Herman Bosman

Microwave absorption on a thin film

With the use of a simple model, it is shown that a thin film of contaminant on a microwave window may absorb up to 50% of the incident power, even if the film thickness is only a small fraction of its resistive skin depth. This unexpectedly large amount of absorption is conjectured to have played a significant role in window failure. The temperature rise in a thin film is estimated.

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.

Heating of a particulate by radio-frequency electric and magnetic fields

With the use of a highly symmetrical model, the heating of a spherical particulate by a predominantly radio-frequency electric field and by a predominantly rf magnetic field is solved exactly using the Maxwell equations. It is found that, in general, heating by the rf magnetic field is dominant whenever δ<a, where δ is the resistive skin depth and a is the radius of the particulate, which may either be nonmagnetic or magnetic. The known analytic scaling laws in the various regimes are recovered, from the static case to very high frequency, subject to λ⪢a, where λ is the free space wavelength of the rf field. The analysis may form a theoretical basis in the heating phenomenology of particulates.

The Ohmic heating of particulates in a lossless medium

This paper provides a general theory on the Ohmic dissipation of electromagnetic energy by a spherical particulate that is embedded in a lossless medium. The particulate may possess an arbitrary electrical conductivity, and both the medium and the particulate may assume general values of permittivity and permeability. Under the assumption that the wavelength of the electromagnetic field in the medium is large compared with the particulate size, we provide an accurate account of the degree of Ohmic heating by the radio frequency (rf) electric field and by the rf magnetic field of the electromagnetic field. It is found that, in general, heating by the rf magnetic field is dominant whenever δ<a, where δ is the resistive skin depth and a is the radius of the particulate. Analytic scaling laws in the various regimes are derived, from the static case to very high frequency, and for ratios of δ∕a ranging from zero to infinity. The calculation is extended to a transient electromagnetic pulse. Also constructed is ...

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.

Power absorption by thin films on microwave windows

A recent theory for power absorption by continuous thin surface films on microwave windows is compared with experimental data, focusing on diamond gyrotron windows. Two measurement techniques, the dc method by Jory and the radio-frequency (RF) method by Heidinger et al., are used to estimate the fraction of the average power absorbed by the thin films, assuming that these films are spread uniformly over the window surface. Both measurements indicate that, on the average, about a fraction of 1% of the incident power is absorbed by the thin films, even though the local absorption coefficient may reach 50%. Discontinuous surface films, where the surface contaminants have a patchy or island structure, are modeled with an equivalent transmission line circuit. The discontinuous film model is also applied to alumina windows with TiN coatings, and the results are compared with the continuous film model.

Heating of Contaminants on Diamond Windows

This paper examines the RF heating of the contaminants on a diamond window. Both heating of an isolated internal graphite impurity, and of a thin film on the window surface, are discussed. Comparison with recent experiments is given. It is found that heating of isolated internal graphite impurities is unlikely to account for diamond window failure. Upon averaging over the entire window, the thin surface film in general absorbs approximately a fraction of one percent of the total incident power. Intense local heating on the surface contaminant is possible, however.

Rapid Onset of Oscillations in a Magnetron with a Transparent Cathode

Summary form only given. The relativistic magnetron is one of the most powerful sources of microwaves developed to-date. However, its typically slow onset of oscillations limits its use in many promising systems such as, for example, in high resolution radars. The slow start is caused by the small amplitude of the synchronous field near the cathode, which hampers the capturing of electrons into spokes to form the anode current. We propose to increase the Ethetas wave field in the interaction region by removing longitudinal strips from a thin-walled tubular cathode, thus making it transparent to this field. Therefore, instead of a field distribution Ethetas ~ sinh[g(r-rc)], we provide a much larger field Ethetas ~ In(gr) near the cathode with radius rc. Here In is the modified Bessel function of order n, the azimuthal index of the operating wave, and g is the transverse wave number. Simulations using the fully 3D electromagnetic particle-in-cell code MAGIC of an A6 magnetron with solid and transparent cathodes show a marked difference in the time for build-up of oscillations. Moreover, in a magnetron with a transparent cathode the emission centers are automatically located periodically on the cathode perimeter, which also promotes rapid onset of oscillations. This was recently demonstrated in [M.C. Jones et al., Rev. Sci. Instrum., vol. 75, 2976 (2004)] for a magnetron using a solid cathode with nonuniform emission centers. Longitudinal currents along the cathode strips produce azimuthal magnetic fields around them that lead to the azimuthally varying tangential magnetic field. The drift velocity, height of electron cyclotron orbits, and, therefore, the synchronism and efficiency depend on this field. As was shown in [M.C. Jones et al., Appl. Phys. Lett., vol. 84, 1016 (2004)], the varying magnetic field also promotes rapid onset of oscillations

Three dimensional PIC simulations of the transparent and eggbeater cathodes in the Michigan relativistic

Summary form only given. A novel relativistic magnetron priming technique consisting of either a main center cathode and several satellite cathodes (eggbeater design) or just the satellite cathodes (transparent cathode design [Fuks and Schamiloglu, 2005]) is investigated using the three-dimensional electromagnetic, particle in cell code ICEPIC. Both these cathode designs rely on RF field penetration into the cathode region to enhance performance. This priming technique is thought to allow for larger amplitudes of the synchronous Ee field in the electron hub region which in turn hastens the capture of electrons into spokes. This technique effectively eliminates mode competition with the Pi mode amplitude dominating the other modes by at least a factor of 25. The A6-3 Michigan magnetron was used for all simulations [Lopez 2002]. For both cathode designs the number, radial placement and orientation of the satellite strips were allowed to vary over several axial magnetic field values so that an optimization with respect to output power and efficiency could be determined at a given magnetic field. Performance optimization was achieved for both cathode configurations at B=3.2 kG with nine satellite cathodes at a radial placement of 1.75 times the main cathode radius. A power output efficiency of 35% and an output power exceeding 550 MW was measured for both designs at optimum parameters. Efficiency and output power decreased as satellite radial placement was increased from optimum. Moreover, it was observed in some cases that satellite cathode orientation (with respect to the slow wave structure) provided an additional ~50 MW of output power. Transparent and eggbeater cathode designs were simulated with 3, 6 and 9 satellite cathodes. It was found that the number of satellite cathodes did affect the magnetron output performance characteristics. For example, unlike the 6 and 9 satellite cathode simulations, the 3 cathode transparent design was robust with respect to cathode placement. Power efficiencies for satellite cathode radial placements at 1.75, 2.0 and 2.3 times the cathode radius all yielded efficiencies in the 30% range and output powers ~500 MW. Only after satellite cathode radius exceeded 2.3 times the main cathode radius did efficiencies start to drop off significantly for the transparent 3 cathode design