Brad W. Hoff

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.

Magnetic Priming at the Cathode of a Relativistic Magnetron

Experiments have been performed in testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high-power microwave performance. Magnetic priming consists of N/2 azimuthal magnetic perturbations applied to an N-cavity magnetron for rapid generation of the desired number of electron spokes for the pi-mode. Magnetic perturbations were imposed by utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 120 apart. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for -mode shots was found to be higher in the magnetically primed case by almost a factor of two. Increases in mean microwave pulsewidth were also observed in the magnetically primed case when compared to the unprimed case (66-ns primed versus 50-ns unprimed). Magnetron starting current for the magnetically primed pi-mode exhibited a reduction to 69% of the unprimed baseline starting current.

Magnetron priming by multiple cathodes

A relativistic magnetron priming technique using multiple cathodes is simulated with a three-dimensional, fully electromagnetic, particle-in-cell code. This technique is based on electron emission from N∕2 individual cathodes in an N-cavity magnetron to prime the π mode. In the case of the six-cavity relativistic magnetron, π-mode start-oscillation times are reduced up to a factor of 4, and mode competition is suppressed. Most significantly, the highest microwave field power is observed by utilizing three cathodes compared to other recently explored priming techniques.

Radio frequency priming of a long-pulse relativistic magnetron

Rapid startup, increased pulsewidth, and mode locking of magnetrons have been explored experimentally on a relativistic magnetron by radio frequency (RF) priming. Experiments utilize a -300 kV, 2-8 kA, 300-500-ns electron beam to drive a Titan six-vane relativistic magnetron (5-100 MW output power in each of the three waveguides). The RF priming source is a 100-kW pulsed magnetron operating at 1.27-1.32 GHz. Tuning stubs are utilized in the Titan structure to adjust the frequency of the relativistic magnetron to match that of the priming source. Experiments are performed on rising sun as well as standard anode configurations. Magnetron start-oscillation time, pulsewidth, and pi-mode locking are compared with RF priming versus the unprimed case. The results show significant reductions in microwave output delay and mode competition even when Adlers Relation is not satisfied

Magnetic perturbation effects on noise and startup in DC-operating oven magnetrons

Previous experiments demonstrated that imposing an azimuthally varying axial magnetic field, axially asymmetric, in dc-operating oven magnetrons causes rapid mode growth (by magnetic priming) and significant noise reduction. This configuration was previously implemented by adding five perturbing magnets on the upper existing magnet of the magnetron. Experiments reported here add five perturbing magnets on each of the two existing magnets of the magnetron, restoring the axial symmetry of the magnetic field, while maintaining the five-fold azimuthal magnetic field symmetry. Compared with the unperturbed magnetic field case, it has been observed that the noise close to the carrier is reduced by up to 20 dB, while the sidebands are not completely eliminated for medium and high currents. Magnetron start-oscillation currents are somewhat higher for this axially symmetric, azimuthally varying magnetic field as compared to the baseline unperturbed magnetic field.

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.

A re-examination of the Buneman–Hartree condition in a cylindrical smooth-bore relativistic magnetron

The Buneman–Hartree condition is re-examined in a cylindrical, smooth-bore, relativistic magnetron using both the conventional, single particle model, and the Brillouin flow model. These two models yield the same result for the Buneman–Hartree condition only in the limit of a planar magnetron. When b/a=1.3, where a is the cathode radius and b (>a) is the anode radius, the difference in the two models becomes significant. When b/a=4 the difference is acute, the Buneman–Hartree magnetic field at a given voltage in the Brillouin flow model exceeds four times that in the single particle model. Such a difference is always present, whether the voltage is relativistic or not. These results are quantified for b/a⪢1 using Davidson’s model, conveniently cast in terms of the normalized gap voltage and normalized magnetic flux imposed on the cylindrical magnetron. A comparison with the University of Michigan/L-3 relativistic magnetron experiment is given.

Negative, positive, and infinite mass properties of a rotating electron beam

An electron rotating under a uniform axial magnetic field and a radial electric field exhibits an effective mass that may be negative, positive, or infinite, in response to an azimuthal electric field. This paper reports simulation results that show instability and stability when the effective mass are negative and positive, respectively, depending on the magnitude and orientation of the radial electric field. Thus, the inverted magnetron would have a much faster startup than the conventional magnetron, an important consideration for pulsed operation. When the effective mass is infinite, the electrons hardly respond to an azimuthal ac electric field.

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.

Experiments on peer-to-peer locking of magnetrons

Experiments on peer-to-peer locking of 2 kW magnetrons are performed. These experiments verify the recently developed theory on the condition under which the two nonlinear oscillators may be locked to a common frequency. Dependent on the coupling, the frequency of oscillation when locking occurs does not necessarily lie between the free running frequencies of the two isolated, stand-alone magnetrons. Likewise, when the locking condition is violated, the beat frequency is not necessarily equal to the difference between these free running frequencies.