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.
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 the loss tangent of the medium, resulting from a distribution of particulates.
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 ␦ Ͻ a, where ␦ 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 ӷa, where is the free space wavelength of the rf field. The analysis may form a theoretical basis in the heating phenomenology of particulates.
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