Technique for forming an RF sample signal for accelerating field feedback stabilization in traveling wave linacs

Feedback loop techniques have been employed for decades in the stabilization of linac RF systems. Systems using these techniques require an input signal which represents the accelerating fields which are to be stabilized. Early implementations have taken a sample of the klystron output power as the source for this signal, thus enabling stabilization of fluctuations in the level and phase of klystron output due to variations in the high voltage pulse, droop in the driver amplifier gain, etc. However, fluctuations produced by beam loading, thermal drift of the linac section, and other effects which occur downstream from the sample point cannot be sensed or corrected using this approach. A technique for combining samples of the input and output power from all of the traveling wave sections driven by a given klystron, enabling the system to respond to perturbations introduced at points throughout the RF system, has been developed. This approach is extended to both constant impedance and tapered section types. Described here are the laws which relate the phases and levels of the sample signals which are to be presented to the summing circuit to form the input to the stabilization loop