Browsing by Subject "Microwave measurements"
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Item Effects of diode gap closure and bipolar flow on vircator microwave generation(Texas Tech University, 1996-05) Young, Douglas T.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.Item Microwave diagnostics on a railgun plasma(Texas Tech University, 1996-12) Grant, Gregory T.This thesis describes microwave interferometry measurements on the High Energy Railgun Apparatus (HERA) at Texas Tech University. Chapter I discusses railguns in general, basic railgun theory, and problems encountered whh railguns. The physical railgun system is described and illustrated as well as the timing and firing system, and the data acquishion system. Chapter II contains a general discussion of interferometry, a detailed discussion of the microwave interferometer used on the HERA railgun, and a discussion and analysis of the expected signal from the detector of the HERA interferometer. Chapter III discusses the acquishon of the experimental data. It describes how the interferometer was "zeroed" before each shot and shows how attenuation and reflection of the interferometer microwave probing beam played a role in the collection of the data. This chapter also describes techniques which could be used to cut down on microwave reflections at dielectric interfaces on the railgun bore. Chapter IV is a detailed analysis of the shot data. This analysis includes a figure for each shot of interest and a table to describe the shot condhions. It also provides calculations and observations based upon the observed waveforms. These calculations are shown for the HERA railgun first whh a 1.8 cm square bore, and then with a 1 cm round bore. Chapter IV also gives resuhs and conclusions of this research. Trends in the data are pointed out and speculations about the significance of these trends are given. Conclusions are drawn and final remarks and recommendations are made.Item Window and cavity breakdown caused by high power microwaves(Texas Tech University, 1998-05) Hemmert, David J.The transmission of high power microwaves through dielectric windows is of essential importance in their use. When an interface window fails due to surface flashover and breakdown, the power can no longer be transmitted and may reflect back into the source, possibly damaging it. In the work reported here, the physical mechanisms of surface flashover and breakdown are investigated for power levels of 10 MW/cm. A 3 MW magnetron and an S-band traveling wave resonator are coupled to produce 100 MW at 2.85 GHz in a high vacuum environment. A window geometry is established to provide a purely tangential electric field along the window surface. High speed diagnostics include forward, reverse, and local field power levels, x-ray emission, and discharge luminosity and imaging. Investigations into other window geometries as well as surface coatings and vacuum-gas interfaces are possible.