Theoretical And Simulation Research On Backward Wave Oscillation In Helix Travelling Wave Tubes

Helix travelling- wave tubes(TWTs) are very important in microwave electronics and have been widely used in many fields. Many factors could affect the normal work of the helix TWTs, such as nonlinear, unstable and parasitical phenomena, among which backward wave oscillation(BWO) is one of the most serious factors. This paper focuses on the study of BWO in the helix TWTs based on the related project. Next, the 1-D small signal model(the coupled- mode theory) and the 2-D small signal model(guided wave excited theory) are used to investigate the factors that affect BWO in the helix TWTs. At last, a database is built.The main work and innovations of this dissertation are as follows:1. Basic theories of BWO in helix TWTs are studied. First we introduce 1-D theory and 2-D theory, and descript the major work in solving the d ifferential equations. Then we arrange the computing methods of the parameters on BWO and starting oscillation current. The theories are the basis of the paper.2. The theories are used to investigate BWO in helix TWTs and predict if they will cause the oscillation. We use the software HFSS to calculate dispersion characteristic, coupling impedance, and attenuation constant of helix TWT to confirm the start oscillation frequency and other parameters. Then we calculate the BWO parameters with the specific working conditions. Next we put the parameters into the differential equations and obtain the start oscillation length by solving the differential equations with numerical method. Finally we predict if the helix TWT will produce the backward wave oscillation. The predicted results are in agreement with the experimental results.3. A database is built. We change one of its high frequency characteristics or other parameters based on a helix TWT, such as pitch, diameter, thickness, width, radius of electron beam, magnetic flux density and magnetic field period to analyze the influence of different parameters on BWO in the helix TWT. Then we verify the conclusions by both 1-D theory and 2-D theory. Finally we sort the data and build the database. These results in the paper provide a strong basis for designing the stable helix TWT.