Mechanical design of the SNS MEBT

Abstract

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. PHYSICS REQUIREMENTSThe 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. TRANSPORT COMPONENTSPhysically 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. Quadrupole MagnetsThe 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 surrou...

RF design studies on a 325 MHz rebuncher cavity for high intensity linear accelerators producing neutrons

“…The geometrical sizes of two rebunchers in the MEBT are the same, and the required maximum effective voltage given by the 2 nd rebuncher is 150 kV. Several types of cavities, such as DTL-type cavity [2,3], CCL-type cavity [3][4][5][6][7] and Quarter Wave Resonator (QWR) [3,[7][8][9], have been taken into consideration in the early stage studies. Compared to the DTL-type and QWR-type cavities, the CCL-type rebuncher cavity is more beneficial for the high frequency cavity performance, i.e., higher quality factor, lower power loss density, and simpler geometry shape.…”

A thermal analysis model for high power density beam stops

“…LBNL has designed and is fabricating the SNS Front End [1], which will accelerate a 38 mA, 6% duty factor H -beam to 2.5 MeV for injection into a 1 GeV linac. The Front End consists of an ion source and low energy beam transport line (LEBT) [2], a radio frequency quadrupole (RFQ) [3,4] and a medium energy beam transport line (MEBT) [5].…”