The Meso-American ball games are corporal manifestations that have come being explored mainly from archaeological and historical perspectives. They have also been recaptured from different initiatives to put them into practice.However to say that the chaaj in mesoamérica is today a recreational and sport alternative, is to keep in little its demonstrated potential. The sacralidad manifested through the corporal movements is offering innovation options of the current societies. There they put at stake the codexes, the archaeological interpretations and the territorial knowledge. In their ludica dimension, the game gathers cultural, sport and pedagogic elements. In the last years it comes registering an unusual promotion. It amid the current times of change that were announced from immemorial times by the mayan knowing: men and women

Abraham, W.

Ayers, J.J.

Cheng, D.W.

Cull, P.

DiGennaro, R.

Doolittle, L.

Gough, R.A.

Greer, J.B.

Hoff, M.D.

Keller, R.

Leung, K.N.

Lewis, S.

Lionberger, C.

MacGill, R.

Minamihara, Y.

Monroy, M.

Oshatz, D.

Pruyn, J.

Ratti, A.

Reijonen, J.

Schenkel, T.

Staples, J.W.

Syversrud, D.

Thomae, R.

Virostek, S.

Yourd, R.

Keller, R

Abraham, W

Ayers, J J

Cheng, D W

Cull, P T

Di Gennaro, R S

Doornbos, J

Gough, R A

Greer, J B

Hoff, M D

Leung, K -N

Lewis, S

Lionberger, C A

MacGill, R A

Minamihara, Y

Monroy, M

Oshatz, D

Pryun, J

Ratti, A

Reijonen, J

Schenkel, T

Staples, J W

Syversrud, D L

Thomae, R W

Virostek, S

Yourd, R

CERN Document Server

Progress with the SNS Front-End Systems

The Front-End Systems (FES) of the Spallation Neutron Source (SNS) project have been described in detail elsewhere [1]. They comprise an rf-driven H{sup {minus}} ion source, electrostatic LEBT, four-vane RFQ, and an elaborate MEBT. These systems are planned to be delivered to the SNS facility in Oak Ridge in June 2002. This paper discusses the latest design features, the status of development work, component fabrication and procurements, and experimental results with the first commissioned beamline elements

Ayers, J. J.

Cheng, D. W.

Gough, R. A.

Greer, J. B.

Hoff, M. D.

Leung, K. N.

Staples, J. W.

