First-turn beam losses in the LAMPF Proton Storage Ring were measured as a function of the left-right position of the carbon foil used to strip neutral hydrogen atoms to H[sup +] for proton injection into the PSR. Two foil thicknesses, 200 and 300 [mu]g/cm[sup 2], were tested. Results indicated that first-turn loss is caused predominately by magnetic field stripping of a small fraction of the H[sub 0] atoms that pass through the stripper foil without being stripped to protons, and the results were not consistent with a mechanism involving protons originating from atoms in the halo of the neutral beam incident on the stripper foil

Frankle, S.

Hutson, R.: Fitzgerald, D.

Macek, R.

Plum, M.

Wilkinson, C.

UNT Digital Library

:LA-UR- 93-1867.Title,Author(s):Submitredto:Los AlamosNAIIONA1 LABORATORY‘\,,, . ‘.,,, -.,’ 1( i,,Srnpper-Foil Scan Studies of the First-turn Beam LossMechanism in the LAMPF Proton Storage Ring (PSR)R. HutSon, D. Fitzgerald, S. Frankle, R. Macek,M. Plum, and C. Wilkinson199.JPurticle Acccleratw Conference,Washington, D,C. May 17-20, 1993i .,l,.m M mm I!*..1 Jnn IIIAJI , ,‘-QE)lFil”wt Mlll(.3N 01 11 {1:, ; ~1If ,1 Ir.11 1,11 l!, I INI lMllt 1)Stripper-Foil Scan Studies of the First-Turn Beam LOSS Mechanism in the LA\lPFProton Storage Ring (PSR)R. Hutson, D. Fitzgerald. S. Frankle, R. !vlacek, M. Plum, and C, WilkjnsonMedium Energy Physics Division, Los .4.lamos National Laboratory, Los Alamos, NM 87545 USAAhrracrFirst-turn beam losses m tie LAMPF Proton Storage Ringwere measured as a function of the letl-right position of thecarbon foil used to strip neutral hydrogen atoms to H+ forproton injection into the PSR. Two foil thicknesses, 200 and300 pg/cm2, were tested. Results indicated that fwst-tu.rn!OSSis caused predominately by magnetic field srnpping of a smaIlfraction of the HOatoms that pass through the stripper fodwithout being stripped to protons , and the results were notconsistent with a mechanism involving protons originatingfrom atoms in the halo of the neutral beam incident on thestripper foil.I. INTRODUCTIONA significant fraction of beam losses in the PSR arepresently duo to protons being lost before completing one turnaround the ring. The cauaa of these tiit-turn losses haa notbeen un&rstood until now. Earlier hypotheses hinged on theicleathat first-turn losses constituted 10SSof protonsin the haloof the injected beam, but mea,mtrcmeraraof the extent of beamhalos did not conclusively support him idea.In an accompanying paper [1], a new hypothesis about thecausa of fret-turn beam Iossaa in the PSR is proposed. It issuggested that unstrippcd HOsemerge from the foil in excitedstates, are subsequently field Srnpped to H+s in the firstbending magnet downstream of tho sttippar foil, and am then],~stbf~ completing the first turn urotlnd b ring becausethey wem stripped to H+ outaida tha acce#rbca phAM spmof the ring.The pmsssntpaper doacribaa tb mstaaurunmtta made tosearch for twidsmx that would diatingtdab ~ theaeItwomdaniama frx tit-harts haaoa ova b othU.U. BEAM INJX!!’M3N INTO THE PSRProtons am istjectd into tJM PSR by magnetic fielddpping 800-MoV W ions to H% and tbea stripping the Hotto protossain ● 2W+g/cm2 carbon foil plud 0sstha ring axis.Roughly 5% of the injected @s thathittha foil are not!itfippCdto ~+s and are tMtsS@ tht’Ott@t the field of thCdowsutmam rini bander and out thswtgh a hoh in tha magnetyoka to a beam atop, This umsgament is illustratedschasnxtically in F@sw 1, which shows tlM injection seasonof tha PSR. For emta PSR pulm, protons am injectad at aconstant raIe for typically 600 pa, and at tha end of inyxuon[hey u-a munedia!mlyextractat.Figu 1. Beam h~jcction into the PSRIll. POSSIBLE ORIGINS OF FIRST-TURNLOSSES AND THEIR CONSEQUENCESA. BeaRsHalosIf the halos of the injected beam are too large, protonsoriginating from smpped H% in the halos WIIIfall outside [hephasaesp=e acceptance of the ring, and WINbe lost qulcklv bycollisions wsth the walls of the Mg vacuum pqx, Because, Inthe horizontal plane, the injected beam has a signi!lcm[lylarger emmance than in tha vertical plane, and because (here I$a large mismatch between the injected beam horizontal- plmephase space eilipaa and the ring lattice eilipsc, protons trornbeam halos in the horizontal beam profile am more llkcly (()cause first-turn baas than am haioa in the verucai protilef), Magnetic Fidd Wpping ufficittd $sA recent Wqgeation [1] about the cause of first-turn l(Ifw\IS t!!! a mad! !hCtMt of unstipgmd H% exit she strrppcr !