In order to study the effect of long range interaction and its wire compensation experimentally, current carrying wires are installed in the CERN Super Proton Synchrotron (SPS). In this paper we summarize the main results of the 2007 wire excitation experiments at 26, 37 and 55 GeV including scans of wire current, beam-wire distance and chromaticity. A strong dependence on the chromaticity and indications of a threshold effect at 37 and 55 GeV were found. The results are compared with simulations, with a simple analytic scaling law and with experimental results from RHIC. Wire driven resonances have been observed through the Fourier spectrum of experimental BPM data and also studied in simulation

Calaga, R

Dorda, U

Fischer, W

Koutchouk, Jean-Pierre

Tomás, R

Wenninger, J

Zimmermann, F

CERN Document Server

LHC-PROJECT-REPORT-110227Aug2008U. Dorda1), R. Calaga2), W. Fischer2), J.P. Koutchouk,  R. Tomas, J. Wenninger, F. ZimmermannEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsWIRE EXCITATION EXPERIMENTS IN THE CERN SPSIn order to study the effect of long range interaction and its wire compensation experimentally, currentcarrying wires are installed in the CERN Super Proton Synchrotron (SPS). In this paper we summarize themain results of the 2007 wire excitation experiments at 26, 37 and 55 GeV including scans of wire current,beam-wire distance and chromaticity. A strong dependence on the chromaticity and indications of athreshold effect at 37 and 55 GeV were found. The results are compared with simulations, with a simpleanalytic scaling law and with experimental results from RHIC. Wire driven resonances have been observedthrough the Fourier spectrum of experimental BPM data and also studied in simulations.Large Hadron Collider ProjectAbstractLHC Project Report 1102CERN,CH-1211 Geneva 23Switzerland1) Technical University of Vienna, Vienna, Austria2) BNL, Upton, USA.CERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008Wire excitation experiments in the CERN SPSU. Dorda, Technical University of Vienna, Vienna, AustriaJ. Koutchouk, R. Tomas, J. Wenninger, F. Zimmermann, CERN, Geneva, SwitzerlandR. Calaga, W. Fischer, BNL, Upton NYAbstractIn order to study the effect of long range interactionand its wire compensation experimentally, current carryingwires are installed in the CERN Super Proton Synchrotron(SPS). In this paper we summarize the main results of the2007 wire excitation experiments at 26, 37 and 55 GeVincluding scans of wire current, beam-wire distance andchromaticity. A strong dependence on the chromaticity andindications of a threshold effect at 37 and 55 GeV werefound. The results are compared with simulations, with asimple analytic scaling law and with experimental resultsfrom RHIC. Wire driven resonances have been observedthrough the Fourier spectrum of experimental BPM dataand also studied in simulations.INTRODUCTIONThe long-range beam-beam interaction (LRBBI) willbecome one of the main limitations of the CERN LHC.Therefore the effect as well as a possible remedy – the wirecompensation – have been, and still are, exhaustively stud-ied in simulations [1, 2, 3, 4]. In order to also gain ex-perimental insight and to benchmark the dedicated simula-tion code BBTrack [5], BBLR wire excitation experimentswere performed in the SPS, and also at RHIC [8]. For thispurpose two sets of wire compensators, each consisting oftwo units like the one shown in Fig. 1, were installed atthe CERN SPS in 2002 and 2004, respectively. During the2007 machine studies these wires were utilized to mimicLRBBI encounters of variable strength under various beamconditions.The experiments were performed on a SPS cycle, wherethe beam is stepwise accelerated from 26Gev to 37GeV andfinally to 55GeV with a few second long plateau at each en-ergy. All experiments were performed within the few avail-able seconds at constant energy and repeated several timesfor one setting. The powered BBLR reduces the dynami-cal aperture and therefore causes beam loss. In