The Front-End Systems (FES) for the Spallation Neutron Source (SNS) project comprise an rf-driven H{sup -} ion source, an electrostatic 2-lens LEBT, a 2.5 MeV RFQ, followed by a 14-quadrupole, 4-rebuncher MEBT including traveling-wave fast choppers. The nominal 2.5 MeV H{sup -} beam has a current of 38 mA at a repetition rate of 60 Hz and 1 ms pulse length, for a macro duty-factor of 6%, and is chopped at a rate of approximately 1 MHz with a mini duty-factor of 68%. The normalized rms beam emittance at the MEBT exit, matching the first tank of a 402.5 MHz Alvarez linac, is measured to be approximately 0.3 {pi} mm mrad. Diagnostic elements include wire scanners, BPMs, fast current monitors, a slit-harp emittance device and RFQ field monitoring probes. The results of the beam commissioning and the operation of the RFQ and diagnostic instrumentation are reported. The entire FES was shut down at LBNL at the end of May 2002 and will be recommissioned at ORNL prior to installation of the drift-tube linac

Aleksandrov, A.

Ayers, J. J.

Doolittle, L.

Greer, J. B.

Keller, R.

Lewis, S.

Lionberger, C.

Monroy, M.

Pruyn, J.

Ratti, A.

Shea, T.

Staples, J. W.

Syversrude, D.

Thomae, R.

Virostek, S.

