INVESTIGATION ON SPECIFIC INTERACTION / PROPAGATING STRUCTURES FOR SUB-THz WAVE PROPAGATION

Abstract

The full report is prepared based on the two major research activities in which one is Graphene based advanced transmission line attenuators and other is high power microwave sources like Gyrotrons. Graphene is a 2-dimensional material with predominant properties and is getting due attention from mm-wave,THz and optical regime research communities . Graphene exhibits a versatile properties of thermal conductivity, tensile strength, variable carrier concentration with electrical conductivity. Its tunable parameters like electrical conductivity and relative permitivity opens many novelties for the design of various microwave circuits and transmission lines. Now the technology is rapidly shifting to millimeter and sub-THz wave frequency bands to make things more compact and as well as it is more approaching towards digitizing or tuning of the circuit parameters on-line without any physical hindrance. considering the facility to tune the carrier concentration and electrical conductivity of Graphene by applying external electric field or magnetic field biasing, it is being used in semiconductor devices at optical frequencies and some active devices at THz wave frequencies. So the graphene’s carrier density nc varies with either its gate voltage Vg or chemical potential mc. As the graphenes properties lies between metals and semiconductors, there are several research activities reported on Graphene Photonics, Plasmonics, and Optoelectronics. Graphene is even being considered for the Surface Plasmon Polaritons (SPPs) propagation through the parallel plate waveguides and cylindrical waveguides at optical frequencies. Significant works were published on Graphene based Patch antennas, mixers, power dividers and traveling-wave amplifiers at THz frequencies. In recent, a few articles reported on the analysis of electromagnetic wave propagation through various graphene based structures at mm-wave frequencies. ii Parallel plate waveguides presumed of using graphene layers and PEC for supporting the quasi-transverse mode of propagation was reported in . Dispersion relation has been discussed in those papers for the TE and TM modes of propagation by considering one plate is of graphene sheet and other is of perfectly electric conductor (PEC) and it was further extended to two parallel graphene layers as parallel plate waveguides(PPWG).A rectangular waveguide with three sides PEC and one broader side as graphene was also discussed for the hybrid modes propagation inside the waveguide in presence of external magnetic bias and that were analyzed using mode matching techniques. Bandpass Filter like characteristics were analyzed using rectangular waveguides loaded with graphene. filter circuits were achieved by making graphene mounted on dielectric slab placed normal to wave propagation in rectangular waveguide is considered for the transient analysis. Some microwave circuits of phase modulators using graphene sheets were reported where the graphene layers are placed in axial direction of wave propagation in a waveguide to achieve the required properties. The concept of traveling wave propagation through graphene sheet in an amplifier like device was also deliberated in. Numerical analysis is reported for analyzing the propagation characteristics of rectangular waveguides with magnetic biasing. The current work focuses on the possibility of developing digitally tunable inline waveguide and coaxial transmission lines based attenuator using embedded graphene sheet. This proposed analysis can be used for designing required attenuator profiles in the transmission lines of various microwave circuits. At present, variable or tunable attenuators at the microwave frequencies are being developed using PIN diode and these will be applicable to medium bandwidth and low power applications. With the present analysis, new design ideas can be explored for developing various transmission line based attenuation structures that can handle ultra-high bandwidths and high power applications. Gyrotrons are most convenient devices for providing Megawatt class of high power EM waves at millimetric and sub-THz wave frequencies and are prominently iii used in Industrial, Scientific, Medical (ISM) applications. Theses Gyrotrons commonly used in the following applications; the study of plasma physics, electron cyclotron resonance heating(ECRH), electron cyclotron current drive(ECCD). In this report, we investigate the design of triode type and diode type Magnetron Injection Guns(MIG), Super conducting magnetic coils for beam gyration and focusing along the axis and Particle in Cell(PIC) simulations on RF cavity. Beam optics simulations are carried out for the design of MIG by using particle tracking solvers or codes. Magneto-static simulations are carried out for the design super conducting magnetic coils to the required magnetic field profiles at the Gun and Cavity regions by using magneto-static solvers.