most of thecases the short measurement intervall was sufficiently longto observe the complete beam loss.Pertinent optics parameters at the wire locations are sum-marized in Tab. 1.It was initially attempted to perform these experimentsat the design LHC tunes (0.31, 0.32), but finally (0.31,0.28) was chosen instead, as the decoupling of the twobeams turned out to be too strong for certain wire exci-tations where the vertical and horizontal tunes approachedeach other.The simulations were performed with the particle track-ing code BBTrack [5], and the border of stability was deter-Figure 1: This SPS prototype compensator contains threewires that can be powered, one at a time, with up to 300 Amintegrated strength.Variable Compensator 1 Compensator 2length l [m] 1.2 1.2location s [m] 5168.6 5170.8βx [m] 47.6 54.1βy [m] 50.8 44.6αx -1.4 -1.52αy 1.46 1.32Dx [m] -0.58 -0.61Dy [m] 0 0σx at 26 GeV [mm] 2.3 2.4σy at 26 GeV [mm] 2.3 2.1Table 1: Optics parameters at the SPS compensator loca-tion, and the corresponding beam sizes for transverse emit-tances of 0.1 μm and δrms ≈ 10−3.mined by following twin particles for 300.000 turns aroundthe ring and examining the corresponding Lyapunov expo-nent.EXPERIMENTSBeam-wire separation scan at 37GeVThe data obtained in two beam-wire separation scanswith a wire excitation of 180 and 240 Am at the workingpoint (0.31, 0.28) are shown in Fig. 2. For a wire strengthof 240 Am first small losses are observed at a distanced ≈ 12 mm (8 σ), but a steeper increase occurs at 9.5 mm(6.3 σ). This two stage increase is reproduced in the sim-ulations of Fig. 3. However, the simulated beam loss forthe 180 Am case is slightly higher than in the experiment,where the onset of beam-loss was found at d ≈ 8.5 mm(5.6 σ, threshold not shown). A simple scaling law [6] sug-gests that the wire excitation should have the same effect onthe beam if the ratio I/d2 is identical. In Fig. 2 additionalpoints indicate the 180 Am data scaled to 240 Am accord-ing to this law, supporting the expected scaling behavior.Figure 2: Experimental beam loss at 180 Am and 240 Amwire exctitation as a function of beam-wire distance, and180 Am data points scaled to 240 Am according to the ex-pected dependence.Figure 3: The simulated beam loss reproduces the twostage increase of beam loss at 240 Am but overestimatesthe one at 180 Am slightly.Wire current scansIn order to obtain experimental hints on the viability ofthe LHC phase-2 upgrade optics proposal “dipole zero”[7] (which features few long-range encounter at a reducedbeam-beam separation), current scans at various beam-wire separations were performed at 26, 37 and 55 GeV.The three 37 GeV data sets shown in Fig. 4 are consis-tent among themselves. At a large normalized beam-wiredistance, the 26 GeV data set exhibits significantly higherlosses already at lowest excitation levels, which must beattributed to the inherently more unstable machine condi-tions at injection energy (field errors, power converter rip-ple etc.). The 55 GeV data are similar to their 37 GeVcounterparts at low current values, but unlike the latter theydo not show any steep increase at about 200 Am.Figure 5 presents the corresponding simulation results,confirming the previous thoughts: While the 37 GeV dataFigure 4: Beam loss as a function of the wire current forvarious beam-wire separations a t 26 and 37 GeV at QH =0.31, QV = 0.28.are reproduced reasonably well, the 26 GeV data are not.The delayed experimental onset of beam loss in the d =4.3 σ case at 37 GeV is not reproduced in the simulation,possibly because in the experiment some potentially unsta-ble regions of the 6D phase space were cleaned during thepreceding 26 GeV plateau. The 6.5–7 σ data reveal similardifferences between 37 and 55 GeV in both simulation andexperiment: While the 55 GeV data show a smooth slowincrease of beam loss, the 37 GeV data are characterizedby distinct slope changes.Figure 5: Simulation of beam loss as a function of wirecurrent for various beam-wire