. The primary conclusion of this thesis is that neither bipolar flow alone nor diode gap closure alone can generate electron beam pinching in a two-dimensional vircator in which the experiment did show electron beam pinching. The second conclusion that is drawn in this work is that diode gap closure is the mechanism that allows vircators to achieve higher output powers. Finally, comparisons between the one- and two-dimensional vircators with diode gap closure or bipolar flow show a dramatic difference in the frequency spectrum of the output microwaves. Suggestions for further work, which include the creation of a model which combines both diode gap closure and bipolar flow, are also included.