The Multi-Turn Extraction (MTE), a new type of extraction based on beam trapping inside stable islands in horizontal phase space, has been commissioned during the 2008 run of the CERN Proton Synchrotron. Both singleand multi-bunch beams with a total intensity up to 1.4 1013 protons have been extracted with efficiencies up to 98%. Furthermore, injection tests in the CERN Super Proton Synchrotron were performed, with the beam then accelerated and extracted to produce neutrinos for the CERN Neutrino-to-Gran Sasso experiments. The results of the extensive measurement campaign are presented and discussed in detail

Arnold Malandain, F

Benedetto, E

Bohl, T

Cettour Cave, S

Cornelis, K

Damerau, H

Follin, F

Fowler, T

Franchi, A

Freyermuth, P

Genoud, H

Giachino, R

Gilardoni, S

Giovannozzi, M

hancock, S

Le Borgne, Y

Manglunki, D

Métral, G

Pereira, L

Ridewood, J

Riva, Y

Schokker, M

Sermeus, L

Steerenberg, R

Vandorpe, B

Wenninger, J

CERN Document Server

FIRST RESULTS FOR THE BEAM COMMISSIONING OF THE CERNMULTI-TURN EXTRACTIONS. S. Gilardoni, F. Arnold Malandain, E. Benedetto, T. Bohl, S. Cettour Cave, K. Cornelis, H. Damerau,F. Follin, T. Fowler, A. Franchi, P. Freyermuth, H. Genoud, R. Giachino, M. Giovannozzi,S. Hancock, Y. Le Borgne, D. Manglunki, G. Metral, L. Pereira, J. Ridewood, Y. Riva, M. Schokker,L. Sermeus, R. R. Steerenberg, B. Vandorpe, J. Wenninger, CERN, Geneva, SwitzerlandAbstractThe Multi-Turn Extraction (MTE), a new type of extrac-tion based on beam trapping inside stable islands in hor-izontal phase space, has been commissioned during the2008 run of the CERN Proton Synchrotron. Both single-and multi-bunch beams with a total intensity up to 1.4 1013protons have been extracted with efﬁciencies up to 98%.Furthermore, injection tests in the CERN Super ProtonSynchrotron were performed, with the beam then accel-erated and extracted to produce neutrinos for the CERNNeutrino-to-Gran Sasso experiments. The results of theextensive measurement campaign are presented and dis-cussed in detail.INTRODUCTIONAlready early in 2001, a new extraction method has beenproposed based on beam splitting into stable islands [1].The main ingredient of the novel extraction is that thebeam is separated in transverse phase space using nonlin-ear magnetic elements (sextupoles and octupoles) to createstable islands. A slow (adiabatic) horizontal tune-variationis programmed to cross an appropriate resonance, in theProton Synchrotron (PS) case the 1/4. The ﬁve equally-populated beamlets resulting from the splitting process areextracted one by one on successive turns, thanks to slowand fast bumps which deﬂect the beam over ﬁve turns to-ward the extraction line. The goal of the extraction is toprovide beam to the Super Proton Synchroton (SPS) at 14GeV/c for the CERN-to-Gran Sasso Experiment and forﬁxed-target experiments, reducing to a minimum the lossesin the PS with respect to the existing extraction. A de-tailed description of the method and hardware implemen-tation required in the PS can be found in [2]. The differ-ent steps which led to the ﬁrst extraction ever realised withthis method are described in the following. In May andJune 2008, beam splitting was resumed using the newlyinstalled sextupoles and octupoles, achieving a loss-freecapture with a single bunch of about 3 1012 protons. Atthe same time the new extraction slow bump was commis-sioned so that it was ready for the extraction tests in July2008 when the new kicker Pulse Forming Networks (PFN),completed and hardware-commissioned, became availablefor the beam tests. Then, on 1 August, ﬁve beamlets withalmost the same intensity were successfully created from asingle bunch of 3 1012 protons and extracted in the ﬁrst partof the transfer line, TT2, to the Super Proton Synchrotron(SPS). The rest of the commissioning period until the endof the SPS-physics run on 3 November 2008 was dedicatedto studying the best longitudinal structure for beam deliv-ery and injection into the SPS, and it included a campaignof measurements of the optical parameters in the transferline between the PS and the SPS machines. These vari-ous steps of the beam commissioning are described in thefollowing sections.NON-LINEAR MAGNETIC