separations and three beamenergies, at the working point QH = 0.31, QV = 0.28Chromaticity scan at d = 6.5 σAll experiments mentioned above were performed at lowchromaticity Q′ ≈ 1. For nonzero chromaticity, the parti-cles experience a periodic momentum dependent tune vari-ation which causes the particle tunes to oscillate periodi-cally in the Qx −Qy plane and thereby to cross resonancelines over and over again. This chromatic effect was stud-ied for a wire excitation of 180 Am at a beam-wire distanced ≈ 9 mm (6.5 σ).Unfortunately the limited available measurement inter-val (a few seconds) was not long enough to reach a satura-tion of the beam loss. While in the case of horizontal chro-maticity variation, at a reduced beam current we observedan exponential decay, heading towards a stable level, thebeam losses for large vertical chromaticity did not showany sign of leveling off.Nevertheless, the available data in Fig. 6 provide a firsthint of the strong sensivity to chromaticity. They are sum-marized in Fig. 6, which also includes the results of a ver-tical chromaticity scan at 55 GeV. In order to confirm thatthe beam loss is due to the combined effect of the wire andchromaticity, one case of high chromaticity, Q ′ = 12, butwithout BBLR excitation was studied as well.Figure 6: Measured beam loss as a function of chromaticityfor d = 6.6 σ at 37 and 55 GeV with varying wire excita-tions. Simulations reproduce a strong impact of the chro-maticity. A quantitative comparison between simulationsand experiment is complicated by the fact that the beamloss did not saturate in the few seconds of the measurementinterval.Current scan at Q′y = 33Finally, a wire current scan was performed for large ver-tical chromaticity Q′y = 33 at 55 GeV and with a beam-wire separation of d = 6.6σ. Figure 7 a) compares theresult with a scan at low chromaticity. The simulations insubfigure b) reveal a similarly strong chromaticity depen-dence. The absolute beam loss is about 2–3 times higherin the simulations, which might again be attributed to thelimited data taking interval.(a) Experimental data (b) SimulationsFigure 7: BBLR current scan at Q′y = 331000 turn dataDuring the 37 GeV measurements 1000 turn BPM datawere recorded. These data were analysed and comparedwith simulated “tracking data” (using an SPS model of sin-gle elements in thin-lens approximation). These data canbe used to extract the spectral components. Figures 8 aand b compare the experimentally observed vertical spec-tral lines with the simulated ones for beam-wire separationsof 8 and 9 mm at a wire excitation of 240 Am. In the sim-ulation a 6D Gaussian bunch launched with a horizontaloffset of 1 mm was tracked over 1000 turns.The 1mm horizontal offset was put to reproduce thelinear coupling. The sextupolar coupling line Qx+Qy isclearly identified in both data and simulations showing aqualitative agreement in the reduction of its amplitude fromthe 9mm case to the 8mm case.(a) Experimental data (b) Tracking dataFigure 8: Vertical spectrumREFERENCES[1] U. Dorda, W. Fischer, V. Shiltsev and F. Zimmermann, “LHCbeam-beam compensation using wires and electron lenses”,LHC-PROJECT-Report-1023, 2007.[2] U. Dorda, “Beam-Beam Issues for ’Phase 1’ and ’Phase 2”’,CARE-HHH-APD IR’07, Nov. 2007.[3] T. Pieloni and W. Herr, “Models to Study Multi Bunch Cou-pling through Head-On and Long-Range Beam-Beam Inter-actions”, EPAC’06 Edinburgh (2006).[4] W. Herr, D.I. Kaltchev, E. McIntosh, and F. Schmidt, “LargeScale Beam-beam Simulations for the CERN LHC using Dis-tributed Computing”, LHC-PROJECT-Report-927, Jul 2006[5] U. Dorda, “BBtrack”, http://ab-abp-bbtrack.web.cern.ch/ab-abp-bbtrack[6] F. Zimmermann, “Scaling of diffusive aperture with wire cur-rent”, unpublished memo, July 2003.[7] J. Koutchouk and G. Sterbini, “An Early Beam SeparationScheme for the LHC Luminosity Upgrade”, EPAC’06 Edin-burgh (2006).[8] W. Fischer et al., “Experiments with a DC Wire in RHIC”,PAC’07 (2007)

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