Transient beam loading effects on energy loss during beam abort in high-current storage ring

Abstract

Beam abort by shutting off the rf cavities is a widely used machine-protection scheme in modern diffraction-limited synchrotron light source storage rings. In this process, the stored beam loses energy turn by turn until it is intercepted by a dedicated absorber. A key parameter in this process is the number of turns after the rf shutdown until the subsequent beam loss, especially for the beam-size blow-up abort scheme. Despite its importance for designing an abort protection system, this quantity has not been characterized at high stored currents. We report measurements of abort turns over a broad current range from 3 to 400 mA in the 3-GeV NanoTerasu storage ring. The results show a clear current dependence: the beam is lost significantly faster at higher stored currents, with the number of turns until beam loss reduced from 435 at 3 mA to 187 at 400 mA. Our results indicate that transient beam loading induced by the aborting beam in empty rf cavities is the primary mechanism responsible for the enhanced energy loss. The number-of-turns behavior is not simply proportional to the beam loading at high current but saturates. Time-resolved cavity pickup signals, together with tracking simulations and analytical modeling, quantitatively reproduce the observed trend. Our experimental results and theoretical modeling demonstrate that transient beam loading significantly influences abort dynamics in high-current fourth-generation storage rings, emphasizing the need to incorporate this effect into machine-protection system design. Our approximate extension of the steady-state cavity-beam response to beam-abort transients reproduces experimental results well, indicating that the number of turns during a beam abort can be predicted purely numerically.