Highly Efficient, Megawatt Class RF Power Sources for Mobile Ionospheric Heaters

Abstract

In this thesis, we consider the development of a highly efficient, grid-less tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in a mobile ionospheric heater. Such a heater has potential advantages over the stationary facilities, such as HAARP (High-Frequency Active Auroral Research Program), found at high latitude. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The beam current is controlled using an annular modulating electrode (mod-anode) placed around the annular emitter on the cathode. This feature removes the traditional semi-transparent grid and the problems associated with interception of current beam at the grid. Three different device configurations based on differing magnetic field confinement were considered. Model A, which has a constant focusing magnetic field and no beam compression, offers the highest interaction efficiency. However, to generate a uniform and constant magnetic field over the whole device length would require the use of a large and bulky solenoid. This makes the setup in the case of Model A much larger (and much heavier). Model B has a magnetic field that is up-tapered from the cathode towards the anode and collector where the bulk part of the solenoid is located. This configuration retains the compression of the electron beam to maintain a high efficiency while keeping the size of the device manageable. It has a lower efficiency than Model A, but it provides a larger cathode area in than in Model A which mitigates cathode loading. In the case of Model C, there no guiding magnetic field and is the most compact, but its interaction efficiency is the lowest among the three device types. Model C also uses two modulating anodes maintained at varying voltages to provide electrostatic focusing of the electron beam. It is still operated in the class D regime by switching the two mod-anodes of Model C on and off together. However, the voltage swing will be much larger compared to Model A.

A theoretical analysis to find the optimal operating point for model A is presented. In particular, the trade-off between the peak current and the duration of the current pulse is analyzed. The beam distributions in axial and transverse momenta and in total electron energies, before and after the decelerating gap, calculated using the Michelle code are presented for Model A. Using static-case Michelle simulation results, the instantaneous and average device efficiencies of the three models were maximized while reducing the device size by studying the influence of electrode geometry (Anode-Cathode Gap, and Anode-Collector gap and shapes) on the device efficiency. After optimizing the device geometry for these three different models, time-domain simulations with secondary electrons were performed. For model A, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector, rather than at the cathode or anode. For model B, most of the secondary electrons are trapped in the collecting region due to an effect called magnetic mirroring from the up tapering of the magnetic field towards the collector region. In Model C, the secondary electrons are largely scattered throughout the tetrode due to the lack of magnetic field confinement making it much harder to prevent the loss of efficiency. In short, three different versions of the grid-less tetrode have been proposed and studied. The optimized version of these devices have efficiencies ranging from 81% to 91.5%. The choice of the optimal design for real systems may depend on a number of tradeoffs. In the situations where the weight and size of a system play a crucial role, Model C could be more preferable with the penalty of lower efficiency. In turn, Model A can offer the highest efficiency, but the solenoids required for maintaining a constant magnetic field along the entire device could be very heavy and bulky. In comparison, Model B offers a middle ground among the three models on compactness and efficiency.