Abstract
In this paper we present two new techniques for frequency-resolved characterization of longitudinal impedances in storage rings. The first method is based on transient measurements of the growth rates and tune shifts of unstable coupled-bunch modes. In the second approach, estimates of the impedances are obtained from analysis of the steady-state synchronous phases of the bunches for uneven fill patterns. These techniques are applicable to measurements of both fundamental and higher-order mode (HOM) impedances and allow characterization of shunt impedances and quality factors of the HOMs. Methods presented here are complementary to laboratory bench measurements of rf cavities, in that the beam-based measurements directly sense the physical impedance in the installed configuration. Experimental results from the Advanced Light Source and BESSY-II are presented showing the use of these techniques to measure complex impedances.
Highlights
The interaction of charged particles in a storage ring or circular accelerator with the ring impedance determines many important accelerator dynamics parameters
Beam-based techniques are an important adjunct to numerical calculations and laboratory bench measurements of rf cavities and components, in that they can measure the actual installed impedance, which is strongly influenced by coupling ports, parasitic components, and environmental factors which can be difficult to include in simulations or laboratory tests
We will illustrate impedance characterization techniques described above with measurements performed at the Advanced Light Source (ALS) and BESSY-II
Summary
The interaction of charged particles in a storage ring or circular accelerator with the ring impedance determines many important accelerator dynamics parameters. By varying bunch length in such measurements one can estimate the parameters of the broad band equivalent impedance [5,6] Such techniques do not resolve individual higher-order modes (HOMs). Frequency-resolved information about the impedance can be extracted from a measurement of the beam transfer function (BTF) [7,8] Such a measurement can be performed only below the instability threshold. Network analyzer sweeps have to be repeated for each unstable mode making BTF approach slow and cumbersome for machines with large numbers of coupledbunch modes Another method for characterizing the impedance is through observation of cavity coupling probe signals excited by the beam [9].
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