Gourber, J P

Henrichsen, K N

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ADJUSTMENT OF TEE WORKING LINES IN THE ISRJ.p. Gourber and K.N. HenrichsenCERN, Geneva, Switzerland(presented in summary by L. Resegotti)AbstractThe various adjustments of the betatron oscil-lation frequencies which have been made since thebeginning of the ISR running-in period are des-cribed in relation with their global effects onthe beam behaviour. The effects of the correct-ing elements (poleface windings, sextupoles,etc.) have been measured and correction methodsdeveloped for the necessary shaping of the"working line" in the QH/QV diagram.IntroductionIn the ISR, the whole available aperture ofthe vacuum chamber is used for injection andstacking. Therefore, the beam properties and inparticular the betatron frequencies (Q values)must be carefUlly controlled over this full re-gion. In the design of the pole profile of thecombined function magnets a sextupolar componenthas been incorporated in order to give constantQ values across the aperture 1 ). At the sametime, poleface Windings (PFW) fed by 24 inde-pendent power supplies and 2 sets of sextupolelenses in mid-F and mid-D locations, respec-tively, have been provided to correct for satu-ration effects and to give a large fleXibilityin the adjustment of the Q values and of theirmomentum dependence. The purpose of this paperis to describe the various Q adjustments whichhave been made from the beginning of the ISRrunning-in period. It is concerned mainly withresults of Ring 1, but the two rings are verysimilar o1. Measuring techniquesThe vertical and horizontal betatron fre-quencies were measured directly with the "Q-meter"2) using a small injected beam which Wasdisplaced across the aperture by RF accelerationas used for beam stacking. In the beginning ofthe running-in period, this displacement wasmade by varying the main magnetic field. At me-dium field levels (15 GeV/c) with small Q cor-rections, this method gives the same reSUlts. Athigher field levels, saturation in the mainmagnet and the larger Q corrections which areneeded cause appreciable discrepancies and thissimple method cannot be used. The effect ofcrossing a resonance was measured by the per-centage intensity loss which resulted for abeam displaced at a constant speed (0.3 cm/s)acro ss the aperture ("aperture scan").2. Effects of the correcting elementsThe Q shifts produced by each set of correc-ting elements (F and D poleface Windings, mid-Fand mid-D sextupoles) have been measured inRing 1 at medium field level, corresponding toa particle momentum of about 15 GeV/c. The 12poleface windings F and the 12 poleface WindingsD were excited in such a way as to add purequadrupolar, sextupolar or octupolar componentsto the field. The calculation of the currentswas based on magnetic measurements and the ISRARGUS computer was provided With the correspond-ing tables. Figure 1 gives the results concern-ing the poleface Windings in the F units. Theamplitudes of the multipolar field componentswere measured using the field display system3 )and expressed by the relative variations of thefield indices and of their first and second de-rivatives with respect to the radial coordinate.The average radial position of the orbit ischaracterized by the corresponding relativechange in proton momentum, 6p/p, with respect tothe centre line.To within a few percent, all these resultscan be represented by expressing the incrementsof QH,V and of their first derivativesQ' 0 QH V1 and second derivativesH ,V o( 6p!p)o2QQ" _ H.V 1 0 bOOH,V - 2 as ~near com ~nat~ons of theo(6p/p)increments of the field indices and of their de-rivatives. As it was expected from the magneticmeasurements, no saturation effects appeared andfor different momenta, the correcting currentshave simply to be scaled linearly. The ISR com-puter has been programmed to set automaticallythese currents according to the required valuesof 6QH V' 6Q~ V' ~Q~ V·, , ,30 Adjustment of the working linesThe Q values were first measured in Ring 1 at15 GeV/c Without any correction o They are repre-sented by a small "working line" in the QIiQvdiagram (see Fig. 2).Resonance lines have been traced on the samediagram up to the 7th order. Crossing a reso-nance line by the working line produces:- a certain loss (depending on the speed) whenthe beam is moved through the correspondingradial position;- a continuous loss (due to a diffusion process)from a stack built across this region: thisloss reduces the lifetime of the beamoIn particular, the conditions of the baremachine at 15 GeV/c were found unsuitable be-cause of the strong third order resonance3 QV = 26 which was in the stacking region andWhich produced more than 60 %loss in the aper-ture scan.- 519 -In the first ISR runs this line was simplyshifted into a resonance-free region of the Qdiagram using a pure quadrupolar correction.More than 2 A were stacked using these newlines - typically 2.75 A in Ring 1 at 15 GeV/con 27th January 1971 (the day of the first beambeam collisions) with the line ANNA, 0, 0* (seeFig. 