The RHIC proton polarimeters can operate in scanning mode, giving polarization profiles and transverse beam intensity profile (beam emittance) measurements. The polarimeters function as wire scanners, providing a very good signal/noise ratio and high counting rate. This allows accurate bunch-by-bunch emittance measurements during fast target sweeps (&lt;1 s) through the beam. Very thin carbon strip targets make these measurements practically non-destructive. Bunch by bunch emittance measurements are a powerful tool for machine set-up; in RHIC, individual proton beam transverse emittances can only be measured by CNI polarimeter scans. We discuss the consistency of these measurements with Ionization Profile Monitors (IPMs) and vernier scan luminosity measurements. Absolute accuracy limitations and cross-calibration of different techniques are also discussed

Bazilevsky, A.

Bunce, G.

Gill, R.

Huang, H.

Makdisi, Y.

Morozov, B.

Nemesure, S.

Russo, t.

Sivertz, M.

Steski, D.

Zelenski,A.

UNT Digital Library

"BROOKHaVENNATlONAL LABORATORYBNL-81865=2009-CPBeam emittance measurements in RHICA.. Zelenski, G.. Atoyan, G.. Bunce, A.. Bazilevskiy, R Connolly,D.. Gassner, R.. Gill, He Huang, G. Mahler, Y.. Makdisi,B. Morozov, S. Nemeshure, T. Russo, D. Steski, M .. Sivertz,S. Tepikian, R Todd, K. YipPresented at the Particle Accelerator Conference(PAC09)Vancouver, B.C., CanadaMay 4-8, 2009Coliider..Accelerator DepartmentBrookhaven National LaboratoryP.O. Box 5000Upton, NY 11973..5000www.bnl.govNotice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC underContract No. DE-AC02-98CHI0886 with the U.S. Department of Energy. The publisher by accepting themanuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,irrevocable, world-wide license to publish or reproduce the published fonn of this manuscript, or allow othersto do so, for United States Government purposes.This preprint is intended for publication in a journal or proceedings. Since changes may be made beforepublication, it may not be cited or reproduced without the author's permission.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor anyagency thereof: nor any of their employees, nor any of their contractors,subcontractors, or their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or anythird party's use or the results of such use of any infonnation, apparatus, product,or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herem to any specific commercial product, process, or serviceby trade name, trademark, manufacturer, or otherwise, does not necessarilyconstitute or imply its endorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof or its contractors or subcontractors.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereofBEAM EMITTANCE MEASUREMENTS IN RIlleA. Zelenski, G. Atoyan, G. Bunce, A. Bazilevskiy, R Connolly, D. Gassner, R. Gill, H. Huang,G. Mahler, Y. Makdisi, B. Momzov, S. Nemeshure, T. Russo, D. Steski, M. Sivertz, S. Tepikian, R.Todd, K. YipBrookhaven National Laboratory, Upton, NY, 11973, USAAbstractThe RHIC proton polarimeters can operate in scanningmode, giving polarization. profiles and transverse beamintensity profile (beam emittance) measurements. Thepolarimeters function as wire scanners, providing a verygood signal/noise ratio and high counting rate. Thisallows accurate bunch-by-bunch emittance measurementsduring fast target sweeps «1 s) through the beam. Veryfum carbon strip targets make these measurementspractically non-destructive. Bunch by bunch emittancemeasurements are a powerful tool for machine set-up; inRHIC, individual proton beam transverse emittances canonly be measured by CNI polarimeter scans. We discussthe consistency of these measurements with IonizationProfile Monitors (IPMs) and vernier scan luminositymeasurements. Absolute accuracy limitations and cross-calibration ofdifferent techniques are also discussed.INTRODUCTIONThe Relativistic Heavy Ion Collider (RHIC) is the firstpolarized proton collider. Siberian snakes weresuccessfully used to avoid resonance depolarizationduring beam acceleration [1]. Polarized beam emittancereduction is of a great importance for polarized protonbeams, since it