Investigation of a high-power virtual cathode oscillator

In this dissertation, three topics are investigated in different degrees: space-charged limited current for a vacuum cylindrical diode, microwave resonance effects in coaxial diodes and microwave propagating mode identification. By introducing a physics approximation method into analyzing the nonlinear Poisson's equation, an analytical expression for the space-charge-limited current density for a one-dimensional (1-D) cylindrical diode has been investigated and developed. Applying the approximation method, we get the relativistic theory corrected current for 1-D cylindrical diodes. This 1-D result is extended to 2-D. Some experiments at TTU show that the interaction between the virtual cathode oscillation and the cavity is a key to determine the microwave frequency and propagating mode. Particularly, we observe that the E-beam plays an important role in cavity formation. Some possible explanations are proposed. The methods to determine microwave propagating modes are reviewed and some suggestions are proposed to identify microwave propagating modes.