UNT Digital Library

PROGRESS WITH THE SNS FRONT-END SYSTEMS*R. Keller,** W. Abraham, J.J. Ayers, D.W. Cheng, P. Cull, R. DiGennaro, L. Doolittle,** R.A. Gough,** J.B. Greer,**M.D. Hoff,** K.N. Leung, S. Lewis, C. Lionberger, R. MacGill, Y. Minamihara, M. Monroy, D. Oshatz,** J. Pruyn, A.Ratti,** J. Reijonen, T. Schenkel,** J.W. Staples,** D. Syversrud, R. Thomae, S. Virostek,** and R. Yourd**Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CAAbstractThe Front-End Systems (FES) of the Spallation NeutronSource (SNS) project have been described in detail else-where [1]. They comprise an rf-driven H- ion source,electrostatic LEBT, four-vane RFQ, and an elaborateMEBT. These systems are planned to be delivered to theSNS facility in Oak Ridge in June 2002. This paper dis-cusses the latest design features, the status of developmentwork, component fabrication and procurements, and ex-perimental results with the first commissioned beamlineelements.1  INTRODUCTIONThe Spallation Neutron Source (SNS) project [2] ispresently in the third year of its construction phase. LBNLis building the front end (linac injector) with its main sub-systems consisting of ion source, low-energy beam-transport section (LEBT), RFQ accelerator, and medium-energy beam-transport section (MEBT). Some parts of thefront end, i.e. the rf power system for the RFQ and theMEBT chopper structures with their power supplies, willbe supplied by LANL; some diagnostic elements by BNL.The SNS Front-End project has been described in detailelsewhere with an ample collection of references [1], andthe present paper emphasizes design changes and progresswith construction. Some basic features, however, are pre-sented here as well, to facilitate easier understanding andaddress recent changes in main SNS parameter values asthey relate to the front end. A 3-dimensional CAD layoutof the front-end beamline is shown in Figure 1, omittingall ancillary systems.The SNS accelerator systems aim at delivering intenseproton-beam pulses of less than 1-µs duration to the spal-lation target at 60-Hz repetition frequency and with anaverage beam power of 1.44 MW. The 1-ms long H- mac-ro pulses that are accelerated in the linac to 1-GeV energyhave to be chopped into ‘mini pulses’ of 645-ns duration,with 300-ns pauses.Chopping is performed in the front end by two separatechopper systems located in LEBT and MEBT, respectiv-ely. The LEBT chopper removes most of the beam powerduring the mini-pulse gaps, and the MEBT chopper re-duces the rise and fall time of the transported beam.The main requirements for the SNS Front-End Systemsare listed in Table 1. The front end is being assembled andcommissioned at the Integrated Testing Facility at LBNLbefore being shipped to ORNL in the summer of 2002.Figure 1. Layout of the SNS front-end beamline.                                      *Work supported by the Director, Office of Science, Office of Basic En-ergy Sciences, of the US Department of Energy under Contract No. DE-AC03-76SF00098.**Author/co-author of other contributions submitted to this conference.Ion Source/LEBTCreate 50-mA beamRFQAccelerate beam to2.5 MeVLEBT/MEBTChop beaminto mini-pulsesMEBTMatch 40-mAbeam into DTLLBNL-47317Table 1. FES Key Performance ParametersIon species H-Output energy (MeV) 2.5H- peak current:MEBT output (mA) 38Nominal ion-source output (mA) 50Output normalized transverse rms emittance (π mm mrad) 0.27Output normalized longitudinal rms emittance (π MeV deg) 0.126Macro pulse length (ms) 1Duty factor (%) 6Repetition rate (Hertz) 60Chopper system:Rise, fall time (ns) 10Off/on beam-current ratio 10-42  ION SOURCE AND LEBTThe H- ion source and LEBT are shown in Fig. 2. Thesource plasma is sustained by pulsed 2-MHz-rf power andconfined by a multi-cusp magnet configuration. A mag-netic dipole filter separates the main plasma from asmaller H- production region (darker area in Fig. 2) wherelow-energy electrons help generating copious amounts ofnegative ions. A heated collar, equipped with eight ce-sium dispensers, surrounds this H- production volume.The outlet plate of the ion source contains another di-pole-magnet configuration that creates a deflecting fieldacross the extraction gap in order to separate extractedelectrons from the ion beam and steer them towards a‘dumping’ electrode biased at 5 kV with respect to theoutlet plate. Because this dumping field steers the ionbeam as well, the entire plasma generator is tilted at anadjustable angle of about 3° against the LEBT axis tocompensate for this effect.Figure 2: Schematic of the ion source and LEBT. Notethat the actual filter and electron-dumping magnetic fieldsare oriented orthogonally to the illustration plane. Thewidth of the ion beam is exaggerated in this schematic toemphasize the focusing action of the double-lens system.The LEBT structure has to serve five main purposes,i.e., beam formation, 2-parameter matching into the RFQ,steering in angle and transverse offset, pre-chopping, andgas pumping. A fully electrostatic system with two einzellenses as focusing elements was chosen for the SNSLEBT. The second one of these lenses is split into fourquadrants which can be biased with d.c. and pulsed volt-ages to provide angular steering as well as pre-chopping.The LEBT can also be offset against the RFQ axis.The last LEBT electrode is part of the RFQ entrancewall, and on its upstream side it carries a diagnostic ele-ment made again from four insulated quadrants. Duringthe pauses in between mini pulses, chopping voltages of±2.5-kV amplitude and 300-ns duration are applied to op-posing pairs of lens quadrants in a rotating pattern, di-recting the chopped beam alternatingly towards each ofthe four separation zones between the diagnostic-electrodequadrants. In this way, any parts of the beam that are