Matthew Franzi

Recirculating Planar Magnetrons for High-Power High-Frequency Radiation Generation

We present designs and simulations of a new class of magnetron, the recirculating planar magnetron. This magnetron has numerous advantages as a high-power microwave generator, including larger cathode and anode area, fast start-up, and compact microwave extraction geometry. The following two geometries are demonstrated by electromagnetic particle-in-cell codes: 1) axial magnetic field with radial electric field and 2) radial magnetic field with axial electric field.

Multipactor susceptibility on a dielectric with a bias dc electric field and a background gas

We use Monte Carlo simulations and analytical calculations to derive the condition for the onset of multipactor discharge on a dielectric surface at various combinations of the bias dc electric field, rf electric field, and background pressures of noble gases, such as Argon. It is found that the presence of a tangential bias dc electric field on the dielectric surface lowers the magnitude of rf electric field threshold to initiate multipactor, therefore plausibly offering robust protection against high power microwaves. The presence of low pressure gases may lead to a lower multipactor saturation level, however. The combined effects of tangential dc electric field and external gases on multipactor susceptibility are presented.

Recirculating-Planar-Magnetron Simulations and Experiment

Microwave oscillation has been measured for the first time in a 12-cavity axial-magnetic-field recirculating planar magnetron, designed to operate in π mode at 1 GHz. The device operates with a -300-kV pulsed cathode voltage and a 0.2-T axial magnetic field, and oscillates at transverse currents exceeding 1 kA when driven by an electron beam pulselength between 0.5 and 1 μs. Microwave pulses were measured at frequencies between 0.97-1 GHz and achieved several hundred nanoseconds in length. Mode competition was observed between the π and 5 π/6 modes.

Microwave window breakdown experiments and simulations on the UM/L-3 relativistic magnetron

Experiments have been performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. In the baseline case, acrylic microwave windows were mounted between three of the waveguide coupling cavities in the anode block vacuum housing and the output waveguides. Each of the six 3 cm deep coupling cavities is separated from its corresponding anode cavity by a 1.75 cm wide aperture. In the baseline case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. In the new window configuration, three Air Force Research Laboratory-designed, vacuum-rated directional coupler waveguide segments were mounted between the coupling cavities and the microwave windows. The inclusion of the vacuum side power couplers moved the microwave windows an additional 30 cm away from the anode apertures. Additionally, the Lucite microwave windows were replaced with polycarbonate windows and the microwave window mounts were redesigned to better maintain waveguide continuity in the region around the microwave windows. No vacuum side window breakdown was observed in the new window configuration at single waveguide output powers of 120+MW (a factor of 3 increase in measured microwave pulse duration and factor of 3 increase in measured peak power over the baseline case). Simulations were performed to investigate likely causes for the window breakdown in the original configuration. Results from these simulations have shown that in the original configuration, at typical operating voltage and magnetic field ranges, electrons emitted from the anode block microwave apertures strike the windows with a mean kinetic energy of 33 keV with a standard deviation of 14 keV. Calculations performed using electron impact angle and energy data predict a first generation secondary electron yield of 65% of the primary electron population. The effects of the primary aperture electron impacts, combined with multiplication of the secondary populations, were determined to be the likely causes of the poor microwave window performance in the original configuration.

Passive mode control in the recirculating planar magnetron

Preliminary experiments of the recirculating planar magnetron microwave source have demonstrated that the device oscillates but is susceptible to intense mode competition due, in part, to poor coupling of RF fields between the two planar oscillators. A novel method of improving the cross-oscillator coupling has been simulated in the periodically slotted mode control cathode (MCC). The MCC, as opposed to a solid conductor, is designed to electromagnetically couple both planar oscillators by allowing for the propagation of RF fields and electrons through resonantly tuned gaps in the cathode. Using the MCC, a 12-cavity anode block with a simulated 1 GHz and 0.26 c phase velocity (where c is the speed of light) was able to achieve in-phase oscillations between the two sides of the device in as little as 30 ns. An analytic study of the modified resonant structure predicts the MCCs ability to direct the RF fields to provide tunable mode separation in the recirculating planar magnetron. The self-consistent solu...

