Through May to September 2008, further beam commissioning of the SPS-to-LHC transfer lines was performed. For the first time, optics and dispersion measurements were also taken in the last part of the lines, and into the LHC. Extensive trajectory and optics studies were conducted, in parallel with hardware checks. In particular dispersion measurements and their comparison with the beam line model were analysed in detail and led to propose the addition of a dispersion-free steering algorithm in the existing trajectory correction program. Its effectiveness was simulated and is briefly discussed

Agapov, I

Fuchsberger, K

Gianfelice, E

Goddard, B

Herr, Werner

Kain, V

Meddahi, M

Mertens, V

Missiaen, D

Risselada, Thys

Uythoven, J

Wenninger, J

CERN Document Server

MACHINE   STUIES     DURING     BEAM    COMMISSIONING    OF     THE      SPS-TO-LHC    TRANSFER      LINESM. Meddahi, I. Agapov, K. Fuchsberger, B. Goddard, W. Herr, V. Kain, V. Mertens,D. Missiaen, T. Risselada, J. Uythoven, J. Wenninger, CERN, Geneva, SwitzerlandE. Gianfelice∗, Fermilab, Batavia, IL, USAAbstractThrough May to September 2008, further beam commis-sioning of the SPS-to-LHC transfer lines was performed.For the first time, optics and dispersion measurements werealso taken in the last part of the lines, and into the LHC. Ex-tensive trajectory and optics studies were conducted, in par-allel with hardware checks. In particular dispersion mea-surements and their comparison with the beam line modelwere analysed in detail and led to propose the addition of adispersion-free steering algorithm in the existing trajectorycorrection program. Its effectiveness was simulated and isbriefly discussed.INTRODUCTIONPrevious injection tests into the LHC allowed to performextensive optics studies [1], and to conclude that the beamlines were operating as expected up to the last beam dump(thereafter, TED). In August 2008, transfer line tests withbeam took place and optics measurements were conductedfor the first time beyond the last TED. This paper sum-marises some of the main findings, while more detailedwork can be found in [2].INVESTIGATION ON COUPLINGOBSERVATIONSThe frame rotation between the transfer lines and theLHC, arising from the use of inclined dipoles, producessome subtle beam dynamics effects at injection which havea variety of implications for operation [3]. The first mea-surements with TI 8 beam of the injection into the LHCrevealed that the coupling is much larger than originallyanticipated and depends on the phase of the measurementoscillation. This effect is reproduced in the MAD-X modeland has been fully described analytically [4]. The initialmeasurements show that the coupling at injection behavesas the full MAD-X model predicts, although the amplitudeis still about 20% larger than expected, which needs furtherinvestigation. The effects should not lead to any major op-erational issues, but the injection steering will be slightlymore complicated than foreseen. The emittance growth atinjection is still expected to be below 2%, and the issue oftail repopulation needs to be taken into account when theSPS scrapers become operational [3]. An overall correctionof the tilt mismatch at the injection point would be possi-ble by skewing several quadrupoles in the transfer line, but∗Work partly supported by Fermilab, operated by Fermi Research Al-liance, LLC under Contract No. DE-AC02-07CH11359 with the UnitedStates Department of EnergyFigure 1: Nominal horizontal dispersion (blue line) andmeasured dispersion values (red triangles) from the startof TI 2 to the left of IP3 in the LHC.this is presently not considered worthwhile in view of theextra complexity introduced for the collimation section, thealignment, the layout and the instrumentation.TI 2 DISPERSION MEASUREMENTSDispersion data measured in TI 2 are shown in Fig. 1and are compared with the model: the dispersion measure-ments agree very well with the model along TI 2. In theLHC, there is a slight dispersion beating. A fit to the dataindicates that almost all the dispersion error in the LHC canbe explained by errors in the TI 2 initial conditions.TI 8 DISPERSION MEASUREMENTSDispersion measurements have been taken in the TI 8beam line and compared with expectation. In Fig. 2, thesimulated dispersion of the TI 8 line is plotted along thebeam line, down to the left of IP7 in the LHC, togetherwith measured dispersion values. The dispersion towardsthe end of the line differs from the model; the largest