Mechanical Design of the Snsmebt*

The Lawrence Berkeley National Laboratory (LBNL) is presently designing and building the 2.5 MeV front end for the Spallation Neutron Source (SNS). The front end includes a medium-energy beam transport (MEBT) that carries the 2.5 MeV, 38 mA peak current, H beam from the radio frequency quadrupole (RFQ) to the drift tube linac (DTL) through a series of 14 electromagnetic quadrupoles, four rebuncher cavities, and a fast traveling wave chopping system. The beamline contains numerous diagnostic devices, including stripline beam position and phase monitors (BPM), toroid beam current monitors (BCM), and beam profile monitors. Components are mounted on three rafts that are separately supported and aligned. The large number of beam transport and diagnostic components in the 3.6 meter-long beamline necessitates an unusually compact mechanical design. 1 PHYSICS REQUIREMENTS The SNS is an accelerator-based user facility that will produce pulsed beams of neutrons for use in scattering experiments. LBNL has designed and is fabricating the Front-End Systems (FES) comprising an ion source, lowenergy beam transport (LEBT) with a pre-chopper, 402.5 MHz RFQ, and MEBT. The FES will accelerate a 38 mA, 6% duty factor, H ion beam to 2.5 MeV for injection into the 1 GeV linac [1]. The MEBT lattice matches the beam from the RFQ through two fast traveling wave choppers into the first tank of the DTL. The 35-cm long choppers perform the final beam chopping that prevents beam from intercepting the septum of the extraction kicker magnet in the accumulator ring during its rise time. A closely spaced lattice with strong focusing is required to minimize emittance growth, due to the nonlinear charge distribution of the beam and the 62-cm drifts required for insertion of the choppers [2]. The layout of the beamline has been optimized to minimize emittance growth while taking into consideration the mechanical implications of closely spaced transport components and diagnostic devices. The MEBT lattice consists of fourteen quadrupole magnets, four rebuncher cavities, two traveling-wave choppers, and a chopper target that intercepts the deflected beam (see Figure 1). Four quadrupoles, one rebuncher, and one chopper are mounted on the first and third rafts, with the remainder of the components arranged symmetrically on the second raft. Six quadrupoles, the first and last magnets on each raft, incorporate dipole steering to correct for misalignments between rafts. Diagnostic devices located between transport components monitor beam quality during operation and enable tuning of the MEBT itself. Figure 1: The MEBT beamline layout. While the beampipes have clear bore diameters of 3 cm on the first and third rafts, those on the second raft have diameters of 4 cm in order to accommodate the vertically displaced trajectory of the chopped beam. 2 TRANSPORT COMPONENTS Physically compact devices have been devised, with careful consideration for mounting and alignment features in order to accomplish the positional accuracy and tight longitudinal spacing necessitated by the goal of minimal emittance growth. 2.1 Quadrupole Magnets The physical envelope of the quadrupoles was tightly constrained by the available longitudinal space and the fit with the beam position monitors (BPM) and beampipes that it surrounds. The design of the MEBT quadrupole is derived from the Los Alamos National Lab (LANL), prototypes developed for APT [2]. The six magnets on the second raft have bore diameters of 4.2 cm. The remaining magnets on the first and third rafts have bore diameters of 3.2 cm. The magnet cores consist of quadrants machined from 1006 low carbon steel. Dowel pins and bolts, spanning the horizontal and vertical centerlines, index and hold together the four quadrants. After conventional machining of the quadrants and surface grinding of the length-wise planes of each core assembly, the pole tip surfaces, mounting feet, and fiducial notches in the outer corners are cut by wire EDM _____________________ * This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. [email protected] 0-7803-7191-7/01/$10.00 ©2001 IEEE. 1574 Proceedings of the 2001 Particle Accelerator Conference, Chicago