Short pulse high power microwave (HPM) devices with Gaussian radiation pattern are attractive for many applications. The importance of the Gaussian microwave beam is that its maximum energy density is concentrated on axis. The purpose of this study is to use a backward wave oscillator (BWO), which is typically known to radiate in the TM01 mode, to produce an HPM Gaussian wave beam. A BWO generates an electromagnetic wave with negative group velocity where the wave is traveling in the direction opposite to the electron beam. Thus, to extract the microwave signal axially, a cavity resonator reflector or cutoff waveguide section are widely used to reflect the TM01 mode backward wave to a TM01 mode forward wave. To provide a HPM Gaussian-like TE11 wave beam, an X-band relativistic BWO (RBWO) is proposed that uses a Bragg reflector instead of a cavity resonator reflector. Therefore, the device of interest is a RBWO consisting of two periodic slow wave structures (SWSs). In the original design, the inner radii of both SWSs were the same. For a uniform electron beam, a Bragg structure that is placed at the cathode (upstream) end, unfavorably perturbed the electron beam. Thus, two designs were proposed to minimize the interaction between the electron beam and the Bragg reflector structure. First, a uniform magnetic field was used with increased radial gap between the electron beam and the structure by optimizing the Bragg structure dimensions. This configuration generated a single frequency at X-band. The second design used a nonuniform magnetic field to control the gap between the electron beam and the SWSs, and to optimize the electron beam-Bragg structure interaction for C-band radiation generation. This is a dual-band RBWO operation with a possibility of generating a TE11-mode with linear polarization at single or dual frequencies (X- and C-bands). The radiation at C-band was generated in the Bragg structure as an asymmetric mode. The only known way to extract such a wave is by using a cutoff neck section. The disadvantage of the cutoff neck reflector is its proximity to the electron beam, and that leads to beam scrape-off and alignment difficulty. A novel idea of combining a periodic SWS with an electromagnetic band gap (EBG) medium for coupling and extracting a Gaussian radiation pattern is presented. Finally, it is well known that the power handling ability of a single RBWO is limited by material technology. Since it is always desired to generate higher microwave power than a single microwave device can produce, an N-way power combiner is proposed as an attractive alternative. A novel structure, which is an oversized circular waveguide to overcome standard waveguides limitations, is designed for power combining and higher order mode output.
Relativistic Backward Wave Oscilator, Bragg Reflector, Dual Band Source, Power Combiner, Gaussian Radiation Pattern
The research performed as part of this Dissertation was partially supported by AFOSR Grants FA9550-11-1-0200, FA9550-15-1-0094, and ONR Grant N000141310565.
Level of Degree
Electrical and Computer Engineering
First Committee Member (Chair)
Second Committee Member
Third Committee Member
Elfrgani, Ahmed. "RELATIVISTIC BACKWARD WAVE OSCILLATOR WITH A GAUSSIAN RADIATION PATTERN AND RELATED TECHNOLOGIES." (2016). https://digitalrepository.unm.edu/ece_etds/74