2). However, their use was limited by:- radial blow up of the beam at injection result-ing in a reduction of the stacking efficiencYt- a strong intensity limitation between 2 and3 A. When one tried to stack beyond this thre-shold, rapid loss occurred followed by a saw-tooth-like sequence of build-ups and losses.To overcome these instabilities, it wasfound necessary to increase the dependence ofboth QH and QV on momentum by adding a sextu-oQH Vpolar correction. Lines with moderate a(~;lp)like CLEO, 1.5, 0 for 15 GeV/c (see Fig. 2) wereenough to prevent the instability of injectionand gave good results for stacking low intensitybeams. In Ring 1, decay rates of less than10-5 min- 1 were measured with beams below 1 A.However, the strong intensity limitation re-mained present and working lines with largeroQH,V 0 to t d Th 1 0 hO ho(~plp) were 1nves 19a e. ese 1nes, w 1Cnecessarily crossed several resonances, were lo-cated in the Q diagram according to the follow-ing criteria:- resonances had to be of high order and crossedbetween injection and the bottom of the stack(practically the centre line);- for injection the working point had to be nottoo close to a resonance to avoid instabili-ties.Many lines were set up in this way; FATA,1.5, 1 (see Fig. 2) being an example for whichonly 5th order resonances are crossed outsidethe stacking region.However, for all these working lines whichOQare not straight the values of o(~;lp) vary Withthe radial position and results were differentwhen the position of the stack was changed.Therefore, in order to obtain straight lines thePFW correction computed in terms of multipolarcomponents was completed by an additional one,determined by a different method. The local gra-dient changes required for suppressing the re-sidual Q-shifts were computed at a discretenumber of radial positions inside the aperture:then the necessary PFW currents for producingthese gradient changes were determined by usingthe results of measurements of the influence of* Each working line has been given a name. Thename itself is characteristic of the Q valueson the centre line and the two figures arethe ~QH and ~QV which are added, using gener-ally the sextupoles.unit currents in individual PFW circuits. Thebest smoothness in the distribution of the cur-rents was obtained When this method was appliedonly for small residual Q-shifts, after havingapproached as much as possible the requiredworking line by using mUltipolar corrections. Atthis level of precision the Q values are influ-enced (to within a few digits in the third deci-mal) by the shape of the closed orbit and, ther~fore, the closed orbit corrections had to be setbefore the final Q adjustments.By reducing the working line of the baremachine to a point in the Q diagram (QH and QVconstant across the aperture), the PFW correc-tion methods provided the starting point forcreating families of working lines correspondingto sets of values of ~Q, ~Q/, and ~QH, whichcould be applied using the ISR computer. A largenumber of working lines were established in thisway for the different momenta chosen for the ISRtests. Some of them had even larger o(~~lp) likeSE22 or included a large octupolar correctionlike OLGA, -OG75, 1.4, two lines which wereestablished at 22 GeV/c (see Fig. 2). A specialinterest has been given to lines in which one ofthe o(~~lp) is negative like IRMA and which canbe located more easily in a resonance free re-gion of the Q diagram.Investigation of these different workinglines is continuing in the ISR. To date thehighest current (6.3 A) achieved at 15 GeV/c wasstacked with the working line 15FA (see Fig G 2).At this intensity level the incoherent Q-shiftsproduced by the stack itself becomes appreciable.Recently, preliminary measurements of thiseffect confirmed the theoretical value containedin the ISR parameter list (~Q = -0.03 for 20 Aat 28 GeV/c).ConclusionThe adjustment and shaping of the workingline in the ISR has been developed to a highdegree of precision and flexibility. The cor-recting method combines, for the poleface Wind-ings, an analysis of the produced field patternin terms of multipolar components and a study ofthe influence of the excitation of each individ-ual circuit. The effects of the correcting ele-ments (poleface Windings and sextupoles) interms of measured Q variations fulfil the re-qUirements of machine operation. The study ofthe beam behaviour for a number of specifiedworking lines has constituted an interestingdiagnostic tool besides permitting significantincreases of stacked intensity and a preliminaryselection of suitable operating conditions forcolliding beam runs GAcknowledgmentsMany people have been associated with thiswork. We should like especially to mentionL. Resegotti who guided our work, and our col-leagues B. Autin, p.J. Bryant, H. Laeger andT.M. Taylor.- 520 -References."..ai...aimaiI, Fig.2 ~ worI<ing lin" r.stoblish&d far ring I1corresponds to Ih& injK\ion orbil: C'l =-0.0182 corresponds 10 0 rod iot displocemenl of +42 mm, from the cenl&r line: <~ I =+ 0.022I ~:i I ~+-I I f-l:;fl tt- o J --I--1 -- I +.,p...,---.0- ----.-.-I..--.-:- I I I7!"i I -rl I -I I 1I--I -I.- o.II 1 .. I -1"-t,-IAt 1 ! ~-1 IH" l 1 11 , I L I,1 -I 1 +-I I ,.-Lj;, I-;- ~ .. I of'3) K.N. Henrichsen and J.p. Gourber - The com-puter controlled system for the forecast ofbeam parameters in the CERN IntersectingStorage Rings - to be presented at thisConference.0.02 lip0.02 IIp/p<linn If= -0.005<t:,n'l f = -0.1 m"lin·Cn l f =-6m'20.0'-0.02-Q04-0.06-0.01 __ ._-/'Fig. 1 EffK\s on QH and Qv of purl! rniltipolar compon~sproduced by Ih& PoI&tac& Windings in Ih& F-unils-0.022) S. Hansen - An automatic digital measure-ment of betatron frequencies - to be pre-sented at this Conference.1) K. HUbner, E. Keil and L. Resegotti -Magnetic field and geometry of the ISR -CERN Internal Report ISR-TH/69-S,10 March 1969.

Adjustment of the working lines in the ISR

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