Linac RF structures for the Spallation Neutron Source

The Spallation Neutron Source (SNS) project is a collaboration among Argonne, Brookhaven, Lawrence Berkeley, Los Alamos, and Oak Ridge National Laboratories. Los Alamos is responsible for the linac that accelerates the H^- beam from 2.5 MeV to 1 GeV. For the baseline design, scheduled for completion in 2005, the linac will deliver to the accumulator ring a beam of 1.1-MW average power. In the SNS linac, a conventional 402.5-MHz drift-tube linac (DTL) accelerates the beam from 2.5 to 20 MeV, at which point 805-MHz structures take over. The 805-MHz linac consists of a coupled-cavity drift-tube linac (CCDTL), followed by a coupled-cavity linac (CCL). The DTL uses permanent magnet quadrupoles inside the drift tubes arranged in FOFODODO lattice; the focusing period is 4βλ long at 402.5 MHz. The CCDTL and CCL use electromagnetic quadrupole magnets external to the rf structure; the FODO lattice period is 12 ph long at 805 MHz. The cavity field profile maintains smooth longitudinal focusing strength per unit length. High cavity stored energy reduces the effect of beam chopping on the cavity fields. The bore radius is 1.25 cm in the DTL, and increases in the CCDTL and CCL in several steps consistent with adequate shunt impedance, to a final value of 2.0 cm. The rf structures are compatible with a future upgrade to a beam power of 4.4 MW