RF window ghost mode analysis

The RF window acts as a barrier between the vacuum and air, gas, or water while allowing RF power to pass through with minimal loss. Resonant modes (called "ghost modes") can occur within the ceramic disk of a window. The frequencies of these modes depend on the material and size of the ceramic. Ceramic disk dimensions must be carefully optimized to minimize reflections and avoid ghost mode resonances within the operating bandwidth. In this paper we present the design of an input window used in an X-band klystron. The dimensions of the window and ceramic disk are optimized to minimize insertion and reflection losses while preventing ghost mode resonances in the operating bandwidth. In addition to this, we ensure that the maximum electric field at the window surface is kept low to reduce the probability of RF breakdowns. Analytical analysis, numerical simulations and experimental measurements of the ghost modes of ceramic disks were carried out. The measured ghost mode frequency was used to evaluate the ceramic dielectric constant. In this article we present simulated and measured results.