reduces the possible depolarisation duringacceleration in the AGS and RIllC along with improvingluminosity. Emittances in RIllC can be measured in anumber ofdifferent ways, such as with IPMs and throughvernier scans. Each technique has its own systematicerrors and limitations. There is no conventional wire-scanner in RHlC~ but beam intensity profiles can bemeasured with p-Carbon CNI polarimeters [2]. In ascanning mode~ the polarimeter counting rate dependenceon the target position can be used for beam intensityprofile as wen as polarization measurements. Recoilcarbon atoms knocked out of the target in elasticscattering are detected in silicon-strip detectors. Therecoil carbon energy and time-of flight are measured forelastic collisions reconstruction. The delayed arrival timeof the recoiled carbons is advantageous since thedetection can be done in a noise-free environment afteraU beam-induced disturbances are gone. Every elasticevent has a time stamp attached and each event is sortedby the bunch crossing. With the high event rate, largestatistical samples can be accumulated in a short time (ofa few hundred milliseconds for the target fast scanthrough the beam), which is used for bunch-by-bunchbeam profile (or emittance) measurements. These*Work supported by DOE under contract No. DE-AC02·98CH 10886and RIKEN ofJapan.zelenski@bnl.govmeasurements have negligible effect to the beam lossesand are a valuable tool for setup and monitoringoIn the 2009 run, polarized beams were accelerated andcollided at maximum RHIC energy of 250 GeV(-$=500 GeV) [3]. The 200 MeV linac injector upgradeand high current polarized source operation (whichallows additional vertical beam scraping after theBooster) as well as work on improving the Booster toAGS injection optics matching resulted in beamemittance reduction at RHIC injection. Assumingconservation of nonnalized emittance from injection at23.7 GeV through acceleration to 250 GeV, the physicalbeam size should shrink by .J.250/23.7) =3.2 times. Thebeam size in the RHIC horizontal IPMs of about 0.5 nun(sigma) is smaner than the IPM resolution~ thereforepolarimeter intensity scans are the only availablemeasurements ofhorizontal emittances.The CN! polarimeters were upgraded for the 2009 run.Two identical target motion mechanisms and detectorsassemblies were installed in new vacuum chambers ineach ring. One polarimeter is used for the verticalpolarization and intensity profile measurements and theother is used for the horizontal profile measurements (orvice versa). As a result the systematic polarization,polarization profiles and emittance measurements wereobtained for both transverse planes. This is in contrast toprevious nms where measurements were limited to oneplane due to long target switch times.POLARIMETER HARDWAREThe use of thin targets in a polarimeter is essential toreduce multiple scattering for recoil carbon ions and keepthe event rate within detector and DAQ capabilities.Carbon strips used in polarimeter are 5-15 urn wide (~5J..l.glcm2 thickness), and contain about 1013 carbon atomsper mm of target length. The target length is 25 nun. Highintensity circulating beam knocks out about 107_108carbon nuclei/s, which cause the eventual targetdestruction, It was experimentally demonstrated thattargets survive in the RIlle beam for at least 100-200measurements at the full beam intensity, whichcorresponds to the one target lifetime of about one-to-twoweeks. Manufacturing of the ultra-thin polarimeter carbon(amorphous graphite) targets requires high skin and is atime-consuming process. Multiple targets (six verticaland six horizontal) are attached to a target ladder toextend the time between maintenances. The precisionprocedure was developed to provide about +1-0.1 mIDtarget alignment accuracy on the ladder, therefore theM.,ti.r St~p~T 2009-02.23_182336Figure 1: Beam position and bunch width measurementfor the 12 bunch store.and counting rate saturation if the target effectivethi~CkJ.!.ess is_too h~_.__".__~__~.__~~,~ ~"_~.,::;::tMt t t:t-'t*....j: WW-%Wl-;m ~ fS¥l:% XIn.u~ ,t. tJ"i'lh1.4n~~ !'rA:EJ/I:'P,,~~".eeu..\pr ;: t. 2tltJq ~~1-q Jij. ~-lEmittanc. Scan - Blu..... l 10597.403 e. .. 