Dielectric materials for window applications

Radio frequency (RF) windows, used in input power couplers for linear particle accelerators, are one of the components limiting the amount of power that can be coupled into an accelerator. The window failures are thought to be due to a nonlinear surface heating caused by multipacting and material imperfections. For continuous wave (CW) and superconducting accelerator applications reliable windows are critical because window failures are devastating to the system. The most common material used for RF windows is high purity polycrystalline alumina; however, alternative dielectric materials are examined in this report, focusing on aluminum nitride as a material for high power RF windows. The desirable material properties of the RF window material are discussed, then previous studies comparing dielectric materials for RF windows are examined. To learn more about the quality and properties of aluminum nitride, the material properties of the aluminum nitride and alumina ceramics used in the electronic packaging industry are also examined. In addition, an analytical study of ceramic windows in fusion reactors, which compares alumina and aluminum nitride is overviewed. Finally, the effects of processing on the properties of alumina nitride are examined and it is concluded that aluminum nitride will be a promising candidate material for RF windows. (C) 2000 Elsevier Science Ltd. All rights reserved.