On the heating of output windows of microwave tubes by electron bombardment

An account is given of an experimental investigation of the behavior of output windows, the particular type chosen for study being the cylindrical type in a cavity resonator. By feeding microwave power into the cavity and measuring electric field and the heat dissipated in the window and in the metal walls, a non-linear window-heating effect was discovered above a certain critical field strength. In this condition, watts dissipated at the window vary as the fifth or higher power of the electric field strength and the resulting thermal stresses may easily destroy it. Further experiments were performed on various modifications of the original structure in order to isolate and study the phenomena responsible for the heating. Some of these tests were made using an axial magnetic field. It was found that there are two distinct mechanisms of heating, one requiring the presence of the magnetic field and the other one not requiring it. Both involve energy exchange between the electric field and the window by free electrons which dance on the window surface synchronously with the alternating field and multiply by a secondary emission or multipactor process above a certain field strength. The critical field strength may be greatly reduced by the presence of contaminating films on the dielectric surface, and may be greatly increased by suitable surface treatment. It is shown that these effects are not restricted to cylindrical windows but will tend to occur at any dielectric surface in vacuum if the tangential RF field is strong enough.