Geoffrey B. Greening

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.

Stability of Brillouin flow in planar, conventional, and inverted magnetrons

The Brillouin flow is the prevalent flow in crossed-field devices. We systematically study its stability in the conventional, planar, and inverted magnetron geometry. To investigate the intrinsic negative mass effect in Brillouin flow, we consider electrostatic modes in a nonrelativistic, smooth bore magnetron. We found that the Brillouin flow in the inverted magnetron is more unstable than that in a planar magnetron, which in turn is more unstable than that in the conventional magnetron. Thus, oscillations in the inverted magnetron may startup faster than the conventional magnetron. This result is consistent with simulations, and with the negative mass property in the inverted magnetron configuration. Inclusion of relativistic effects and electromagnetic effects does not qualitatively change these conclusions.

Harmonic Content in the Beam Current in a Traveling-Wave Tube

In a klystron, charge overtaking of electrons leads to an infinity of ac current on the electron beam. This paper extends the klystron theory of orbital bunching to a traveling-wave tube (TWT). We calculate the harmonic content of the beam current in a TWT that results from an input signal of a single frequency. We assume that the electron orbits are governed by Pierces classical three-wave, linear theory. The crowding of these linear orbits may lead to charge overtaking and, therefore, harmonic generation on the beam current, as in a klystron. We analytically calculate the buildup of harmonic content as a function of tube length from the input, and compare the results with the CHRISTINE code. Good agreement is found. Also found is the surprisingly high level of harmonic contents in the electron beam current, even when the TWT operates in the small signal regime. A dimensionless bunching parameter for a TWT, X = (2Pin/(PbC))1/2, is identified, which characterizes the harmonic content in the ac beam current, where Pin is the input power of the signal, Pb is the dc beam power, and C is Pierces gain parameter.

Stability of Brillouin flow in the presence of slow-wave structure

Including a slow-wave structure (SWS) on the anode in the conventional, planar, and inverted magnetron, we systematically study the linear stability of Brillouin flow, which is the prevalent flow in crossed-field devices. The analytic treatment is fully relativistic and fully electromagnetic, and it incorporates the equilibrium density profile, flow profile, and electric field and magnetic field profiles in the linear stability analysis. Using parameters similar to the University of Michigans recirculating planar magnetron, the numerical data show that the resonant interaction of the vacuum circuit mode and the corresponding smooth-bore diocotron-like mode is the dominant cause for instability. This resonant interaction is far more important than the intrinsic negative (positive) mass property of electrons in the inverted (conventional) magnetron geometry. It is absent in either the smooth-bore magnetron or under the electrostatic assumption, one or both of which was almost always adopted in prior analyt...

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.

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.

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.

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.

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.