100.14 G..VW~r..·".:'!!!:..!--_-__"_~,Ii~~,. ;~c"'"- J"~ .~:\W!l. !!. 'i'l _~,~ ~~1 ':r ·.··.' "',,~, - ::: ;II £'~,.~ ~."" , ,Vl~~ti'jU"'it"~ ;.;. 4!i2t'O~af8-", n1'eA~" 3'8.6"'i.'('tiJtlCP:"", "A~f1J2il:nPeA" ',',' 25:H_G-t ift ~e.e~llWidth ... (:l.o.fi77'+. etI.tm'3 am",)800After beam exposure the carbon target expands and thetarget is loosened, which may corrupt the profilemeasurement. Cross-checks with different (fresh,unexposed) targets were used to provide more reliablemeasurements.A proton-carbon CNI polarimeter similar to the RHICpolarimeter is used for AGS polarization measurements.The polarimeter operates in a mst continuous sweep modefor 0.8s while beam is stored in the AGS at 23.7 GeY.This measurement produces very reproducible horizontaland vertical beam intensity profile measurements for thesingle AGS bunch. These measurements are used forcross-checking of the AGS IPMs measurements andmachine set-up to minimize beam emittances [4].POLARIZATION AND BEAM INTENSITYPROFILE MEASUREMENTSThe carbon target width of 5-15 /lm is much smallerthan the beam size. Therefore intensity and polarizationprofiles can be measured by the target scan. In scanningmode the counting rate dependence on the target positioncan be used for the beam intensity profile measurementsin addition to polarization measurements. Usingwaveform digitizers (WFD), every elastic event has atime stamp attached and each event is sorted by bunchcrossing. The target position is defined by a synchronousstep-motor counting in one WFD channel.With high event rates, large statistics are accumulatedin a very short time for fast target scans. Typical targetvelocity for emittance measurements is about 14 mmls. Atthis velocity the target is swept through the beam in lessthan one second at injection, where beam size is '0" ~1.5mm, and even faster at store energies. Accumulatedstatistics during this fast sweep are sufficient to measurebunch-by-bunch beam width with statistical accuracy of+/-2%. An example measurement is shown in Fig. 1.The application for emittance measurements imports ~­functions from the on-line RHIC optical model andcalculates beam emittances. The accuracy of ~-functionvalues is about 10%.The absolute accuracy of these emittancemeasurements is limited by the target quality/straightnesstarget positioning accuracy is limited only by the targetstraightness. A combination of linear and rotationalmotion in the target mechanism provides the targetreplacement and polarization scans operation. Use ofprecision linear stages and rotational vacuum feed-throughs gives target position accuracy of+/-O.2nun.Two identical polarimeter vacuum chambers arelocated in the warm RHIC sections, which are separatedfor two circulating beams and have separate vacuumsystems. The chambers are situated between two isolationvalves and each is equipped with an ion-pump incombination with Ti-sublimation pump and a NEG-cartridge pump, which are isolated by 8.0" gate-valves.This vacuum system allow polarimeter maintenance suchas target and detector replacement in short 12 hrshutdowns. Silicon detectors and carbon strip-targets cannot sustain any baking, but with the powerful pumps andout-gassing by the beam (beam scrubbing) the vacuum inthe polarimeter was reduced to 1.3.10-9 Torr level, whichis acceptable for RIDC operation.Time-of-flight and recoil carbon energy measurementsare required for elastic scattering identification. Silicon-strip detectors are used in the polarimeters, which allowsmeasurements of energy and arrival time of recoilCarbons in the RHIC ring vacuum environment. At fullRHIC design intensity, the bunch width is about 8 ns andbunch spacing is 106 ns. To avoid prompt background,carbon nuclei should arrive at the detectors in the timewindow between two bWlches.Figure 2. AGS beam intensity profile measured by theCNI polarimeter scan intensity 1.5·1011 protonlbunch.Width: 0=0.98 mm, horizontal emittance-15mnm mrad.IONIZATION PROFILE MONITORSFigure 3. Beam emittances (measured with !PM) growth(18 to 22 mnm mrad) to during the 10 hrs store (toppicture). Beam intensity decay (bottom picture).3°1;".". _-----,-20\" ..:. ~ .,~:.~:.?:\ •.:~;..