The collimation system of the SNS transfer lines

The High Energy Beam Transport (HEBT) and Ring to Target Beam Transport (RTBT) lines of the Spallation Neutron Source (SNS) include collimation systems that protect the line itself, as well as the systems downstream. Due to the one-pass mechanism in a transfer line, collimation can only provide protection against accidental losses while shaping the beam is reserved for multi-pass systems as rings. Efficiency has to be improved by increasing the impact parameter in the collimator in the first passage. In the HEBT line the use of strippers for collimation of the H^- beam enhances the final capture efficiency but introduces some limitations in the layout geometry. In the RTBT, protection is provided by the optics of the line and collimator optimization is reduced to considerations of length and the amount of free space available. We revisited the design of the collimation systems for both transfer lines attending to various criteria including loss scenarios and flexibility. With the revised design, we estimated the final performance and cleaning efficiency under nominal conditions for both systems