dis-crepancy is observed at the end of the TI 8 matching sec-tion into the LHC (at MQIF876), with a dispersion beat-ing propagating into the LHC. The difference between themeasured and nominal dispersion values, normalised to the√βx, is plotted in Fig. 3. The measurements indicate thatthe oscillation seems to originate around Q5.L8, and thebeating amplitude increases after Q5.Proceedings of PAC09, Vancouver, BC, Canada TH6PFP040Beam Dynamics and Electromagnetic FieldsD01 - Beam Optics - Lattices, Correction Schemes, Transport 3793-4-3-2-1 0 1 2 0  1000  2000  3000  4000  5000  6000Dx [m]s [m]’Dispersion with 2008 v2 rematched optics’’TI 8 measured dispersion’’LHC dispersion measurements - August 08’Figure 2: Nominal horizontal dispersion (red line) andmeasured dispersion values (crosses) from the start of TI 8to the right of IP7 in the LHC.-0.2-0.15-0.1-0.0500.050.10.150.20 5 10 15 20(Dx-Dx)/sqrt()modelmeasx x(2 )Dispersion measurement TI 8/LHCQ4, Q5Figure 3: Horizontal dispersion difference between themeasurements and the model, normalised to√βx.INVESTIGATION ON POSSIBLEMAGNETIC ERRORSA campaign of magnet checks was performed to ver-ify all magnet currents and fields. Calibration curves werechecked, the magnetic field of matching quadrupoles of theend of the line was measured in the tunnel and alignmentswere verified. No significant errors were found; in partic-ular all quadrupole tilts were of the correct magnitude andsign. In parallel the survey team remeasured the positionof the magnetic elements from MQIF868 to the end of theline, with particular care to the dipoles tilted by 19 degrees.It was found that since the last alignment campaign done in2007, elements at the end of the line have moved radially,some of them by up to 1− 2 mm, which is not too surpris-ing for a relatively new tunnel.These investigations and measurements allowed to estab-lish a MAD-X file of errors for all magnets of the line.Simulation showed that the field and alignment errors wereindeed acting on the dispersion, but not to the amplitudeobserved.-4e-05-3e-05-2e-05-1e-05 0 1e-05 2e-05 3e-05 4e-05 5e-05 0  500  1000  1500  2000  2500  3000kick [rad]s [m] ’Corrector kicks during TI 8 operation’’Simulated corrector kicks using MICADO’Figure 4: Corrector strengths used to correct the bare tra-jectory with 14 correctors, and used for the nominal oper-ating trajectory.-0.003-0.002-0.001 0 0.001 0.002 0.003 0.004 0  500  1000  1500  2000  2500Delta_x [m]s [m] TI 8 difference trajectoryX_measured - X_simulatedFigure 5: Corrected bare trajectory from the model with allerrors in and as measured in the TI 8 line.TRAJECTORY ANALYSISBeam time was allocated on 17 September 2008 torecord the TI 8 bare trajectory. The measured trajectoryexcursion showed amplitudes within the specification (±4mm) along the beam line, at the exception of the MQIF874and MQIF876 locations. The origin of the large trajectorydeviation was quickly identified as a radial displacement ofthe MQIF872 by dx= 2mm. TI 8 survey measurementsindeed confirmed that the entrance of MQIF872 was dis-placed by 2 mm and the exit by 1.4 mm. The strengthof the 14 correctors used with MICADO are plotted ver-sus the strength of the operational trajectory correctors inFig. 4. The strengths show a good agreement between thetwo cases. When all the measured beam line element errorsare added to the model and the bare trajectory is correctedtowards the end of the line using MCIAH872, the differ-ence between the resulting simulated bare trajectory andthe measured one is plotted in Fig. 5. The beginning of theline shows the largest discrepancies and the model will beimproved with the addition of magnetic corrections appliedduring operation in order to optimise the trajectory at thisTH6PFP040 Proceedings of PAC09, Vancouver, BC, Canada3794Beam Dynamics and Electromagnetic FieldsD01 - Beam Optics - Lattices, Correction Schemes, Transport-4-3-2-1 0 1 2 0  1000  2000  3000  4000  5000  6000Dx [m]s [m]’Simulated dispersion with 2008 v2 optics and correctors on’’TI 8 dispersion measurements’’LHC dispersion measurements - August 08’Figure 6: Simulated horizontal dispersion after trajectorycorrection (14 correctors) and measured dispersion values.location. Along the beam line, at this stage, the agreementis considered satisfactory. These results allowed us to de-velop confidence in the beam line model with errors and totherefore exploit this model for further dispersion analysis.DISPERSION EFFECTS FROM ERRORSAND CORRECTOR