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{sup -} 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

DeMello, A.

Doolittle, L.

Luft, P.

Oshatz, D.

Staples, J.

Zachoszcz, A.

UNT Digital Library

LBNL-47343MECHANICAL DESIGN OF THE SNS MEBT*D. Oshatz†, A. DeMello, L. Doolittle, P. Luft, J. Staples, A. ZachoszczLawrence Berkeley National Laboratory, Berkeley, CA, USAAbstractThe Lawrence Berkeley National Laboratory (LBNL) ispresently designing and building the 2.5 MeV front endfor the Spallation Neutron Source (SNS). The front endincludes a medium-energy beam transport (MEBT) thatcarries the 2.5 MeV, 38 mA peak current, H- beam fromthe radio frequency quadrupole (RFQ) to the drift tubelinac (DTL) through a series of 14 electromagneticquadrupoles, four rebuncher cavities, and a fast travelingwave chopping system. The beamline contains numerousdiagnostic devices, including stripline beam position andphase monitors (BPM), toroid beam current monitors(BCM), and beam profile monitors. Components aremounted on three rafts that are separately supported andaligned. The large number of beam transport anddiagnostic components in the 3.6 meter-long beamlinenecessitates an unusually compact mechanical design.1 PHYSICS REQUIREMENTSThe SNS is an accelerator-based user facility that willproduce pulsed beams of neutrons for use in scatteringexperiments. LBNL has designed and is fabricating theFront-End Systems (FES) comprising an ion source, low-energy beam transport (LEBT) with a pre-chopper, 402.5MHz RFQ, and MEBT. The FES will accelerate a 38 mA,6% duty factor, H- ion beam to 2.5 MeV for injection intothe 1 GeV linac [1].The MEBT lattice matches the beam from the RFQthrough two fast traveling wave choppers into the firsttank of the DTL. The 35-cm long choppers perform thefinal beam chopping that prevents beam from interceptingthe septum of the extraction kicker magnet in theaccumulator ring during its rise time. A closely spacedlattice with strong focusing is required to minimizeemittance growth, due to the nonlinear charge distributionof the beam and the 62-cm drifts required for insertion ofthe choppers [2]. The layout of the beamline has beenoptimized to minimize emittance growth while taking intoconsideration the mechanical implications of closelyspaced transport components and diagnostic devices.The MEBT lattice consists of fourteen quadrupolemagnets, four rebuncher cavities, two traveling-wavechoppers, and a chopper target that intercepts thedeflected beam (see Figure 1). Four quadrupoles, onerebuncher, and one chopper are mounted on the first andthird rafts, with the remainder of the components arrangedsymmetrically on the second raft. Six quadrupoles, thefirst and last magnets on each raft, incorporate dipolesteering to correct for misalignments between rafts.Diagnostic devices located between transport componentsmonitor beam quality during operation and enable tuningof the MEBT itself.Figure 1: The MEBT beamline layout.While the beampipes have clear bore diameters of 3 cmon the first and third rafts, those on the second raft havediameters of 4 cm in order to accommodate the verticallydisplaced trajectory of the chopped beam.2 TRANSPORT COMPONENTSPhysically compact devices have been devised, withcareful consideration for mounting and alignment featuresin order to accomplish the positional accuracy and tightlongitudinal spacing necessitated by the goal of minimalemittance growth.2.1 Quadrupole MagnetsThe physical envelope of the quadrupoles was tightlyconstrained by the available longitudinal space and the fitwith the beam position monitors (BPM) and beampipesthat it surrounds. The design of the MEBT quadrupole isderived from the Los Alamos National Lab (LANL),prototypes developed for APT [2]. The six magnets onthe second raft have bore diameters of 4.2 cm. Theremaining magnets on the first and third rafts have borediameters of 3.2 cm. The magnet cores consist ofquadrants machined from 1006 low carbon steel. Dowelpins and bolts, spanning the horizontal and verticalcenterlines, index and hold together the four quadrants.After conventional machining of the quadrants andsurface grinding of the length-wise planes of each core_____________________* This work is supported by the Director, Office of Science, Office ofBasic Energy Sciences, of the U.S. Department of Energy underContract No. DE-AC03-76SF00098.