Study On Novel Helix Traveling-wave Tubes With Sheet Beam

Two main goals for the development of traveling-wave tubes(TWTs) for practical applications in radar, high-data-rate communications, and compact high power sources are higher-frequency operation and higher output power. The round helix slow wave structure(SWS) possess the advantages of wide operating bandwidth and moderate interaction impedance, making it the dominant SWS for microwave amplification in medium power TWT and microwave power module(MPM). However, as the operating frequency increases to V band, the circular helix is hard to be fabricated due to its intrinsic 3-D structure. To operate on higher frequencies, the rectangular cross-section helix has been proposed and the sheath model has been adopted to study the high-frequency characteristics of this novel planar helix SWS. The obtained dispersion equations based on the sheath model is of guiding significance to some extent. However, the sheath model simplifies the rectangular helix SWS as the homogeneous system, which ignores the effects of helix tape width on the high frequency characteristis of the helix thus results in theoretical errors in the dispersion equations. As the important SWS for plane TWT, it is urgent to provide more accurate theoretical basis for the practical design of the rectangular helix TWT. For this purpose, we performed the analysis of the rectangular helix SWS by using the improved tape model. The tape model can provide a more accurate evaluation of the helix characteristic because it accounts for the contributions of space harmonics with consideration of the finite conductor width, that is absent in the simpler sheath model. The improved dispersion equations have been obtained by rigorous electromagnetic analysis, which reflect accurately the propagation characteristics of electromagnetic waves in this novel plane SWS to a great extent. At the same time, the round helix TWT usually adopted the round electron beam as the energy source of microwave amplification. However, the intense space charge effect inherent in the round electron beam has discouraged the further enhancement of output power of the TWT. Thus it is urgent to explore the new SWS and the corresponding working mechanism. For this purpose, we investigate the properties of electromagnetic wave propagation in a waveguide with circular outer conductor and eccentric circular inner helix. The closed form characteristic equation has been established based on the rigorous field matching method by the combined applications of the orthogonality of helix modes and Graf’s addition theorem for modified Bessel functions, the effects of the misalignment on the cold parameters of this non-concentric waveguide has been analysed in detail. The obtained characteristic equation can provide theoretical basis of the multi non-concentric round helixes SWS in the aplication of planar TWT using the sheet electron beam.The innovations of this thesis are mainly concluded as follows1. In order to better understand the properties of electromagnetic wave in the rectangular helix SWS, we study the rectangular helix SWS by using the improved tape model. With the expansion of surface currents in helix and the applications of modified Marcatili’s method as well as average power flow matching method at the boundaries, we obtained the dispersion equations and the interaction impedance of the rectangular tape helix immersed in free space. It is shown that, compared with the results of simplified sheath model by former researchers, higher accuracy has been obtained between the calculation results of the theory and the data obtained from HFSS. Also, the pass band and stop band can be easily estimated by using the improved characteristic equations. The above obtained results will help to get a deep-going understanding of the rectangular helix and provide the theoretical foundation for its application in planar TWT.2. The rectangular helix SWS loaded with dielectric and incorporating metal shielding has been studied. Equations for the slow-wave properties including normalized phase velocity and average interaction impedance have been acquired with the tape model. Numerical calculations for the normal loaded and also the inverted helix demonstrate the higher accuracy of tape helix model, and the validity of the tape modle has further been demonstrated by comparison with the experiments of the S band rectangular helix. The effects of dielectric and metal shielding parameters on the RF characterisitc are discussed.3. Based on the linearlized field theory, with the tape model for the rectangular helix, the hot dispersion equations of the transverse antisymmetric modes in the rectangular helix TWT are obtained. The complex hot dispersion equation governing the gain and propagation of waves are numerically calculated to evaluate the effects of the structure dimentsions and the beam parameters on the gain coefficient. It is shown that the square helix TWT possesses higher peak gain but limited bandwidth than that of the equivalent round helix TWT. Meanwhile, the gain and bandwidth increase as the aspect ratio increases when the pitch and pitch angle are fixed. Compared with the equivalent circular helix TWT, the rectangular helix TWT with medium and high aspect ratio hold the comprehensive advantages of gain and bandwidth.4. As the fundamental work for the study of the multi-helixes SWS, the cold parameters including the dispersion properties and the average interaction impedance of a sheath eccentric round helix in the circular waveguide are investigated. With the applications of orthogonal properties of the helix modes and Graf’s addition theorem for the modified Bessel functions, the closed form characteristic equations concerning the helix modes are derived rigorously. The obtained characteristic equations show that the misaligned structure only admits the field solutions existing in the form of the superposition of hybrid modes. Numerical calculations show that the normalized phase velocity and the average interaction impedance in low frequency band of the dominant mode decreases as the misalignment increases. Meanwhile, the cold parameters in medium and high frequency band are infulenced slightly by the misalignment. The obtained equations would be instructive for the analysis of multi-helixes SWS interacting with the sheet electron beam.