STRENGTHSAdding all magnetic errors to the line model, togetherwith the trajectory corrector strengths used along the beamline to establish the trajectory, indicated that the misalign-ments and the trajectory correctors were, by themselves, re-producing the larger dispersion measured towards the endof the TI 8 line, Fig. 6. However the dispersion after the in-jection point still diverges from the revised model, althoughnow to a lesser extent. A ”dispersion-free” steering algo-rithm (thereafter, DFS) was implemented in the operationalsteering program [5]. It was applied to the measured TI 8and LHC sector 87 trajectory and dispersion. The resultsindicate that the large dispersion error in the LHC cannotbe corrected using DFS in TI 8, unless a very large trajec-tory oscillation of 6-10 mm is launched in TI 8 to producea ’compensating’ dispersion wave.SUMMARYThe model of the line has been refined with the additionof all known field and alignment errors, together with theactual corrector settings used for the measurements.For the trajectory, the measured alignment offsets repro-duce fairly well the measured ’bare’ trajectory in the line.Using MICADO to correct the trajectory, the corrector set-tings found in MAD-X and the actual operational settingsalso agree well. The beginning of the line shows some un-explained behaviours which need further clarification.The dispersion behaviour with all the errors included -with the trajectory correctors powered- shows the same am-plitude and phase of perturbation that was measured alongTI 8. There are still differences in the beating pattern in theLHC, but the magnet errors and corrector strengths in theLHC proper were not included, and these may explain partof the effects.It may be possible already to improve the beating withthe realignment of the TI 8 quadrupoles, especially in theradial plane. In addition, an algorithm for ’dispersion-free’steering was tested with MAD-X and showed encouragingresults. However, DFS alone can only correct a fraction ofthe dispersion error observed in the LHC.In conclusion it seems now as if the perturbation to thedispersion at the end of the TI 8 line is caused by the ac-cumulation of these small errors (alignment and steering)along TI 8 which have to be corrected by some strong pow-ering of corrector magnets. The main sources of dispersionat the end of the TI 8 line (MQIF876) seem to be nearerto the start of the line. The same model explains this andreproduces the measured trajectory. The larger amplitudeof the dispersion beating in the LHC, downstream of theTI 8 line, remains to be understood. Machine developmenttime has been requested in order to perform detailed mea-surements in 2009. The misalignment and magnetic errorsof the LHC ring elements seen by the injected beam will beadded in the model, together with the strength of the LHCcorrectors.The limited number of BPMs in the transfer lines madethe analysis more difficult, especially when trying to dis-entangle dispersive and trajectory effects. Therefore, 4 ad-ditional BPMs have been installed at the end of TI 8 inorder to have beam instruments at each quadrupoles in thisregion. Also, the acquisition system of all installed TI 8BPMs has been upgraded to allow dual plane measure-ments. The same improvements will be performed in TI 2during the 2010-2011 shutdown. Finally regular alignmentchecks / re-alignment campaigns will be planned.ACKNOWLEDGMENTMany colleagues contributed to these measurements andinvestigations, in particular O. Bru¨ning, S. Fartoukh, M.Giovannozzi, L. Jensen, M. Jones, R. Jones, M. Lamontfrom CERN and R. Calaga from the USLARP collabo-ration. Finally the excellent support from the numerousteams involved in preparing and running the LHC is ac-knowledged.REFERENCES[1] V. Mertens et al., Beam commissioning of injection into theLHC, these proceedings.[2] I. Agapov et al., TI 8 transfer line optics studies, LHC Per-formance Note 004.[3] B. Goddard et al., Geometrical alignment and associatedbeam optics issues of transfer lines with horizontal and ver-tical deflection, LHC-Project-Report-719, 2004.[4] K. Fuchsberger et al., Coupling at injection into the LHCfrom reference frame rotation, LHC Performance Note 003.[5] J. Wenninger, Dispersion free steering for YASP and disper-sion correction for TI 8, LHC Performance Note 005.Proceedings of PAC09, Vancouver, BC, Canada TH6PFP040Beam Dynamics and Electromagnetic FieldsD01 - Beam Optics - Lattices, Correction Schemes, Transport 3795

