"windowtron" RF breakdown studies at SLAC

Summary form only given, as follows. The current design for the X-band Next Linear Collider (NLC) requires electric field gradients significantly higher than SLAC\´s operating S-band Linear Collider (SLC). Finding methods to achieve these higher gradients and minimize processing time is the focal point of the study. The test platform for the RF breakdown study is a "windowtron" which consists of a transmission cavity with demountable nose tips situated between two windows. The nose tips can be made from various materials, and different cleaning and machining techniques can be applied. The RF power is supplied to the cavity by a 50 MW X-band (11.4 GHz) klystron. Automation is currently being developed for this study in order to provide a consistent comparison between experiments. The breakdown criterion, previously based on gas or visible light, has been changed and is now based on the percentage of missing transmitted energy within each RF pulse. A fast interlock system is utilized that prevents damage by successive pulses, and enables a higher degree of control during experiments. Current experiments include comparing breakdown damage inside a high Q and low Q cavity, where the low Q cavity has eight times the energy available inside the cavity. This information is relevant to constant gradient accelerator structures where the group velocity and impedance vary inside the structure. Other experiments include high temperature (450/spl deg/C) vacuum bakeout compared to heat tape baking, the effect of multiple hydrogen braze cycles, and a comparison between different RF processing methods.