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Electromagnetic and Multipactor Stresses on RF Windows in Drift Tube Linear Accelerators


Radio frequency (RF) particle accelerators are a key component to many scientific and industrial endeavors, so their reliable operation is of great concern. An obstacle to reliable operation has been the performance of the RF windows that act as barriers between the vacuum environment of the accelerating cavities and atmospheric conditions in the transmission line. Failures of these windows have a large impact on accelerator availability, and recovering from a window failure is a multiple day event. In order to mitigate these failures, an analysis of the stresses placed on the window has been performed. This dissertation presents simulations of the stresses placed on the window due to the electromagnetic environment and the multipactor effect with a discussion of how to minimize those stresses.

The electromagnetic environment was first analyzed using the commercially available CST Microwave Studio software. An analysis of the window geometry in isolation from the accelerating cavity is performed first to minimize the stresses placed on it. Then the analysis of the whole system with the cavity present was performed using an approach that was developed to allow transient simulations of the highly resonant cavity. Simulations show how the signal develops over time, thus allowing adjustments to be made to the driving signal to minimize stresses. Simulations also demonstrate the effect of harmonics and their excitement of higher order modes, and the effect of an off-resonant cavity is briefly discussed.

Multipactor is a parasitic phenomenon whereby large electron populations accumulate and is common to accelerator systems. The principles and simulation techniques of multipactor are first presented, and benchmarks are presented to illustrate the validity of the simulations. The conditions of both traveling and standing waves are examined for the effect on multipactor development and electron collisions with the window. The broad analysis of many standing wave conditions allows for mitigation of multipactor in consideration of the signal analyzed in the electromagnetic simulations. Effects of window material and charge accumulation on the window are discussed, and the consideration of the harmonics and higher order modes analyzed previously are shown to have negligible effect on multipactor.

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