BERLinPro 7-CELL SRF CAVITY OPTIMIZATION AND HOMs EXTERNAL QUALITY FACTORS ESTIMATION

The main scope of this work is the optimization of the Superconducting Radio Frequency (SRF) accelerating cavity design for the Berlin Energy Recovery Linac Project (BERLinPro). BERLinPro shall serve as a demonstrator for 100-mA-class Energy Recovery Liniac (ERL) with CW LINAC technology. High-current operation requires an effective damping of Higher-OrderModes (HOMs) of the 1.3 GHz main-linac cavities. Consequently it is important, at the SRF cavity design optimization stage, to calculate the external quality factors of HOMs to avoid beam break up (BBU) instabilities. The optimization of the SRF cavity design consists of two steps. In the first step, the cavities’ end half-cells are tuned with respect to field flatness, effective shunt impedance and geometrical factor of the fundamental accelerating mode using robust eigenmode simulations. The second step involves frequency domain simulations and the extraction of external quality factors of HOMs from transmission S-parameter spectra using a vector fitting procedure and an automated traveling poles elimination (TPE) scheme to remove non-static poles.