Ahmed Ibrahim Nashed
Design, Analysis, and Optimization of a Cherenkov Radiation Based Sub-mm/THz BWO
Safieddin Safavi-Naeini and Sujeet Chaudhuri (Adjunct)
Lies between the microwave and optical frequency bands; the Terahertz (THz) radiation is defined as an electromagnetic radiation that has a frequency extended from the microwave band to the infrared band (0.1 – 10 THz). Along with the high data rate communication offered in this frequency band, several applications; such as biomedical imaging and spectroscopy, are designed to use the special characteristics of material responses to THz radiation. The keystone in developing a commercial based THz application is the availability of a stable, compact, high power, and reasonably inexpensive sub-millimeter and THz radiation source. There are two main types of THz radiation either; a Continuous Wave (CW) radiation or pulsed radiation.
Within the last decade, the advance of the modern micro-fabrication techniques along with the huge advance in the modern computer architecture, gives rise to introducing and optimization several THz radiation sources. These sources can be categorized as; Laser based sources, Vacuum Electronic Devices (VEDs) and solid state source. Among these sources, the VED can be considered as the most powerful source, which can fulfill both the compact size requirement alongside the sufficient generated power. Generally speaking, the VED devices are consists mainly of three parts; the electron gun, the focusing circuit and the Slow Wave Structure (SWS). The focus of this thesis is the design of the SWS.
In this work, the Photonic Crystal PC structure is used to design a novel Double Defected PC (DD-PC) based SWS. Compared to other SWS, the DD-PC structure has no axial discontinuity, thus the DD-PC fabrication can be done with fewer complications. Furthermore, the generated electromagnetic radiation extraction point is located away from the electron beam, thus the electron beam-wave interaction process is not affected. Moreover, since the DD-PC based SWS is an all metallic structure, its capabilities in handling significant THz power generation is satisfied. The design of the DD-PC, operates at the 130GHz, was performed using a non-uniform Finite Difference Frequency Domain (FDFD) method. The main difference between non-uniform FDFD and the conventional FDFD method is the use of multi-level meshing along with the complex material constant. These two modifications allow the non-uniform FDFD method to calculate the modal solution of the waveguide structure accurately and efficiently. Due to the complexity of the structure and electron beam-wave interaction, theoretical analysis of an electron beam-wave interaction is difficult; therefore, the analysis is done using numerical simulation. The numerical simulation, applied to analyze electron beam-field interaction, can be divided into two connected simulations: the electromagnetic field simulation and the particle dynamics simulation. The Finite Difference Time Domain (FDTD) analysis is applied as the electromagnetic field simulation and at the same time, the Particle In Cell (PIC) is used to calculate each particle's physical parameters. The FDTD/PIC simulation was used as a test bench for analyze the THz generation via electron beam-wave interaction in the Backward Wave Oscillator (BWO) proposed using the DD-PC based SWS. Furthermore, the FDTD/PIC tool was used to optimize the BWO parameters for maximum THz radiation.
To alleviate the high electron beam requirement in the DD-PC based SWS, the Axial loaded Double Defected Photonic Crystal (ADD-PC) based SWS was implemented to work at 200GHz. Starting from the band gap structure; the design procedure, optimization curves was implemented at first using the HFSS code. Then, to test the designed ADD-PC based SWS, the simulation was performed using the FDTD/PIC CAD tool. Three main ADD-PC based SWS were used for the optimization process. Furthermore, the FDTD/PIC CAD was converted from the MATLAB implementation to a C++ implementation, thus the CAD simulation time was significantly reduced. Thus, the FDTD/PIC was used as a performance optimization tool, to tweak the generated electromagnetic field frequency to the pre-designed frequency. Finally, to show the frequency scalability of the proposed design, a design of a BWO operates at 650GHz was implemented using the HFSS Floquet’s mode analysis, and then verified using the HFSS probe-wheel experiment. The FDTD/PIC simulation results indicate that with the proposed BWO, an output power of almost 8W at 650GHz with 1.8% conversion efficiency can be achieved.