notintercepted by the diagnostic electrode are prevented fromhitting the RFQ vane tips whose accurate shapes couldotherwise gradually be eroded by sputtering.The LEBT-electrode shapes were optimized by simu-lating proton beams, using the 2-d positive-ion codeIGUN [3] in a novel way that allows introduction of finiteion temperatures into the calculation without experiencingunrealistic deformations of the plasma meniscus [4]. De-tails of the beam-formation and electron-dumping proc-esses were modeled [5] using the 2-d code PBGUNS [6]with actual H- ion species input and handing the trajectorydata over to the 3-d code SIMION [7]. These simulationswere helpful to determine the source tilt-angle and provethat no lateral axis offset was needed to obtain low emit-tances.The ion source and LEBT have been commissioned,and average beam pulse-currents of 50 mA have beenobtained at 6% duty factor and transported through theLEBT. Our simulations predict transmission values >85%through the RFQ with the actual emittances measured byan Allison scanner and shown in Figure 3. Peak beam-current values up to 68 mA were measured at the begin-ning of the pulses.Figure 3: Transverse emittances of a 50-mA beam, takenat the LEBT exit.The remaining issues for the ion source are the layoutof the rf impedance matcher and facilitation of the dis-charge ignition. The most promising approach consists inusing low-amplitude cw rf-power of 13.56-MHz frequen-cy to create a steady-state, low density plasma with  neg-ligible H- abundance, in addition to the main 2-MHzpulsed power. Details of a newly developed matcher de-Vertical0.3 π mm mradHorizontal0.33 π mm mradsign are given elsewhere [Staples**]; efforts are under-way to finalize the implementation of this scheme.3  RFQThe SNS RFQ is 3.72-m long overall and consists offour modules built as composite structures with an outerGlidCop shell and four oxygen-free copper vanes. Peaksurface fields reach 1.85 Kilpatrick, and the total rf powerconsumption is 800 kW during pulses. Water-cooled π-mode stabilizers separate unwanted dipole modes fromthe main quadrupole mode. Static frequency tuning isachieved by 20 slug tuners per module, and dynamic tun-ing by adjusting the temperature difference between vanetips and the outer walls of the modules.Figure 4 shows the assembled first module before thefinal brazing operation. This module has now been com-missioned, reaching the full rf gradient. The resonancefrequency with slug tuners at nominal positions is veryclose to the design frequency of 402.5 MHz, and the fieldflatness is better than ±1% peak-to-peak. The dynamictuning procedure involving regulation of the vane-to-walltemperature difference was successfully tested with thismodule. The other three modules are all in advancedstages of fabrication; Module #2 has been conditioned tofull rf amplitude as well.Figure 4: End-on view of the assembled RFQ Module #1prior to the final brazing operation. The upstream ends ofthe four vanes are seen at the center, with π-mode stabi-lizers penetrating the vanes horizontally and vertically.The first rf-accelerated SNS beam was achieved on thefirst day after connecting RFQ Module #1 to the LEBTtank. These experiments resulted in validating the struc-ture modeling efforts and the LEBT-chopper design. Asan example, Figure 5 shows the simulated and measuredbeam transmission values as a function of cavity excita-tion. The mini-pulse rise and fall times generated by theLEBT chopper amounted to 25 ns, twice as fast as hadbeen assumed for the design of the MEBT chopper target.Figure 5: Simulated (red squares) and measured (greencrosses) transmission values of RFQ Module #1, normal-ized to 100% maximum, for an input beam of 35-mA.4  MEBTThe 3.67-m long Medium-Energy Beam Transport(MEBT) structure shown in Figure 1 has three main func-tions, i.e., matching the beam from the RFQ exit planeinto the MEBT chopper and its target, cleanup chopping,and guiding the remaining particles into the Drift-TubeLinac currently being built by LANL. Matching in bothtransverse and in the longitudinal direction is provided by14 quadrupole magnets, arranged in three families, andfour rebuncher cavities. An anti-chopper will direct allparticles back on axis that were deflected by the chopperduring the rising and falling pulse flanks and not inter-cepted by the target.The MEBT will also contain diagnostic elements suchas beam-position monitors that will also gather phase in-formation, profile monitors, and two fast current trans-formers.All MEBT elements are grouped on three rafts that canbe individually aligned. At present, most major beamlinecomponents, including power supplies, have been fabri-cated and received at LBNL; power tests of the first re-buncher cavity are scheduled for June 2001.REFERENCES[1] R. Keller for the FES Team, “Status of the SNS Front-End Sys-tems, Paper MOP5B04, EPAC 2000, Wien (2000).[2] T. Mason, “The Spallation Neutron Source: A Powerful Tool forMaterials Research,” Paper MOAL04, these conference proceed-ings, PAC ’01, Chicago, IL (2001)[3] R. Becker, “New Features in the Simulation of Ion Extraction withIGUN,” EPAC ‘98, Stockholm  (1998).[4] J. Reijonen, R. Thomae, and R. Keller, “Evolution of the LEBTLayout for SNS,” Paper MOD19, Linac 2000, Monterey (2000).[5] R. Welton at al., “Simulation of the ion source extraction and low-energy beam transport systems for the Spallation Neu-tron Source,” submitted to ICIS ’01, Oakland (2001).[6] J. Boers, “PBGUNS,” Thunderbird Simulations, 626 Bradfield Dr.,Garland, TX 75042[7] D. Dahl, “SIMION 3D v.7.0,” Idaho National Engineering andEnvironmental Laboratory Idaho Falls, ID 83415 (2000)

Progress with the SNS front-end systems

Jairzinho Francisco Panqueba-Cifuentes

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