ELEMENTSThe stable islands are created by a set of dedicated non-linear elements: in total two independent octupoles andfour sextupoles connected in two independent groups havebeen installed, together with new power converters, duringthe PS shutdown 2007-2008 [3]. The magnetic multipoleeffective strengths have been cross checked by measuringthe horizontal and vertical non-linear chromaticity for agiven set of currents. The measurements have been then ﬁtwith a MADX model of the PS to deduce the expected cur-rent provided by each power converter. One example of themeasurements for an octupole is shown in Fig. 1 togetherwith the ﬁt from the model. The results of this campaign-2.0e-02 -1.0e-02 0.0e+00 1.0e-02p/p0.10.150.20.250.30.350.4measured tunesMeasured Qx (June 9-10 2008) OMT55@-100AFit: Qx= 0.242 + 4.67 x + 225.6 x^2 + 3897 x^3-2.0e-02 -1.0e-02 0.0e+00 1.0e-02p/p0.150.20.250.30.350.4Measured Qy (June 9-10 2008) OMT55@-100AFit: Qy= 0.276 + 4.51 y - 155.29 y^2 + 182 y^3matching with PS MADX-PTCnon-linear lattice model thatmatches meas. chromaticityof the bare PSMatching OMT39 =-93.6 A Matching OMT39 =-101.9 AFigure 1: Example of non-linear chromaticy measurementfor one of the MTE octupoles.proved that all the sextupole plus one octupole strengthsare well within the speciﬁcations, whereas the second oc-tupole is delivering a lower gradient by about 20%. Furtherinvestigations showed that the magnet gradient is withinspeciﬁcation [3] and the power converter is also deliveringthe required current. The reduction in octupolar strength ismost probably related to a magnetic feed-down effect pro-duced by an orbit distortion in the section of the magnet.TU6RFP022 Proceedings of PAC09, Vancouver, BC, Canada1578Accelerator Technology - SubsystemsT12 - Beam Injection/Extraction and TransportSLOW AND FAST BUMPOnce the beamlets are generated, they are separated bymeans of a tune variation to accomodate between two is-lands the blade of the extraction septum. At this stage, aslow bump 7.5 ms long1 pushes the beamlets towards theseptum blade. The jump of the blade is produced by a setof kickers which ﬁre during the last ﬁve turns, with one PSturn ≈ 2.1 μs long. The new slow bump has been designedto ensure at the same time the bump closure over 7.5 msand to optimise the beam clearance in the extraction region.These two goals have been realised by installing six inde-pendent bumpers. Fig. 2(left) shows the measured orbitat the maximum bump height with respect to the expectedbump, demonstrating a very good agreement between thetwo. During the bump rise time, the beam is no more cen-tered with respect to the main magnets over about 12 sec-tions in the region of the extraction. The PS is composedof 100 combined function dipoles (Main Units, MU) in-terleaved by 100 Straight Sections (SS) in which auxiliarymagnets are installed. The control of the linear and non-linear working point is performed by powering four extracoils mounted on top of each of the MU poles and one sup-plementary winding of the main coil. The feed-down ofthose ﬁelds induces a variation of the horizontal tune ofthe order of 1.5 10−3.This variation reduces the clearanceavailable between the islands for the septum blande at ex-traction. A tune correction, as presented in Fig. 2(right),has been implemented to compensate for this effect. The0 20 40 60 80 100pick-up number [Straight Section] -5051015202530horizontal position [mm] DESIGN BUMPMEASUREMENT833 834 835 836 837 838 839 840            time [ms]0.2550.2560.2570.2580.2590.260.261Measured horizontal tune before correctionafter correctionFigure 2: Beam trajectory at the maximum of the extractionbump (left) and tune compensation during the bump rise(right).extraction proper is realised by ﬁring ﬁve kickers (KFA):those in SS13, SS21 and SS9 deﬂect the beam constantlyfor ﬁve turns creating a closed bump, whereas those in SS4and SS71 are used on top of the others to extract the cen-tral island during the last turn. The commissioning of thenew kickers, with the PFNs available since the beginning ofJuly 2008, started with the strength calibration tests. Thesuccessful results are presented in Fig. 3(left), in which thepredicted trajectory distortion induced by one of the