Beam dynamics in the spallation neutron source linac

Being an 1.44MW machine, the beam loss requirement on the SNS linac is less than 1 W/m and controlling halo particle generation is of great importance. Beam dynamics aspects of the SNS linac design are presented considering various halo generation mechanisms. A halo generation mechanism in the non-periodic lattices such as the SNS linac MEBT (Medium-Energy Beam-Transport between RFQ and DTL) is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6-m-long MEBT chopper section is responsible for halo formation. The proposed mitigation measures are modifying the MEBT optics and introducing adjustable collimators in the MEBT. The transient beam behavior of the LEBT and MEBT choppers is also studied for the fate of partially chopped beams.