†daryl_oshatz@lbl.govassembly, the pole tip surfaces, mounting feet, andfiducial notches in the outer corners are cut by wire EDMto a profile tolerance of 0.076 mm. The epoxyimpregnated quadrupole coils fit closely around thequadrants with power and cooling water connections onboth horizontal sides of the core. Six of the fourteenmagnets contain back-leg dipole steering coils. Thedesign of the core and coil connections (see Figure 2)allows the magnet to be disassembled into two halves andreassembled around a beampipe or BPM, withoutchanging the position of the magnetic center with respectto the fiducial surfaces.Figure 2: 3.2-cm bore quadrupole and 3- cm BPM.All quadrupoles have been fabricated at LBNL, andmagnetic measurements have shown the field quality andintegrated focusing strength of all magnets to be wellwithin specification.2.2 Rebuncher CavitiesThe 402.5 MHz rebuncher cavities have beam aperturediameters of 3 cm on the first and third rafts and 3.6 cmon the second raft. JP Accelerator Works, Los Alamos‚NM, is designing and building the type-TM010 pillboxcavities. Each cavity assembly consists of a copper-plated304 stainless steel body and lid. The bolted connectionbetween these two parts contains stainless steel spacers, asilver-plated inconel C-seal to ensure RF contact, and aviton O-ring that creates the vacuum seal. Coarsefrequency adjustment is achieved by varying the thicknessof the spacers. Stepper motor driven slug tuners willachieve finer tuning with continuous adjustment duringoperation. All power, water cooling, and vacuumpumping connections are located on the outside diameterof the cavity, enabling the assembly to fit within the 13-cm beamline insertion length. One 3-cm cavity has beendelivered and is currently undergoing testing at LBNL.2.3 Chopper StructuresThe fast traveling wave chopper assemblies, to besupplied by LANL, consist of two meander-line structuresspaced 1.8 cm apart [4]. The meander-line pattern ischemically etched in the copper coating of a dielectricplate that is epoxy-bonded to an aluminum backplate.Water cooling dissipates approximately 19 watts of heatgenerated per assembly. Vacuum chambers mounted onthe first and third rafts contain the chopper structures.The assemblies are hinged at one end and mounted to anadjustable micrometer feedthrough at the other end toenable pitch adjustment during tuning.2.4 Chopper TargetDuring SNS operation the deflected beam will depositan average power of less than 400 watts on the choppertarget. However, the peak power density in the core of thebeam will exceed 200 kW/cm2. This high instantaneouspower and the limited longitudinal space available in thebeamline to spread out the chopped beam posedsignificant thermal and mechanical design challenges [2].Material selection was limited by the fact that copper andits alloys could not be exposed to the beam, as copper-65has a neutron production threshold of 2.1 MeV forincident protons.The design utilizes a grazing angle of 75° from normalto decrease the instantaneous heat flux. The TZM (Mo-0.5Ti-0.1Zr) faceplate contains cooling channels 1 mmwide by 4 mm deep, machined by EDM on a 2 mm pitch.The TZM backplate contains water inlet, outlet, andexchange plenums that circulate water through the coolingpassages at a flow velocity of 15 ft/sec. The faceplate isbrazed to the backplate and cut by wire EDM to create a1-mm thickness of material between the front surface andthe bottom of the cooling passages (see Figure 3).Beam at 75º Angleof IncidenceTZM MolybdenumAlloy Face PlateTZM MolybdenumAlloy Back PlateWater CoolingPassages (1.0 x 4.0mm with 1.0 mmhotwall)Braze Joint(Palculsil 25)InletOutlet70 mm24 mm102 mmWater CoolingManifold Sealed withO-rings to Back Plate Plenums in the BackPlate Direct WaterFlow ThroughChannels (4 PassCircuit)Figure 3: The MEBT chopper target.The target was designed for a peak operating current of52 mA. At a peak current of 52 mA, a 1-msec long beampulse would raise the front surface of the target to atemperature of 206° C. Differential, transverse, thermalexpansion would create a Von Mises Stress in thefaceplate of 271 MPa, 65 % of the yield strength of TZM,including a derating for temperature and fatigue loading.The reduction of the FES peak current requirement to 38mA by the SNS collaboration should reduce the peaksurface temperature to 106° C and the peak surface stressto 93 MPa [3]. The target assembly is suspended in itsvacuum beambox from a stepper motor driven bellowsfeedthrough and can be vertically adjusted during tuningby increments as small as 0.02 mm.3 BEAM DIAGNOSTIC DEVICES3.1 Beam Position MonitorsSix quadrupole magnets