(II!in relatively loosely bund excited stmernthat can he tICIdstripped to H+s in the magnetic field of the firsI down~frctm~ring ‘nmding magnet. Howaver, since an excltcd H“ hil$ Jfinite Iifetinsa in ● magnetic field, it WI1lnot be lmm:dlawl~strapped to a proton whan it enters the fringe field (It themagnet, md. u ● conaeqttasm, its trajectory in [he m.agnc[before being wzippad will not ba exactly tha 9ame M (ha[ (It Aproton. A rigniflcant tsumh of the protons rcsultlng I r{ Imexcitad Ho stri~ing will, than, find thamsalves (Nits Idc [hcacceptance pha~ spaca of the ring, and wIil be lot( hvcollisions with the beans pipa wall before maklrrg d tIillrevolution around tha rin~.Iv. ‘rm MEASUREMENTSA. ‘MethodsOur approach to gathering evidence in support of aparticular first-turn loss mechanism hinged on twocomparisons. The first was to look at the change in fwst-tumlosses vs. the change in ring beam as the stripper foil wasscanned horizontally across the injected Ho beam. Therelationship between the amount of ring beam and the first-tum losses was then compared with the patterns expected foreach of the two r techanisrns described above.The second comparison was between the magnitude offirst-turn losses for two different stripper foil thicknesses. Iffield stripping of excited HOs is the primary cause of losses,then the losses should be less for thicker foils since thestripping efficiency is greater, leaving fewer excited HOs to befield stripped in the bending magnet.Both the fu.’-scan and the foil-thickness comparisonsinvolved measuring two quantities, ring beam and fwst-turnlosses, A toroidai current monitor in the ring was used tomeasure the ring beam current while beam losses weremeasured with a system often loss monitors spaced uniformlyaround the periphery of the ring tutmel.8, Stripper Foil ..ScanStudiesThree different foils, 200 yg/cm2 thick and 10 mm wide,200 vg/cn? thick and 16 mm wide, and 300 I@c& thick and16 mm wide, were scanned woss the beam. Each foil wasmoved in i-mm horizontal steps across the injected H* beamwhose horizontal rms width at the foil was 6 mm.At each foil position wc measured both the ring beamcument and the first-turn losses. The amount of beam injectedinto ring varied as the foil was moved to cover differentamounta of the incoming HObeun.If the beam-halo mechanism is the cause of first-tu.mlosses, then the loss rate will be directly proportional to theamount of beam halo that is covered by the stripper foil. If oneuses a stripper foil wide enough to completely cover theinjected beam, then, as the stripper foil is moved from aposition completely outaidc tha bounds of the bans to whereit starts covering Iha b halo on ma Ma, one would see israpid rige of the IOSS rtto. Aftmr the halo on one sido is~omplcteiy covered by tlM SItrippar foil, moving the foil tocover more ~)f the injectd km would result in more ringbeam, but it would not caww a significant incrcrua in theamount of first-turn loss. As the foil in moved farther tu also(:r)vcr (be ~~ h~o on the othr)r side. the lose rate would,]p!tin increase untd the halos on both sides are covered, atwhich point first-turn 1rse rata wodf be a maximum.There is ewdence, from beam Wwing in the hmizontalplane, that the injti”ed beam very nearly fills the acceptedphase $pace of’the titt~, and that boarn halos can contmhute tofirst-turn Iomcs if the uyected km is not steered properly m:the ring axia [2]. Iiowcver, there is no conclusive cvidcmcthat, when the beam is properly steered. beam halos conmbutesigmficantly to fwst-rum losses.On the other hand, consider the case in wh~ch losses arecaused by field stripping of excited HOs. Since both [henumber of H-s aod H“s are directly proporrlonal [o thenumber of @s incident on the foil one would expect to tindthat the amount of first-turn loss IS Ilnearly related [o the ringbeam current.Figure 2 illustrates, fur the two mechanisms descr]lwdabove, the expected qualitative relationship between the t-lrst-turn losses and the Iing beam as the foil IS moved from Jposition where none of the injected Ii” beam IS beingintercepted by tie foil to a position where the foII IS centeredon and completely covering the beam.Figure 2. Expected Patterns of First-Turn Beam Loss VJBeam Injected into the PSR for the Two Loss MechamsmsC. Foil TLickmsscs StudiesAlso of intermt is the difference between first-turn lossesfor strrpper foils of different thicknesses. Thicker foils WIIIstrip more of the injected beam, leaving fewer Hos andcorrespondingly fewer excited H*s. If excited HOs are thecause of fmt-tum losses, then thicker foils should result in theproduction of fewer excited HOs, In this case losses WIII heproportional to the amount of beun injected into the nng I(,on the oher hand, protons originating from the tnjcctcd hcmnhalos cause first-turn losses, the ratio C’ .