Needs and Solutions for Machine Impedance Reduction

Particle beams with highest possible beam intensities are requested nowadays, hence in modern circular accelerators, the consideration of beam coupling impedance issues is of increasing relevance. Classical sources of beam coupling impedance are RF-systems, injection and extraction kickers, but also beam diagnostic elements such as wire scanners where the object itself forms an undesired cavity, and beam pipe transitions, namely tapers or steps. Optimally, for any machine, impedance mitigation shall take place already during the design phase. However, for older existing machines, often considerable hardware changes would be required to obtain a significant impedance reduction. In these cases, the required geometry changes for reducing beam coupling impedance are costly to be carried out, hence retro-fitted solutions such as impedance shields or damping mechanisms are required. For both approaches, different impedance mitigation strategies are available and their selection also depends on the needs, i.e., the type of problem that is arising. Single bunch instabilities, for example, require an optimization of the object geometry with the goal to reduce R/Q, whereas multi bunch instabilities or heating will ask for different measures. In this presentation, we will explain about typical sources of beam coupling impedances and how they could be circumvented. Possibilities to reduce beam coupling impedance in existing machine elements are included and applied solutions, such as impedance shields or HOM dampers will be presented as well.