A HIGHLY EFFICIENT, MEGAWATT CLASS CONSTANT IMPEDANCE TUNABLE POWER EXTRACTION CIRCUIT FOR MOBILE IONOSPHERIC HEATERS
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Ionospheric modification (IM) refers to changes in the ambient properties of the ionosphere that are produced by humans. The ability to control and exploit ionospheric processes helps us understand naturally occurring phenomena like aurora, scintillations, and airglow. It also helps us improve trans-ionospheric communication and develop new applications that take advantage of the ionosphere as an active plasma medium. Ionospheric Heaters broadcast high power radio waves, typically in the radio frequency (RF) band, to modify the ionosphere in a controlled manner. These facilities are permanent terrestrial installations and do not presently support study at different latitudes. However, past IM experiments conducted at high latitudes across the world indicate a strong dependence of ionospheric processes on the geomagnetic latitude.
Mobile Ionospheric Heaters will allow for the first time, quantitative exploration of the ionosphere at different geomagnetic latitudes. These mobile structures must be relatively smaller the the existing arrays (small enough to fit on the barge of a ship) and highly efficient at the same time. The size and efficiency of the terrestrial heating units prevent their reuse in mobile structures. These factors motivate the need for developing novel heater units. Our research focused on a new high power, high-efficiency RF source that consists of a gridless tetrode RF tube and a highly efficient power extraction circuit. My research addresses the characterization and optimization of the power extraction circuit.
The power extraction circuit in the RF device under study converts the kinetic energy of a temporally modulated electron beam into electromagnetic energy. The beam is collected on an electrode surface and the resulting current is passed through the circuit. The circuit generates a potential in response to the current, and that potential is used to decelerate the beam. The circuit must be tunable to cover the desired broad frequency range (3 - 10 MHz), and the decelerating voltage should be independent of frequency to maintain high efficiency.
In this relatively low frequency range lumped element circuits are considered instead of cavities given the size limitations imposed. To achieve high power a high voltage beam is required. Consequently, matching the relatively high electron beam impedance (5 kΩ) to the load antenna (50 Ω) creates situations where high Q resonant circuits are necessary to achieve the high efficiencies required. We design a pi-circuit to achieve the impedance matching and validate the design experimentally.
Although experiments validate our design, measured efficiencies are too low because of parasitics in the circuit elements. The parasitics include the proximity effect and self-resonance in the elements. We model these effects that enhance the losses and limit the efficiency of the circuit.
Our research finds that the impedance transformation from beam to load (two orders of magnitude) imposes severe restrictions on single-stage design. Additionally, measurements of high Q components aren’t very reliable at higher frequencies. We propose a two-stage power extraction circuit that resolves both of these problems. Simulation results show a circuit design that is expected to yield the desired efficiency. The findings from the research in this thesis would help in the eventual construction and testing of the MW RF heating system that would facilitate a mobile heater for IM research in the future.