In the SuperKEKB accelerator, higher-order-mode (HOM) power absorbed by the HOM dampers of the superconducting (SC) cavities limits the attainment of the design electron beam current of the High Energy Ring. As regards cylindrical ferrite dampers installed in the SC cavities, we observed that their effective loss factors considerably depended on the bunch fill pattern. To investigate this behavior in detail, wakefield simulations were conducted. HOM power is generally evaluated from the numerically calculated coupling impedance, which provides only the total beam power loss within the structure. However, when multiple absorbers are present, the absorbed power of each absorber remains unknown. To address this limitation, we introduce calculation methods that utilize the local electromagnetic fields to evaluate the absorbed power. In this study, the theoretical basis of the methods is presented and applied to the SuperKEKB SC cavity.
In electron coolers devices, at low energies the electron beam is immersed in a continuous solenoid, which provides a cold and tightly focused beam in the cooling and transport sections. At low energies, the electron motion satisfies the adiabatic criterion when the typical length of the magnetic field variation is much larger than the Larmor spiral length. At relativistic energies, this condition generally cannot be fulfilled, so a special method of transporting the electron beam is required. This article describes one such method, which involves the use of special “resonant” elements to guide the electron beam along the desired trajectory.
