Coaxial ferrimagnetic based gyromagnetic nonlinear transmission lines as compact high power microwave sources

Coaxial ferrimagnetic Nonlinear Transmission Lines (NLTL) can be used as pulsed, solid-state, High Power Microwave (HPM) sources. An NLTL can operate in either pulse sharpening or microwave generation mode depending on the initial magnetization of the ferrimagnetic material and the applied magnetic field. Present research at Texas Tech University is focused on creating an array of NLTLs operating in microwave generation mode for use as a compact HPM system. The microwaves produced by NLTLs are due to the motion of magnetic moments in the ferrimagnetic material through damped gyromagnetic precession. This motion is described by the Landau-Lifshitz Gilbert (LLG) equation and is the primary method for analyzing design requirements. Since the precession frequency is proportional to the applied field, the gyromagnetic ratio constant, and inversely proportional to the phenomenological damping factor, the NLTL is a tunable frequency HPM source. The permeability of the ferrimagnetic material can be altered by applying a biasing magnetic field thereby allowing for an adjustable delay on each line by controlling the phase velocity. The NLTL system uses a pressurized spark gap to generate a 15 kV to 40 kV voltage pulse with a rise time on the order of 2 ns to drive coaxial transmission lines insulated with pressurized sulfur hexafluoride (SF$6$). The ferrimagnetic material is in the form of toroidal ferrites around the inner conductor of the coaxial line. The resulting source is frequency adjustable from 1 GHz to 4 GHz with an output power of 40 MW. A two-way transient high power guided combiner was designed and tested and results are presented. An NLTL model was developed using a Finite Element Method (FEM) simulation in COMSOL. The simulation solves the LLG equations for a coaxial geometry allowing for calculation of effective fields and estimation of the operating frequency.