Impedance Calculation and Verification in Storage Rings

概要

In a storage ring, the (short-range) longitudinal wake, or equivalently, the broad-band impedance, can induce the collective effects of bunch lengthening, energy spread increase, and the microwave instability. In a high current ring, such as SuperKEKB, these effects can negatively affect the machine performance. Consequently, in such rings a concerted effort is generally undertaken to design and build a smooth vacuum chamber, in order to minimize the impedance. The bottom-up approach of obtaining a ring impedance--performing wake calculations beginning with drawings of vacuum chamber objects--was probably first used in the 1980's in designing the vacuum chamber of the damping rings of the Stanford Linear Collider (SLC). Yet even now, so many years later, the calculation and verification of the impedance in a ring can be challenging and have aspects of an art form. In this talk I will review the calculations and measurements at the SLC. I will also discuss the case of Dafne, and that of the ATF at KEK.