kickersfor a given strength has been compared with the expectedone. Then the kicker timings and strengths have been ﬁnelyadjusted to achieve the bump closure and the correct bumpamplitude (see the results in Fig. 3, right).1duration counted from zero bump current up to the maximum current.0 3 6 9 12 15 18 21 24 27 30 33 36 39pickup number [straight section]-50-40-30-20-1001020304050Relative horizontal displacement [mm]KFA21 @ 65 kVKFA21 @ 65 kV MADXkick0 20 40 60 80 100    pickup number [Straight Section]-40-2002040horizontal position [mm]slow bump slow + fast bump turns 1-4slow + fast bump turn 5Figure 3: Tests of the new KFA21 calibration (left). Thetrajectories during the turn before the ﬁring of the fast kick-ers, the ﬁrst four extraction turns, and the last turn show theprecise closure of the slow and fast bump (right).EXTRACTION AND LOSSOPTIMISATIONThe ﬁrst real MTE extraction has been performed on 1August, when a single bunch beam of about 3 1012 protonshas been sent to an external dump in the PS-SPS transferline. Fig. 4 (left) shows the measured horizontal beam pro-ﬁle of the beamlets in the PS at the end of the splittingprocess and prior to extraction. A ﬁt of the ﬁve beamlets isalso superimposed. Fig. 4 (right) shows the intensity signalof a pick-up in the PS-SPS transfer line of the extractedbeam. Each of the ﬁve peaks corresponds to a beamletextracted over a single turn. The distance between themcorresponds to the PS revolution time of 2.1 μs. As ex-pected, the ﬁve peaks feature about the same intensity, aclear sign that the equally populated islands were correctlyextracted. The rest of the commissioning was devoted tothe preparation of the operational magnetic cycle, the studyof the extraction with multi-bunch beam and the determi-nation of the most suitable longitudinal structure to be in-jected in the SPS. This has been another crucial point ofthe study. The beamlet formation, in fact, is not sensitiveto the number of bunches in the machine, but it is sensi-tive to the beam momentum spread, since via chromaticitya large momentum spread creates a large tune modulation,inducing trapping/detrapping phenomena and reducing thecapture efﬁciency. The losses at extraction, however, de-pend on the bunch spacing due to the ﬁnite rise time of thekickers. In particular, the kicker rise time is longer thanthe h=8 PS bunch spacing, the rise time being about 350ns and the bunch spacing about 260 ns. This implies that a-40 -20 0 20 40x [mm]0200400600800Horizontal beam profile [arbitrary units]measured profilecore (fit)          intensity: 22%4 islands (fit)   intensity: 19.5%global fit0 1 2 3 4 5 6 7 8 9 10time [μs]01234TT2 pick-up signal   [arbitrary units]turn # 1        turn # 2        turn # 3        turn # 4        turn # 52.1 μsFigure 4: Transverse proﬁle of the beamlets before extrac-tion (left) and intensity signal of a pick-up in the PS-SPStransfer line of the extracted beam.Proceedings of PAC09, Vancouver, BC, Canada TU6RFP022Accelerator Technology - SubsystemsT12 - Beam Injection/Extraction and Transport 1579part of the beam will be intercepted by the extraction sep-tum during the kicker rise. The situation becomes worse ifthe h=16 harmonic is preferred or if the beam is debunched:the losses are nearly doubled going from 0.6% of the cir-culating intensity for h=8 up to 0.9% and 1% for the h=16and debunched cases, respectively [2]. A detailed series ofstudies, presented in [4], has been done by injecting a beamin the SPS with different longitudinal structures from thePS and assessing the dependence of the losses as a functionof the harmonic number and the RF voltage at extraction.The outcome is that a debunched beam is the most suit-able for the SPS, even if this choice does not minimise thelosses in the PS. For these reasons, a MTE debunched ex-traction, see Fig. 5(left), has been prepared after the endof the SPS run 2008, increasing the extracted intensity to1.4 1013 protons. The extraction efﬁciency, expected to beup to 97-98%, turned out to be on average about 93%, butwith peaks up to 99%. These ﬂuctuations were correlatedwith a beam instability due to a slightly negative value ofthe chromaticity just prior to the resonant crossing. For thenominal