Review of the 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.

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

https://escholarship.org/content/qt37n7912d/qt37n7912d.pdf?t=pgbro1

Conservative fourth-order finite-volume Vlasov–Poisson solver for axisymmetric plasmas in cylindrical (r,vr,vθ) phase space coordinates

A new design for an inverted magnetron is presented and modeled both analytically, using a single particle smooth bore relativistic approach, and numerically, using a massively parallel electromagnetic particle-in-cell code, Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) code. Analysis and simulation confirm that the inverted magnetron design presented here is capable of oscillating in the π mode at axial magnetic fields of the order of ~0.1 T. ICEPIC simulations demonstrate that the inverted magnetron is capable of fast start-up, mitigation of mode competition, π-mode dominance, and high output power, of the order of 1 GW in some cases. Moreover, these performance features spanned over a variety of magnetic fields and input voltages. In simulations, the inverted magnetron design presented here demonstrated that end-loss current, a common source of energy leakage in relativistic magnetrons, has been eliminated as a source of energy loss. However, radio frequency output power efficiencies only remained comparable with standard relativistic designs. This was due to poor energy exchange between the particle and field. Thus, a refinement of the slow wave structure may be necessary.

Numerical Simulations of a Relativistic Inverted Magnetron

Effects of secondary electron emission (SEE) on the performance of a 6-cavity relativistic magnetron with transparent cathodes are probed through particle-in-cell simulations. Appropriate relations for the secondary electron yield have been developed and used. For comparisons, separate simulations have been performed with- and without electron cascading. Simulation results seem to indicate SEE to be detrimental to the power output due to deviations in the starting trajectories of secondary electrons, and the reduced fraction with synchronized rotational velocity. A higher reduction in output power is predicted with electron cascading, though mode competition was not seen at the 0.65 T field. A possible solution to mitigating SEE in magnetrons for high power microwave applications would be to alter the surface properties of emitting electrodes through irradiation, which can lead to graphitic film formation.

Assessing the role of secondary electron emission on the characteristics of 6-cavity magnetrons with transparent cathode through particle-in-cell simulations

Shaped cathodes enhance key magnetron performance parameters, such as output power, power efficiency, and impedance. Simulations using these novel designs showed a dramatic increase in the range of magnetic field in which the magnetron functions, a doubling of output power, an increase in efficiency, an elimination of mode competition, and halving of the startup time.

L. Bowers

Papers

Virtual Prototyping of Novel Cathode Designs for the Relativistic Magnetron