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RESULTS OF THE SNS FRONT END COMMISSIONINGAT BERKELEY LAB *A. Ratti, J.J. Ayers, L. Doolittle, J.B. Greer, R. Keller, S. Lewis, C. Lionberger, M. Monroy,J. Pruyn, J. W. Staples, D. Syversrud, R. Thomae, and S. VirostekE. O. Lawrence Berkeley National Laboratory, Berkeley, CA, USAA. Aleksandrov for the SNS Accelerator Physics Group and T. Shea for the SNS Beam DiagnosticsCollaborationOak Ridge National Laboratory, Oak Ridge, TN, USAAbstractThe Front-End Systems (FES) for the Spallation Neu-tron Source (SNS) project comprise an rf-driven H- ionsource, an electrostatic 2-lens LEBT, a 2.5 MeV RFQ,followed by a 14-quadrupole, 4-rebuncher MEBT includingtraveling-wave fast choppers. The nominal 2.5 MeV H-beam has a current of 38 mA at a repetition rate of 60 Hzand 1 ms pulse length, for a macro duty-factor of 6%, andis chopped at a rate of approximately 1 MHz with a miniduty-factor of 68%. The normalised rms beam emittance atthe MEBT exit, matching the first tank of a 402.5 MHzAlvarez linac, is measured to be approximately 0.3 π mmmrad. Diagnostic elements include wire scanners, BPMs,fast current monitors, a slit-harp emittance device andRFQ field monitoring probes. The results of the beamcommissioning and the operation of the RFQ and diagnos-tic instrumentation are reported. The entire FES was shutdown at LBNL at the end of May 2002 and will be re-commissioned at ORNL prior to installation of the drift-tube linac.1. INTRODUCTIONThe Spallation Neutron Source [1] is being built by acollaboration of six US Laboratories** led by Oak RidgeNational Laboratory which will operate the facility at theirsite. The project construction started in 1998, and itscompletion is expected by 2006.The collaborative nature of this project is reflected inthe initial commissioning activities of the front end (linacinjector) at LBNL described here, where members of theAccelerator Physics group at ORNL and the Beam Diag-nostics teams from BNL and LANL were contributinghardware, software, and technical support.The front end was built at LBNL and sequentially testedas the subsystems were completed and installed. Early re-sults obtained at the various stages were reported at previ-ous conferences, starting with the ion source and LEBTcommissioning [2] and the RFQ construction [3]. A sepa-rate paper at this conference describes the installation ofthe RFQ and its own commissioning with beam [4, 5]which started in January of 2002 and ended in March.The 402.5-MHz Low Level RF system is the subject ofanother paper submitted to this conference [6].The commissioning phase at LBNL demonstrated allrelevant parameters of the front end and supported the deci-sion to ship the equipment to ORNL on schedule in June.A key part of the commissioning activities was devotedto the beam instrumentation systems that were developedin a collaborative effort by several labs. The wire scannerhardware was built at BNL, while the rest of the beamlinedevices were built at LBNL. The electronics for the com-mercial Bergoz current transformers were developed atBNL, and LANL delivered the electronics for the wirescanners. A laser-based beam-profile monitor, althoughnot part of the project baseline, was delivered by BNL andtested on the front end beam [7].Beam was successfully transported to the end of theMEBT on the first shot on April 4, 2002. The beammeasurements described here went on through May 31when 50 mA were recorded behind the MEBT, see Fig. 1,well in excess of the nominal 38 mA.Figure 1. Faraday cup signal of a 50-mA beam trans-ported through the MEBT.2. BEAM SIMULATIONSThe simulation tools used for space-charge affectedbeam modelling were TOUTATIS for the RFQ and PAR-MILA for the MEBT. An actually measured emittancefrom the LEBT was converted into a 6-dim phase-space_________________* This work is supported by the Director, Office of Science, Office ofBasic Energy Sciences, of the U.S. Department of Energy underContract No. DE-AC03-76SF-00098.**The SNS project is being carried out as a collaboration of six USLab-oratories: Argonne National Laboratory (ANL), BrookhavenNational Laboratory (BNL), Thomas Jefferson National AcceleratorFacility (TJNAF), Los Alamos National Laboratory (LANL), E. O.Lawrence Berkeley National Laboratory (LBNL), and Oak RidgeNational Laboratory (ORNL). SNS is managed by UT-Battelle, LLC,under contract DE-AC05-00OR22725 for the U.S. Dept. of Energy.distribution and taken as input file for TOUTATIS. Insimulated output-beam spectra as a function of RFQ gra-dient, low-energy particles were accurately tracked andcould later be identified on measured RFQ emittances.3. DIAGNOSTIC BEAMLINEA diagnostic beamline consisting of an emittance slitand collector harp was developed for use with the SNSFront End commissioning. This device was used for beamstudies at the end of first the RFQ and later the MEBT.The slit consists of two 13.5” long jaws made frommolybdenum TZM and arranged with a total opening an-gle of 10° and a slit opening of 0.002”. Both jaws are ac-tively cooled with passages machined into back plates thatare brazed to either jaw. The slit was designed with thehelp of a time-resolved ANSYS model that accurately de-picts the deposition of heat below the surface of the mate-rial. It can handle the full power, 60-Hz, 2.5-MeV beamwith a pulse length of up to 50 µs.The collector is a customised NTG 32-wire harp with0.5-mm wire spacing. It includes a back plate to suppresssecondary electrons. A 32 channel, 2 MS/s, 16 bit com-mercial data-acquisition board is used to process the sig-nals from the wires and the back plate. Measured dc biassignals were subtracted from the raw data, and slit scatter-ing effects observed with this system as shown in Fig. 2were subtracted during the final data analysis. All compo-nents were fiducialized on a CMM, providing accuratealignment of the slit to the beam and the harp to the slit. In order to cover the observable phase space, the slitand collector positions are controlled by two separate mo-tors travelling at a fixed speed ratio of 1:7. This resultedin an overall reduction of the number of electronic chan-nels needed for the data acquisition. The motion of thesetwo stages, as well as the data acquisition itself, werefully controlled in the EPICS environment. The devicecan be rotated by 90° to measure in both transverse direc-tions.Figure 2. Slit scattering effects of about 1% of peaksignal measured on the emittance device.A beam stop capable of withstanding the full beampower at full pulse length was also developed using thesame jaw design.  