Optimization of the collimation system for the SNS accumulator ring

The collimation system for the Spallation Neutron Source (SNS) accumulator ring will dissipate about 2 kW of beam power and is designed for a capture efficiency close to 95% of the halo. Preliminary studies, assuming no errors and a uniform distribution of losses, indicated that a maximum level of uncontrolled loss of 0.01% of the total beam is achievable. On the other hand, the energy lost in the primary scraper could concentrate uncontrolled losses in areas of maximum dispersion. We use Monte Carlo simulations to clarify some beam dynamic issues that could compromise the high efficiency required. The material interacting with the beam and the shape of the three structures forming the collimator system have been carefully chosen to maximize the collimation efficiency and minimize activation. Moreover, a realistic distribution of losses around the machine allows identification of potential hot points. This paper describes the latest version of the collimation system and summarizes the main results of these numerical studies.