Steven C. Exelby

Additively Manufactured High Power Microwave Anodes

Additively manufactured components were successfully fielded for the first time in a relativistic crossed-field device. Anode structures for a relativistic planar magnetron were 3-D printed from a photopolymer using a stereolithography printing process. One anode was electroplated with copper (RPM-12b), whereas the other was thermal sprayed with copper (RPM-12c). The coating thicknesses at the vane tips were approximately 0.18 and 0.23 mm, respectively. The performance and durability of these structures were evaluated in comparison with a solid aluminum anode (RPM-12a) fabricated via conventional machining. The experimental parameters were cathode voltages between -150 and -300 kV, voltage pulse lengths of 200 to 600 ns, axial magnetic fields of 0.13 to 0.31 T, peak anode currents from 1 to 7 kA, and a base operating pressure of 9 × 10-6 torr. The 3-D printed anodes demonstrated microwave performance comparable to the aluminum anode, generating microwave powers in excess of 150 MW, with an average instantaneous peak total efficiency of 27% ± 10%. After 100 shots on each structure, neither anode showed any signs of operationally induced damage. The anodes did, however, have a higher rate of postshot outgassing, emitting 32% and 23% more CO2 per shot, respectively.

Harmonic frequency generation in the multi-frequency recirculating planar magnetron

The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a variant of the Recirculating Planar Magnetron (RPM), a crossed-field, high power microwave (HPM) source developed at the University of Michigan1. The MFRPM geometry offers potential advantages over traditional cylindrical cavity magnetrons, such as a large cathode area, with the added benefit of producing multiple frequencies using a single HPM source. Previous research involved the design, fabrication, and demonstration of the MFRPM-6+8, a prototype consisting of a 6-cavity 1-GHz slow-wave structure (SWS) and an 8-cavity 2-GHz SWS. Experiments have successfully extracted simultaneous dual-frequency oscillations (1-GHz, 20 MW and 2-GHz, 7 MW) from the MFRPM-6+8 prototype driven by the Michigan Electron Long Beam Accelerator with a Ceramic insulator (MELBA-C) using a -300kV, 0.3-1.0 μs pulse applied to the cathode.

Experimental microwave power extraction in the Multi-Frequency Recirculating Planar Magnetron

The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a variant of the Recirculating Planar Magnetron (RPM), a crossed-field oscillator capable of producing high-power microwaves (HPM). The first multi-frequency RPM, termed the MFRPM-6+8, consists of a 6-cavity 1-GHz and 8-cavity 2-GHz set of planar cavity arrays coupled by cylindrical electron beam / RF recirculation bends. Recent experimental work involved installation of an axial microwave power extraction assembly. Experiments are currently underway using a -300 kV, 200-300 ns voltage pulse, a 0.1-0.3 T axial magnetic field, and a Mode-Control Cathode (MCC) with a velvet electron emitter.

Experimental harmonic characterization in the Multi-Frequency Recirculating Planar Magnetron

The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a crossed-field oscillator capable of producing high-power microwaves (HPM). The MFRPM-6+8 is the first prototype MFRPM, which consists of a 6-cavity 1-GHz and 8-cavity 2-GHz set of planar cavity arrays coupled by cylindrical electron beam / RF recirculation bends. Recent experimental work conducted using a -300 kV, 200-300 ns voltage pulse, 0.1-0.3 T axial magnetic field, and Mode-Control Cathode (MCC) showed evidence of potentially significant harmonic 2- and 4-GHz frequency generation from the 1-GHz cavity array. Experiments are currently underway to characterize these harmonics.

Additively manufactured anodes in a relativistic Planar Magnetron

Recent experiments on the UM Recirculating Planar Magnetron (RPM), have explored the use of 3-D printed components in a HPM system. The system was driven by MELBA-C, a Marx-Abramyan system which delivers a -300 kV voltage pulse for 0.3-1.0 us, with a 0.13-0.31 T axial magnetic field applied by a pair of electromagnets. Anode blocks were printed from Water Shed XC 11122 photopolymer using a stereolithography process, and prepared with either a spray-coated or electroplated finish. Both manufacturing processes were compared against baseline data for a machined aluminum anode, demonstrating improvements in pulsed magnetic field penetration, while maintaining similar performance for power output, oscillation frequency, and mode stability. Residual gas analysis indicated similar outgassing behavior, with a higher incidence of gas spikes for the plastic anodes. With over 100 shots on each 3-D printed structure, there was no evidence of beam-induced degradation.

Additively manufactured structures for high power microwave devices

Additive manufacturing techniques, like 3D printing, offer many benefits to the design and manufacture of high power microwave (HPM) components, including the ability to conduct rapid prototyping. A good prototype, however, must be a suitable approximation of the production device. To that end, we have evaluated metallized 3D printed plastic anodes for use in a recirculating planar magnetron. While other authors have examined a metallized plastic structure in an HPM device, our work is the first use of a 3D printed structure, and the first where the electron beam impacts the 3D printed anode.

Multi-frequency recirculating planar magnetrons

The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a type of Recirculating Planar Magnetron (RPM) designed for tunable or simultaneous oscillation at more than one primary frequency. Like the RPM, the MFRPM is a crossed-field, high power microwave (HPM) source that integrates the design advantages of the RPM with the added benefit of producing multiple frequencies using a single HPM device. Design of an L-S-band prototype is underway for a 0.2 T axial magnetic field and a -300 kV voltage pulse.

Experimental progress on a prototype multifrequency recirculating planar magnetron

The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a variant of the Recirculating Planar Magnetron, a crossed-field, high power microwave source1. The MFRPM integrates the same advantages over traditional cylindrical cavity magnetrons as UMs original RPM with the added benefit of producing multiple frequencies using a single HPM device. Previous research involved the design and fabrication of the MFRPM-6+8, a prototype MFRPM consisting of a 6-cavity 1-GHz slow-wave structure (SWS) and an 8-cavity 2-GHz SWS. Experiments have successfully demonstrated simultaneous dual-frequency oscillations in the unloaded MFRPM-6+8 prototype driven by the Michigan Electron Long Beam Accelerator with a Ceramic insulator (MELBA-C) using a -300kV, 0.3-1.0 μs pulse applied to the cathode.