BEAM BEHAVIOR THROUGH THE SNS CHOPPER SYSTEM

The chopping system must meet several requirements: 1) the cleanliness of the beam gap must be 1 in 10 4 to minimize activation or damage of the ring extraction elements, 2) beam loss along the linac, including the chopper transients, must not exceed 1 W/m, and 3) the beam power dissipation on the MEBT chopper target must not exceed 400 W. Here, we look into the efficacy of the chopper-system in meeting these criteria using four different timing scenarios for the two choppers. The Spallation Neutron Source (SNS) linac consists of a 2.5-MeV RFQ, a Medium Energy Beam Transfer (MEBT) line, a 402.5-MHz DTL, followed by 805-MHz CCL and SRF structures that accelerate beam to a final energy of 1 GeV. It is designed to inject 1.4 mA average H beam into the storage ring. To minimize beam loss during ring extraction we “chop” a 300-ns notch out of the beam at 1 MHz. This notch is preserved throughout the linac and ring filling to provide a clean extraction gap. 2 LEBT CHOPPER Chopping is accomplished in two stages. A segmented Einzel-lens in the low-energy beam transport (LEBT) line chops the beam by deflecting it between the RFQ vanes. A traveling-wave parallel-plate structure in the MEBT deflects beam vertically onto a target. By design, [1] this chopping system provides a gap to beam current ratio of 10 -4 . In this paper, we investigate four chopper-timing scenarios and examine the expected beam behavior.