In this case, the jaws are set at an open-ing angle of 3.3° and with zero gap width, and a backplate is used to catch any stray beam. The beam stop canbe installed in the vacuum box in place of the slit.4. BEAM COMMISSIONING RESULTS4.1 Beam Transmission MeasurementsAs in the commissioning of the RFQ, MEBT beamtransmission measurements were made as a function of theRF power in the RFQ. The results were in excellentagreement with TOUTATIS simulations. Beam loss inthe MEBT itself was negligible as expected.Figure 3. Calculated and measured beam transmissionthrough the RFQ as a function of rf power.4.2 Transverse Emittance MeasurementsFig. 4 shows an example of a measured MEBT emit-tance exhibiting a minute amount of halo. Normalizedrms emittance sizes as a function of beam current aregiven in Table 1.Figure 4. Measured horizontal MEBT emittance withhand-traced halo border at about 10-3 of peak value.4.3 Beam Profile MeasurementsBeam profiles were measured in five locations along theMEBT, and the results compare well with the beam dy-namics calculations. The deviations from expected profilewidths are within ±10%. Fig. 5 shows a typical wire pro-file measurement, where at first a horizontal wire crossesthe beam, then a diagonal one, and finally a vertical wire.Figure 5. Example of a beam profile measurement.Table 1. Measured  MEBT emittancesNo background subtraction except for slit scatteringCurrent Rms Size ConditionmA π mm mrad24 0.276 hor/unbunched33.4 0.281 hor/unbunched27 0.261 vert/unbunched27 0.270 vert/bunched5. ENDURANCE TESTA round-the-clock endurance test was performed to ver-ify the initial robustness of the system. The system wasrun at high duty factor, initially above 1%, and graduallyincreasing to about 3%. One week of testing resulted inno failures caused by beam operations.6. PLANS FOR OPERATION AT SNSThe Front End was disassembled starting on June 3, andshipped to Oak Ridge during the following six weeks. Ithas already been reassembled at the SNS site, and a vac-uum check has confirmed the integrity of all components,including windows. Low level RF measurements con-firmed the proper positions of all RFQ drive loops thathad remained installed on the cavity during shipment. Thediagnostic beam-line will be used in Oak ridge to supportthe Front End commissioning activities later this fall.These activities include commissioning the final power rfsystems and testing the MEBT choppers, both supplied byLos Alamos, as well as integrating all local infrastructuresuch as controls and timing. The re-commissioning of theFront End will be completed by the end of 2002.7. CONCLUSIONSThe SNS Front End commissioning was highly suc-cessful and produced important results. All major parame-ters that were specified for the injector were individuallydemonstrated, including high-duty-factor operation andbeam production, transverse emittances, and beam pro-files. Issues that were not fully explored were the longitu-dinal emittance, due to the lack of an appropriate diagnos-tic device, and the ion-source antenna lifetime, which wastested to over 100 hours of operation only because timeconstraints did not allow a full test to the limit.Among the concepts that were proven or further vali-dated, probably the most visible ones are the effectivenessof the cesium-enhanced H- production, electron removal atlow energy, and beam transport in an electrostatic LEBT.During commissioning, a proof-of-principle demonstra-tion of the laser-based beam profile monitor was conductedby the ORNL and BNL collaborators [8].8. ACKNOWLEDGEMENTSThe success with such a complex system would havenever been possible without the effort and commitment bya large group of people. Among them are the SNS Con-trols group, in particular Dave Gurd, Ernie Williams andErik Bjorkland; the SNS Beam Instrumentation groupwith Saeed Assadi, Wim Blockland and Dave Purcells, theLANL Beam Instrumentation group with Mike Plum,John Power, Matt Stettler, Chris Rose, Wym Christen-sen, Lisa Day; and staff from BNL, Peter Cameron andCraig Dawson; in addition, the SNS Accelerator Physicsgroup with Marion White and John Galambos, as well asPaul Gibson, Rob Welton, and Martin Stockli gave valu-able contributions.Other essential contributions came from the EH&Sgroup at LBNL, including Pat Thomas, Ken Barat, TomCaronna and Ted DeCastro. The priceless help from theFront End Electronics and Mechanical Engineering groupsis also acknowledged, mainly by Daryl Oshatz, AllanDeMello, Bill Abraham, Pete Cull, Tim Kuneli, MarkRegis, and Nathan Ybarrolaza.9. REFERENCES[1] M. White, “The Spallation Neutron Source,” these pro-ceedings.[2] R. Keller et al., “Progress with the SNS Front End Sys-tems,” proceedings of the 2001 Particle Accelerator Con-ference, Chicago, IL, USA, June 2001, 70-72.[3] A. Ratti et al., “The design of a high current, high dutyfactor RFQ for the SNS,” Proc. European Particle Accel-erator Conference, Vienna, Austria, June 2000, 495-497.[4] A. Ratti et al., “The SNS RFQ Commissioning,” theseproceedings.[5] A. Ratti et al. “Fabrication and Testing of the First Mod-ule of the SNS RFQ,” Proc. XXth International Linac Con-ference, Monterrey, CA, USA, August 2000, 911-913.[6] L. Doolittle et al., “The SNS Front End LLRF System,”these proceedings.[7] T.J. Shea et al, “SNS Accelerator Diagnostics: Progressand Challenges,” Proc. 2001 Particle Accelerator Con-ference, Chicago, IL, USA, June 2001, 512-516[8] R. Connolly, “Laser Beam Profile Monitor Developmentat BNL for SNS,” these proceedings.

Results of the SNS front end commissioning at Berkeley Lab

Results of the SNS front end commissioning at Berkeley Lab

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