Transverse beam resonance in the superconducting linac of the Spallation Neutron Source

Abstract

A weak transverse resonance in the Spallation Neutron Source (SNS) superconducting linac is identified in computer simulations, and is believed to be one of the mechanisms behind measured beam losses. This weak resonance is induced by the nonlinear dodecapole component of the linac quadrupole magnets. It occurs when the linac focusing lattice has a transverse phase advance close to 60°. By reducing the phase advance to approximately 50° to avoid the resonance, we observe significant reduction in beam loss in the SNS superconducting linac. We present theory and computer simulation results supporting the notion that a suppression of the 60° resonance may contribute to reduction in the beam loss.