~:!.,i~~;~l~,•.:,:~~~:.~:~~;j;;~:~~~:~ ..~r :~.. .I,~ ....\~Jt.......tr..J,rw4:l..N .... ~. '" .a:, .1G/1RHIC IPMs are based on ionization of residual gas andcollection of electrons, which are swept by an electricfield that is transverse to the collector strips and parallelto the beam. Micro-channel plates (MCPs) are used toamplifY the signaL A uniform magnetic field parallel tothe electric field is applied to counter the defocusingeffects of the space-charge and recoil momentum [5]. Theionization cross-section is proportional to ~Z2 so gold ionbeams produce a sufficient number of electrons for singlehooch detection. For proton beams only average bunchwidth measurements are possible. The collector strips arespaced about 0.5 mm apart, which limits their resolutionfor smaller beam sizes. RIlle vertical IPMs are situatedin a region with large J3-function ofabout 80m; horizontalIPMs are situated in a region with smaller 13 of -25m. At250 GeV, the proton beam size shrinks to 0' ~ 0.5 mm asmeasure at J3=-25m with the eN! polarimeter. Thehorizontal IPM measurement accuracy is limited by aninsufficient resolution. Additionally, recoil electronmomenta of about 100 eV introduce Larmor precessionswith radii of about 0.5 mm. In the vertical IPMs the beamsize is larger, 0' -0.8 mm. From cross-calibration withCNI polarimeter scan measurements, correction factorsfor the finite IPM resolution were calculated and appliedto the IPM emittance measurements. Continuous periodicIPM measurements provide complimentary informationabout the beam emittance evolution during the store time(see Fig. 3).0"SUMMARYThe p-Carbon CNI polarimeters at the AGS and RHICprovide fast, practicaUy non-destructive polarization andintensity profile measurements. A polarimeter upgrade hasbeen completed for the 2009 polarized proton run, whichallowed "4complete" polarization measurements includingpolarization profile scans in both horizontal and verticalplains. Accurate beam intensity profiles measurements(beam emittances including bunch by bunch emittances)were additionally used as a complimentary diagnostics formachine set-up, operation and cross-calibration with theother techniques.The Linac injector upgrade reduced emittances from 151t mm mrad to about 5 1t rom mrad.Higher polarized source intensity allowed reduction of thelinac pulse duration, which reduced the multiplescattering in the stripping foiLEven with short linac pulses, the beam intensity issufficient to use vertical scraping in the Booster (about50%) to reduce vertical and horizontal beam emittances.Small emittances in AGS help to reduce depolarization.The ongoing work on injection optics matching to AGSresulted in smaller « 15 1t mm mrad) beam emittances, asmeasured with the CN! polarimeter scans in AGS [4].Similar emittance values were measured in theluminescent profile monitors in the ATR (AGS- To-RHIC) beam line and at injection to RHle (by the CNIpolarimeter scans). These emittances depend on beambunch intensity and AGS set-up and periodical tuning andmonitoring is required to maintain the smallest possibleemittances for injection to RIllC.Accurate measurements with CNI polarimeter scansdemonstrated emittance preservation for acceleration to100-250 GeV in RHIC, and resolved inconsistenciesbetween IPM and vernier scan measurements. Vernierscan measurements produced averaged vertical andhorizontal emittances of about 16-20 1i"·mm-mrad.Smaller beam emittances obtained after upgradescontributed to the RIlle luminosity increase in run 09.ACKNOWLEDGEMENTSWe would like to thank the staff of Collider AcceleratorDepartment Beam Component, Vacuum and ControlGroups for great effort to complete polarimeter upgradeproject on time. We acknowledge the financial support ofthe RIKEN-BNL Research Centre.REFERENCES[1] T. Roser, AlP Couf. Proc. 980, p.lS, (2008).[2] l. Nakagava et ai., ibid., p.380, (2008).[3] M.Bai et aI., this conference[4] H.Huang et aI., this conference[5] RConnoHy et al.~ Proc.PAC 2005, p.230, (2005).

Beam emittance measurements in RHIC

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Beam emittance measurements in RHIC

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