Regenerative multibunch beam breakup instabilities and countermeasures for a high-intensity electron accelerator and the superconducting Darmstadt linear electron accelerator

Abstract

Regenerative multibunch beam breakup instabilities are a well-known phenomenon in recirculating linacs where particle bunches pass multiple times through the same superconducting rf cavities with extremely high quality factor. This is in particular true for energy recovery linacs. Parasitic electromagnetic modes excited in the cavities can affect bunch dynamics in such a way, that on its subsequent passes it excites the modes further and a positive feedback loop is formed. Direct bunch tracking and a stability analysis technique can be used to study the instability. Usually only dipole modes are considered. In the present work, similar approaches are applied to monopole and quadrupole modes and illustrated with simulation results for the S-DALINAC and MESA facilities. An approximated stability analysis technique with better performance for the case of multiple recirculations is proposed. Countermeasures including betatron phase advance adjustment and additional betatron coupling are considered and a universal criterion for assessment of their effectiveness is proposed. A simple model of a damped oscillator with feedback is proposed as a universal example illustrating the phenomenon in general.