Innovative Crossed-Field Devices for the Generation of High Power Microwaves

Abstract

Modern High Power Microwave (HPM) initiatives pursue challenges in fundamental science, such as fusion research and particle accelerators, as well as industrial applications and homeland security. RADAR, telecommunications, and counter-IED (improvised explosive device) measures also rely on HPM. Crossed-field devices, like the magnetron and magnetically insulated line oscillator (MILO), are a subclass of microwave sources capable of delivering HPM. This dissertation describes the theory, simulation, and design processes applied to produce novel contributions in two separate projects, one a relativistic magnetron and the other a MILO. The magnetron is an inherently narrowband source, which is undesirable for applications such as counter-IED technologies. Past Recirculating Planar Magnetron (RPM) concepts have proven multispectral microwave generation in magnetrons, and the Harmonic-RPM was designed to expand and further understand these capabilities. In the innovative configuration of this dissertation, the HRPM implements a 1 GHz, L-Band Oscillator (LBO) and a 2 GHz, S-Band Oscillator (SBO) on the same side of the planar cathode, both that are made frequency-agile by leveraging the novel phenomenon of harmonic frequency locking. An experimental investigation of harmonic frequency locking between the LBO and SBO demonstrated the LBO can be used to control the SBO frequency and phase through harmonic beam content, and the SBO responds to this excitation at varying degrees depending on its quality factor. In the low quality factor experiment, the HRPM was driven at 255 ± 19 kV, 1.23 ± 0.32 kA, producing microwave bursts up to 40 MW with shot-averaged pulse duration of 77 ± 17 ns at 7.3 ± 2.4% total efficiency. When the HRPM was properly tuned to excite the SBO on resonance in the low quality factor experiment, the shot-averaged SBO power was 28 ± 9 MW at 2.102 GHz ± 1.5 MHz. Harmonic frequency locking enabled tuning of the SBO over a range of 33 MHz in this experiment, corresponding to 1.6% tunability. By reversing electron rotation direction by the magnetic field, it was shown that the SBO was no longer influenced by the harmonic content of the LBO-modulated beam. The MILO is a variant of the magnetron, differentiating itself in its method of producing the magnetic field for synchronous interaction. The magnetron uses permanent magnets or pulsed solenoidal coils, whereas the MILO magnetic field is established by large, pulsed currents along the central axis of the device. The vast majority of MILO devices in the literature operate at a low impedance (V/I) of roughly 10 Ω and typically 50-60 kA, resulting in efficiencies commonly in the single digits of percent. The MILO investigated in this dissertation was the first to demonstrate oscillations at less than 10 kA currents, at -240 kV for an impedance of 25-30 Ω. Microwave bursts were observed up to 25 MW at 1.5% efficiency with shot-averaged frequency and pulse duration of 993 ± 7 MHz and 118 ± 43 ns, respectively. The shot-averaged output power was highly irreproducible at 10 ± 7 MW, and is significantly lower than simulation estimates. These experiments were compared with a contemporary theoretical treatment of Brillouin flow in the coaxial MILO geometry, which revealed consistent device operation in a unique condition near the Hull cutoff condition.