Three-Dimensional Simulations of Magnetic Priming of a Relativistic Magnetron

Using a hybrid approach, 3-D simulations of magnetic priming of a relativistic magnetron have been performed. The primed magnetic field values were calculated using a magnetostatics code (Magnum) and then imported into a particle-in-cell code (Magic PIC) and run for the case of a six-vane relativistic magnetron. The magnetically perturbative structures chosen for implementation in these simulations were sets of three high-permeability wires of various lengths, which would be placed within the cathode, the anode, or, in the combined case, both the cathode and anode. In the best-performing cathode-wire case (three 4-cm wires), magnetic priming was found to reduce the start-oscillation time of the magnetron to 50% that of the unprimed case. When wires were embedded in both the cathode and the anode, the best-performing case (4-cm cathode wires and 4-cm anode wires) was found to start oscillating at 30% of the start-oscillation time of the unprimed case. The cases of magnetically primed magnetrons were found to exhibit slightly reduced equilibrium power levels, compared with the unprimed case.

Microwave Power and Phase Measurements on a Recirculating Planar Magnetron

Calibrated microwave power and phase measurements are presented for the first recirculating planar magnetron prototype consisting of two coupled six-cavity 1-GHz planar cavity arrays. The results are presented for a solid cathode and two mode-control cathodes (MCCs) with aluminum or velvet electron emitters. The measurements were conducted using a prototype coaxial microwave power extraction scheme. The experimental operating parameters included: pulsed cathode voltages between -250 and -300 kV, voltage pulselengths of 200-600 μs, axial magnetic fields of 0.1-0.32 T, and entrance currents of 1-10 kA. The results showed improved oscillator frequency locking for the MCCs and increases in power and efficiency using the velvet electron emitter.

21.1: Recirculating-planar-magnetrons for high power, high-frequency radiation generation

We present results for a new class of magnetrons that utilize two planar sections coupled by recirculating bends.[1] The Recirculating Planar Magnetron (RPM) has the advantages of large cathode and anode areas for high currents and improved heat dissipation capability. There are two major embodiments of the RPM: 1) axial-B/radial-E and 2) radial-B/ axial-E fields. The axial magnetic field embodiment can be operated in either conventional (cathode inside) or inverted (cathode on outside) configurations. Simulations show that the recirculating planar magnetron has advantages for both high power and high frequency radiation generation. Simulations and ongoing experiments will be presented

A Pi-mode extraction scheme for the axial B-field recirculating planar magnetron

A recirculating planar magnetron (RPM), operating in the π-mode and utilizing a compact, waveguide-based extraction scheme was simulated using ICEPIC. At an applied voltage of 300 kV and B-field of 0.130T, output power was 430 MW at 43% efficiency. The oscillator was found to operate at frequency of 2.23 GHz.

Coaxial all cavity extraction in the Recirculating Planar Magnetron

Recent experiments on the first Recirculating Planar Magnetron (RPM) prototype, the L-band RPM-12a, have demonstrated successful generation of microwave power in the vicinity of pi-mode at 1 GHz using a -300 kV pulsed voltage and 0.2 T axial magnetic field. However, these initial experiments were designed primarily to demonstrate feasibility and validate the RF dispersion relationship. This prototype has not demonstrated the efficient extraction of microwave power into waveguides. A novel approach to power extraction using a coaxial adaptation of the “all cavity extractor” is currently under development at the University of Michigan. The Coaxial All Cavity Extractor (CACE) is a highly compact axial extraction technique that is specifically designed to accommodate the planar slow wave structures of the RPM. Concepts and design of the extractor are presented as well as a new experimental prototype RPM-CACE, a 12-cavity device simulated to yield 450 MW with 60% efficiency at 1.9 GHz.