V.B. Neculaes

Low-noise microwave magnetrons by azimuthally varying axial magnetic field

A technique has been demonstrated to significantly reduce the noise in microwave oven magnetrons. The technique employs permanent magnets to generate an azimuthally varying axial magnetic field. Noise measurements are reported which show dramatic reductions in the noise of kW oven magnetrons operating near 2.45 GHz. The noise reduction near the carrier is some 30 dB. Microwave sidebands are reduced or eliminated. Noise reduction occurs at all anode currents, but is particularly significant at low current near the start-oscillation condition.

Cathode priming of a relativistic magnetron

A cathode priming technique of a relativistic magnetron is analyzed via a three-dimensional particle-in-cell simulation. By imposing a threefold azimuthal variation on the emitting cathode of a six-cavity relativistic magnetron, the electrons are prebunched at birth. This leads to fast startup of the pi mode with three electron spokes. Suppression of unwanted modes during startup is observed in the simulation.

Projection ablation lithography cathode for high-current, relativistic magnetron

Initial results are presented of an innovative cathode operating in a relativistic magnetron powered by an accelerator with parameters: −0.3 MV, 1–10 kA, and 0.5 μs pulse length. This cathode is fabricated by ablating a pattern on the cathode using a KrF laser. This projection ablation lithography (PAL) cathode has demonstrated fast current turn-on and microwave startup times have decreased from an average of 193 to 118 ns. The pulselength of 1 GHz microwave oscillation has increased from a 144 ns average to 217 ns. With these improvements in microwave startup and pulse length, the microwave power has approximately remained the same compared to the previously used cloth cathodes. A new triple-azimuthal emission region is tested as means of prebunching the electrons (“cathode priming”) into the three spokes desired for pi mode operation in a six-cavity magnetron. The Tri-PAL cathode priming results in the fastest startup and highest efficiency of relativistic magnetron microwave generation.

Simulation of rapid startup in microwave magnetrons with azimuthally varying axial magnetic fields

A method is proposed whereby the startup of a magnetron may be hastened. For a N-cavity magnetron operating in the pi mode, the formation of the N/2 electron spokes is much more rapid when an azimuthally varying axial magnetic field of N/2 periodicity is employed. Electromagnetic particle-in-cell code simulations are presented that show electron prebunching by the azimuthally varying axial magnetic field, long before the pi mode is excited.

Modeling and experimental studies of magnetron injection locking

A phase-locking model has been developed from circuit theory to qualitatively explain the various regimes observed in magnetron injection-locking experiments. The experiments utilize two continuous-wave oven magnetrons: one functions as an oscillator and the other as a driver. The model includes both magnetron-specific electronic conductance and frequency-pulling parameter. Both time and frequency domain solutions are developed from the model, allowing investigations into the growth and saturation as well as the frequency response of the output signal. This simplified model recovers qualitatively many of the phase-locking frequency characteristics observed in the experiments.

Low-noise microwave oven magnetrons with fast start-oscillation by azimuthally varying axial magnetic fields

A technique has been proven to significantly reduce the noise and to hasten the startup in microwave signals from dc operating oven magnetrons. The technique is based on imposition of an azimuthally varying axial magnetic field in magnetrons. For an N-cavity magnetron operating in pi-mode, the optimal number of periods of azimuthal variations in the axial magnetic field is N/2. This technique demonstrates the fastest startup and lowest start-oscillation current by prebunching the electrons. Measurements show dramatic reduction in noise of 600-800 W oven magnetrons operating dc near 2.45 GHz. The sidebands are eliminated (*30 dB) and the noise within 50 MHz from the carrier is reduced by 30 dB. Noise reduction is observed at low (up to 60 mA), medium (60-200 mA) and high currents (200-300 mA), for both aged and fresh oven magnetrons. The reduction in power and in electronic efficiency is of order of 10%, in this low-noise, fast startup operation.

Relativistic magnetron driven by a microsecond E-beam accelerator with a ceramic insulator

Relativistic magnetron experiments performed on a six-cavity device have generated over 300 MW total microwave power near 1 GHz. These experiments were driven by the long-pulse electron beam from an accelerator with parameters as follows: voltage of *300 kV, current of 1-10 kA, and typical pulselength of 0.5 ms. This paper reports investigations of high-power microwave generation, mode competition, and pulse shortening for the relativistic magnetron with a ceramic insulator compared to a plastic insulator. The ceramic insulator improves the vacuum by a factor of ten (to 10/sup *7/ torr range) and flattens the voltage of the accelerator. Relativistic magnetron performance with the ceramic insulator shows increased microwave power and pulselength over the plastic insulator. Effects of RF breakdown in the extraction waveguide on peak microwave power and pulselength are also investigated by utilizing SF/sub 6/ in one or both of the extraction waveguides.

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.

Rapid kinematic bunching and parametric instability in a crossed-field gap with a periodic magnetic field

Single particle orbit considerations show that the cycloidal orbits of electrons in a gap with crossed electric and magnetic fields lead to rapid spoke formation if the external magnetic field has a periodic variation. This spoke formation is primarily a result of kinematic bunching, which is independent of the radio frequency electric field and of the space charge field. A parametric instability in the orbits, which brings a fraction of the electrons from the cathode to the anode region, is discovered. These results are examined in light of the recent rapid startup, low noise magnetron experiments and simulations that employed periodic, azimuthal perturbations in the axial magnetic field.

Simulations of magnetic priming in a relativistic magnetron

Two-dimensional simulations have been performed on a six-vane relativistic magnetron with uniform axial magnetic fields versus azimuthally varying axial magnetic fields, defined as magnetic priming. Electron phase-space plots show rapid growth of the /spl pi/-mode when the axial magnetic field has three-azimuthal perturbations: it takes 36 ns for the /spl pi/-mode to dominate in the uniform magnetic field case versus only 13 ns for the /spl pi/-mode to dominate in the case with magnetic priming imposed. RF electric field plots versus time show the suppression of the 2/spl pi//3-mode when magnetic priming is imposed.