Power Absorption by Dielectric Contaminants in High Power Microwave Systems

Abstract

The heating of dielectric microwave transmission windows places an effective upper limit on the amount of power that may be transmitted through the window. Lossy surface films are known to form on diamond gyrotron windows, while thin films of TiN are intentionally deposited on alumina klystron windows to protect against multipactor. A uniform 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. Typical values for power losses due to surface films are on the order of 0.1%. This paper also provides the most general theoretical treatment to date on the degree of ohmic heating of discrete particulates by the rf electric field and the rf magnetic field of an electromagnetic wave, with the only assumption being that the wavelength is large in comparison with the particulate size. In general, heating by the rf magnetic field is dominant whenever the resistive skin depth is less than the radius of the particulate. The analysis may form a theoretical basis in the heating phenomenology of particulates