Development of a Novel Slow Wave Structure for Relativistic Backward Wave Oscillators

Abstract

In this dissertation, the development of BWOs with arbitrarily corrugated slow wave structures (SWSs) that will provide improved efficiency with higher output power and ease of fabrication is presented. A comprehensive theoretical formulation has been developed in order to analyze the BWO with arbitrarily corrugated SWSs. The chosen corrugated SWSs are, (1) trapezoidally corrugated SWS (TCSWS), (2) semicircularly corrugated SWS (SCCSWS), (3) triangularly corrugated SWS (TrCSWS), and (4) circular-edge disk-loaded SWS (CDSWS). The radial functions defining the proposed SWSs have been derived in terms of Fourier series in an attempt to apply the Rayleigh-Fourier method for the analysis. The zero-beam current dispersion characteristics of fundamental as well as higher order TM modes have been calculated analytically which are verified with the cold test experimental values and the numerically obtained values using the commercial EM solver, CST. Comparisons of the analytical and numerical results with the results found from the experiments show an excellent agreement which provides a high degree of assurance in the validation of the developed theory and the simulated results. Microwave radiation is generated when an electron beam with sufficient energy and current propagates through the SWS to initiate the instability. To study the temporal growth rate (TGR) for the fundamental TM01 mode, the dispersion equation is solved for the beam current of 0:1 ð€€€ 1:0kA and the beam energy of 205ð€€€665kV . For the TM01 mode, the TGR of the BWOs with the proposed SWSs, which provides a qualitative index of the strength of the microwave generation, is compared with those of the BWO with sinusoidally corrugated SWS (SCSWS) for different beam parameters. The study reveals that, while the performance of SCCSWS is significantly inferior to SCSWS, TCSWS, TrCSWS, and CDSWS; the TCSWS, TrCSWS, and CDSWS exhibit an average of 1.48% , 1.98% , and 7.71% higher peak TGR (PTGR) respectively, compared to conventionally used SCSWS. In order to substantiate the improved PTGR obtained analytically, the particle-in-cell (PIC) based simulation is performed and the time dependent output power, field analysis, and phase space plots are studied rigorously. With a beam voltage of 341kV and a beam current of 0:8kA, a maximum conversion efficiency of 32.82%, 16.41%, 33.74%, 51.16%, and 26.41% has been achieved for TCSWS, SCCSWS, TrCSWS, CDSWS and conventionally used SCSWS respectively, from which it can be deduced that the BWO with the proposed CDSWS provides maximum conversion efficiency with peak output power of 139:56MW. Moreover, the CDSWS also provides an advantage in the fabrication process and is less prone to RF breakdown since it has no sharp edges in the inner wall where the electric field intensity can be infinitely high.