in the MEBT will containshorted stripline BPM’s (see Figure 4) [5].Figure 4: A sectional view of the BPM assembly.The 316 stainless steel assemblies are externally TIGwelded. The TZM central conductors of the SMAmicrowave feedthoughs are spot welded to the ends of thestriplines. Radial shims between the quadrupole pole tipsand precisely machined flats on the outside of the boretubes will center and hold the BPM assemblies in themagnets. A welded bellows, on one end, enables insertionof adjacent beampipes and prevents the assembly frombecoming over-constraint during bolting. All 3-cm and 4-cm bore diameter BPM’s have been fabricated and tested.3.2 Beam Profile MonitorsDedicated beamboxes for beam profile monitoring areprovided in five locations in the MEBT [5].Measurements of beam profile will be made at low dutyfactor with crawling wire scanners. Run-timemeasurements may be made at full duty factor by laser-based photodissociation (LP). The MEBT beamboxesinclude provision for both types of diagnostics and have alongitudinal dimension of only 5 cm. A slot machined ata 45° angle by wire EDM guides the traveling wirescanner frame. Four 25 mm aperture viewports providetwo orthogonal lines of sight to the beam for the LPsystem. Brookhaven National Lab (BNL) will provideflange mounted wire scanners for acceptance tests atLBNL and calibration of a possible, future LP system.Prototypes of both systems are currently underdevelopment at BNL [6].3.3 Other Diagnostic DevicesThe MEBT includes two current transformers tomeasure beam current, one before and one after thechopper target [5]. Both BCM’s have been fabricated.LBNL is currently designing other diagnostics devicesto be utilized during commissioning, including a fullpower beam stop and a low-duty-factor emittance scanner.The beamstop and emittance slit designs will allowtemporary installation of either device in the anti-chopperbeambox on the third raft of the MEBT.4 MECHANICAL SYSTEMS4.1 Support and AlignmentThe MEBT raft design utilizes tight fabricationtolerances and coordinate measuring machine (CMM)fiducialization to achieve accurate positioning of transportcomponents. The bores, mounting feet, and fiducialtargets of quadrupole magnets and rebuncher cavities aremapped by CMM prior to installation on the rafts. Boltsand shims fasten and align components to the precisionground mounting surfaces of the raft weldments. CMMmeasurements will verify the alignment of elements onthe assembled rafts to an accuracy of 0.025 mm. In theevent that removal of a magnet or cavity is required formaintenance or repair purposes, the shims will remainfixed to the raft to enable repeatable assembly.A six-strut system kinematically mounts each raft to thecarbon steel support frame. Flexible bellows spoolsbetween rafts allow separate alignment of each raft, undervacuum, to an RMS accuracy of 0.04 mm. Fiducialtargets on individual components will remain accessiblefor the verification of stability or location of elementsafter replacement.4.2 Vacuum SystemThe limited conductance down the MEBT beampipenecessitates a distributed pumping strategy. Eight VacIon55 Starcell® ion pumps are expected to achieve an averagevacuum in the mid-10-7 Torr range [2]. Two V300HTturbo pumps backed by TriScrollTM 300 scroll pumps onthe chopper beamboxes will pump the beamline to arough vacuum adequate to start the ion pumps.5 CONLUSIONThe mechanical design of compact beam transport anddiagnostic devices has accommodated the challengingphysics requirements of the MEBT. Most elements havebeen fabricated, and assembly work will begin thissummer in order that the MEBT can transport beamduring acceptance tests at LBNL in early 2002.6 REFERENCES[1] R. Keller, et al., “Progress with the SNS Front-EndSystems,” PAC ’01, Chicago, June 2001.[2] J. Staples, D. Oshatz, T. Saleh, “Design of the SNSMEBT,” Linac ‘00, Monterey, August 2000.[3] S. Virostek, D. Oshatz, J. Staples, “A ThermalAnalysis for High Power Density Beam Stops,”PAC’01, Chicago, June 2001.[4] S. Kurennoy, J.F Power, R.J. Roybal, D.L. Schrage,“Progress with SNS Fast Beam Chopper,” PAC01,Chicago, June 2001.[5] L. Doolittle, T. Goulding, D. Oshatz, A. Ratti, J.Staples, “SNS Front End Diagnostics,” Linac ’00,Monterey, August 2000.[6] P. Cameron, “Carbon Wire Profile Monitors in theSNS,” PAC ‘01, Chicago, June 2001.

Mechanical design of the SNS MEBT

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Mechanical design of the SNS MEBT

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