Abstract
Measuring a synchronous phase shift as a function of beam current is commonly done in accelerator physics to estimate the longitudinal impedance of a storage ring vacuum chamber. This measurement is normally done with RF techniques that do not have enough accuracy to detect small phase shifts typical to the newer storage rings. In this paper we report results from a new method for precise measurement of a synchronous phase shift. Our method involves downmixing from the RF frequency to a kHz range and then using an audio DSP lock-in amplifier for the actual phase detection. This paper describes the idea and the advantages of a new method as well as its practical implementation in the apparatus we build for precise synchronous phase measurements in the Stanford Linear Collider damping rings. The results of those measurements are also presented. [S1098-4402(98)00020-2]
Highlights
Measuring a synchronous phase shift as a function of beam current gives the so-called loss factor, which is related to the ring impedance and affects the longitudinal beam dynamics
There are elaborate computational methods to calculate the impedance by summing up the contributions of individual vacuum chamber elements, and the measurement of the loss factor provides a check of the calculations and the vacuum chamber manufacturing and assembly procedure
We have designed and built a new apparatus that allows for precise synchronous phase measurement in the storage rings
Summary
Measuring a synchronous phase shift as a function of beam current gives the so-called loss factor, which is related to the ring impedance and affects the longitudinal beam dynamics. The synchronous phase shift measurement can be difficult in practice, especially for the newer storage rings. The loss factor tends to be small, resulting in a total phase shift on the order of a degree or less which is hard to measure accurately with traditional experimental techniques. A similar problem arises when the loss factor is relatively high but measurements are performed at low current to avoid the effect of bunch lengthening or instabilities. We will describe our successful use of this apparatus to measure the synchronous phase shift in the Stanford Linear Collider (SLC) damping rings
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