extraction case, with an efﬁciency of about 98%,the beam loss pattern has been compared to the ﬁve turnextraction currently in use, the Continuous Transfer Ex-traction (CT) [5], which should be replaced by MTE. Inthe CT case, the beam is split in ﬁve slices by an electro-static septum in SS31. As expected, (see Fig. 5, right), theMTE beam losses are concentrated in the extraction region,whereas for the old extraction losses of typically 5-6% ofthe circulating intensity are spread out over the entire ma-chine circumference [5]. Losses during the CT extractionare generated ﬁrst by the interaction of protons with theblade of the electrostatic septum in SS31. Moreover, par-ticles scattered at the septum generate losses in the regionbetween SS40-45 and SS72-76.-6x10-6 -4x10-6 -2x10-6 0 2x10-6 4x10-6 6x10-6 8x10-6Time020406080Intensity [A. U.][s]0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100Straight section050100150200250300Beam loss/section [A. U.] SFTPRO LossesMTE LossesElectrostatic septumExtraction septumFigure 5: Intensity of a MTE debunched beam extractedover ﬁve turns measured by a transformer in the PS-SPStransfer line (left). Beam losses at each PS SS for the MTEbeam compared to a CT extracted beam for the same inten-sity (right). The sizable reduction is apparent.SPS INJECTION TESTSDuring the last part of the CNGS run, it was ﬁnally pos-sible to inject a MTE h=16 bunched beam in one of theCNGS SPS cycles. The total intensity injected was about1.4 1013 protons. The two batches, each one of about 0.71013 protons, each one ﬁve PS turns long (presented inFig. 6) had the last PS ejected turn with a larger intensitythan the other four. This was due to a change in the beamradial position induced by the synchronization between thetwo machines. The variation of the radial position changesthe capture process as the non-linear magnetic componentschanges due to different feed down effects. The protonbeam in the SPS could be injected, accelerated and ex-tracted towards the CNGS target. Neutrinos could also beproduced during the last night of the CNGS run. Losses inthe SPS were measured during the CNGS cycle and com-pared to the beam injected with a similar method currentlyin use [4]. Larger losses have been observed for the MTE,corresponding to about 23% of the injected intensity [4], tobe compared with 8% for the other method. The injectionlosses were likely a consequence of an optical mismatch.Extraction losses were due to an instability causing a verti-cal emittance blow-up.0 5 10 15 20time [μs]024681012Beam intensity   [ x10    protons]2     injection from the PS1    injection from the PS10ndstFigure 6: SPS ring transformer showing a double batchMTE injected beam.CONCLUSIONSThe MTE commissioning proved for the ﬁrst time thepossibility to extract beamlets produced by beam splittinginto stable islands. MTE extracted beam was delivered tothe SPS for the last night of the CNGS run, with about aquarter of the nominal intensity. Later in the PS run, themaximum intensity extracted was raised to 1.3-1.4 1013,with extraction losses in the PS limited to about 1% of thecirculating beam in the best case.REFERENCES[1] R. Cappi et al., Phys. Rev. Lett. 88 (2002) 104801.[2] M. Giovannozzi (editor) et al., “The CERN PS multi-turn ex-traction based on beam splitting in stable islands of transversephase space”, Design Report, CERN-2006-011.[3] S. Gilardoni et al., TU6RFP023, these proceedings.[4] T. Bohl, “Observations in the SPS of beam with various lon-gitudinal parameters and extracted from the CPS using CT”,CERN-BE-Note-2009-013. T. Bohl, “Low intensity beam ex-tracted from the CPS via CT or MTE with various longitudi-nal parameters”, CERN-BE-Note-2009-014. T. Bohl, “Lowintensity MTE beam in the SPS”, CERN-BE-Note-2009-015.[5] S. Gilardoni et al., “Beam losses in the PS during CT extrac-tion”, BEAM’07, Geneva, October 2007, CERN-2008-005TU6RFP022 Proceedings of PAC09, Vancouver, BC, Canada1580Accelerator Technology - SubsystemsT12 - Beam Injection/Extraction and Transport

First Results for the Beam Commissioning of the CERN Multi-Turn Extraction

First Results for the Beam Commissioning of the CERN Multi-Turn Extraction

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