~ssesto n ng hcnrnwill be largely indqmndent of foil th. dkness. Wc hasc i)urcomparison of fmt-tum losses vs. foil thickness on the d:IIJfor the two 1&mm wide foils of 200 Kg/cmL and 300 I@ m:thickness.V. RESULTS AND CONCLUSIONSA, Smipper Foil Scan StudiesResults of the foil scan studies are summarlml III I IXUIC3, The plots show, for each of the three foils u~cd, IIICmagnttude of fht-tum low vs. ring bcarn current iis the !(III1$ warmed acrom the beam. The plots txacc out Ihc f Ir\t 1111n1(NS history ns the foils am scanned from the far Icti \Iclc ~t IIIC..beam {negative x values), to the beam center, and finally tothe far right side (positive x values).The 10-mm-wide foii results in Figure 3a show an almostlinear relationship between fwst-tum losses and ring beambotlt as the foil is moved from the far left side of the beam tothe center of the beam and continuing to the right where mostof the beam once again misses the foil. This result is exactlywha! would be expected if first-turn losses are caused by fieldstripping of HOs.Figure 3a. First-turn loss vs. Ring Beam for 200 I.@cn#, 10-mm-wide Carbon Stripper Foil!100I ‘1- l-l-mFigure 3b, First-turn loss vs. Ring Beam for 200 f.@cm2, 16-mm-wide Carbon Stripper Foil#. +14,0 1Figure 3c. First-turn loss vn. Ring Beam for 300 1.LgiCrn2,16-mm-wide Carbon !itripptw Foilfoil cases there is a range of foil positions near beam centerfor which most of the injected beam is covered resultlng In anessentid]y constant rtng beam current for several adJacent fodpositions. An unexpected feature of the results for these twowider foils IS that first-turn losses change signl~cantly overthe range of positions for which the fol! is roughly centered onrhe beam. We believe tha! this asymmetry m the pattern of thefirst-turn vs. beam current relationship is probably due [osome comblnat]on of an asymmetry In the phase spaceacceptance of the ting and/or a spill-locatlon sensitivity ut theloss monitors, although we have not yet made detiwled iludlesor calculations to verify this idea,B. Foil Thicknesses SrudiesFigures 3b and 3C indicate that, for the same amount ofring beam current, f~st-tum losses with the 300 kg/cm2 foilare about 45% of the losses with the 200 pg/cm2 foil.Assuming that field stripping of excited HOs is the cause offirst-turn losses, and estimating the fraction of the lnJecteclHOs that survive unstrapped after passing through a foil, wepredicted that losses with the 300 @cm2 foil wou14 heroughly 30% of the losses with the 200 yg/cm2 one. “fhisprediction is significantly lower than was observed. However,there is considerable uncertainty about th: effect of fwlthickness on in the energy level distribution of exclteci H()states, The number of HOsthat are field stripped is sensluve [oth)s distribution so the observed discrepancy hetwt?cnmeasurement and prediction is not surprising, [f protons tromthe halos of the injected beam were the cause (Ii iir~(. [urnlosses, the losses would be the same fraction of’ the Injectedbeam independent of foil thickness.Therefore, wc conclude that ttrI’ results of the tIIIlthickness studies are consistent with the hyprxhesls [Ilfi!t“lcldstripping of excited @s is the cause of first-turn losses.VI. REFERENCE[I] R. Hutson and R. Macck, “First-’Turn Lcsses Ir: theLAMPF Fmton Storage Ring (FSR)”, these pr[xccdlngi[2] R Macck, et al , “Analysis of beam Iosscs at I’SR ‘.Conference record of the 19&9 EPAC Confermcc, V(II .?,pp. 1252- ~254.Results sh wn in Flaw-u 3b and 3C for the 16-rrm-widefoils also show a generally linear relationship hctween henmloss and ring beam aa the foil ic warmed toward and awnyfrom the injected hcam center. However, in them two widc-

Stripper-foil scan studies of the first-turn beam loss mechanism in the LAMPF proton storage ring (PSR)

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Stripper-foil scan studies of the first-turn beam loss mechanism in the LAMPF proton storage ring (PSR)

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