Machine studies during beam commissioning of the SPS-to-LHC transfer lines

Meddahi, M.

Agapov, I.

Fuchsberger, K.

Goddard, B.

Herr, W.

Kain, V.

Mertens, V.

Missiaen, D.

Risselada, T.

Uythoven, J.

Wenninger, J.

/Fermilab, /CERN

UNT Digital Library

Machine studies during beam commissioning of the SPS-to-LHC transfer linesM. Meddahi, I. Agapov, K. Fuchsberger, B. Goddard, W. Herr, V. Kain, V. Mertens,D. Missiaen, T. Risselada, J. Uythoven, J. Wenninger, CERN, Geneva, SwitzerlandE. Gianfelice∗, Fermilab, Batavia, IL, USAAbstractThrough May to September 2008, further beam commis-sioning of the SPS-to-LHC transfer lines was performed.For the first time, optics and dispersion measurements werealso taken in the last part of the lines, and into the LHC. Ex-tensive trajectory and optics studies were conducted, in par-allel with hardware checks. In particular dispersion mea-surements and their comparison with the beam line modelwere analysed in detail and led to propose the addition of adispersion-free steering algorithm in the existing trajectorycorrection program. Its effectiveness was simulated and isbriefly discussed.INTRODUCTIONPrevious injection tests into the LHC allowed to performextensive optics studies [1], and to conclude that the beamlines were operating as expected up to the last beam dump(thereafter, TED). In August 2008, transfer line tests withbeam took place and optics measurements were conductedfor the first time beyond the last TED. This paper sum-marises some of the main findings, while more detailedwork can be found in [2].INVESTIGATION ON COUPLINGOBSERVATIONSThe frame rotation between the transfer lines and theLHC, arising from the use of inclined dipoles, producessome subtle beam dynamics effects at injection which havea variety of implications for operation [3]. The first mea-surements with TI 8 beam of the injection into the LHCrevealed that the coupling is much larger than originallyanticipated and depends on the phase of the measurementoscillation. This effect is reproduced in the MAD-X modeland has been fully described analytically [4]. The initialmeasurements show that the coupling at injection behavesas the full MAD-X model predicts, although the amplitudeis still about 20% larger than expected, which needs furtherinvestigation. The effects should not lead to any major op-erational issues, but the injection steering will be slightlymore complicated than foreseen. The emittance growth atinjection is still expected to be below 2%, and the issue oftail repopulation needs to be taken into account when theSPS scrapers become operational [3]. An overall correctionof the tilt mismatch at the injection point would be possi-ble by skewing several quadrupoles in the transfer line, but∗Work partly supported by Fermilab, operated by Fermi Research Al-liance, LLC under Contract No. DE-AC02-07CH11359 with the UnitedStates Department of EnergyFigure 1: Nominal horizontal dispersion (blue line) andmeasured dispersion values (red triangles) from the startof TI 2 to the left of IP3 in the LHC.this is presently not considered worthwhile in view of theextra complexity introduced for the collimation section, thealignment, the layout and the instrumentation.TI 2 DISPERSION MEASUREMENTSDispersion data measured in TI 2 are shown in Fig. 1and are compared with the model: the dispersion measure-ments agree very well with the model along TI 2. In theLHC, there is a slight dispersion beating. A fit to the dataindicates that almost all the dispersion error in the LHC canbe explained by errors in the TI 2 initial conditions.TI 8 DISPERSION MEASUREMENTSDispersion measurements have been taken in the TI 8beam line and compared with expectation. In Fig. 2, thesimulated dispersion of the TI 8 line is plotted along thebeam line, down to the left of IP7 in the LHC, togetherwith measured dispersion values. The dispersion towardsthe end of the line differs from the model; the largest dis-crepancy is observed at the end of the TI 8 matching sec-tion into the LHC (at MQIF876), with a dispersion beat-ing propagating into the LHC. The difference between themeasured and nominal dispersion values, normalised to the√βx, is plotted in Fig. 3. The measurements indicate thatthe oscillation seems to originate around Q5.L8, and thebeating amplitude increases after Q5.FERMILAB-CONF-09-321-APC-4-3-2-1 0 1 2 0  1000  2000  3000  4000  5000  6000Dx [m]s [m]’Dispersion with 2008 v2 rematched optics’’TI 8 measured dispersion’’LHC dispersion measurements - August 08’Figure 2: Nominal horizontal dispersion (red line) andmeasured dispersion values (crosses) from the start of TI 8to the right of IP7 in the LHC.-0.2-0.15-0.1-0.0500.050.10.150.20 5 10 15 20(Dx-Dx)/sqrt()modelmeasx x (2 )Dispersion measurement TI 8/LHCQ4, Q5Figure 3: Horizontal dispersion difference between themeasurements and the model, normalised to√βx.INVESTIGATION ON POSSIBLEMAGNETIC ERRORSA campaign of magnet checks was performed to ver-ify all magnet currents and fields. Calibration curves werechecked, the magnetic field of matching quadrupoles of theend of the line was measured in the tunnel and alignmentswere verified. No significant errors were found; in partic-ular all quadrupole tilts were of the correct magnitude andsign. In parallel the survey team remeasured the positionof the magnetic elements from MQIF868 to the end of theline, with particular care to the dipoles tilted by 19 degrees.It was found that since the last alignment campaign done in2007, elements at the end of the line have moved radially,some of them by up to 1 − 2 mm, which is not too surpris-ing for a relatively new tunnel.These investigations and measurements allowed to estab-lish a MAD-X file of errors for all magnets of the line.Simulation showed that the field and alignment errors wereindeed acting on the dispersion, but not to the amplitudeobserved.-4e-05-3e-05-2e-05-1e-05 0 1e-05 2e-05 3e-05 4e-05 5e-05 0  500  1000  1500  2000  2500  3000kick [rad]s [m] ’Corrector kicks during TI 8 operation’’Simulated corrector kicks using MICADO’Figure 4: Corrector strengths used to correct the bare tra-jectory with 14 correctors, and used for the nominal oper-ating trajectory.-0.003-0.002-0.001 0 0.001 0.002 0.003 0.004 0  500  1000  1500  2000  2500Delta_x [m]s [m] TI 8 difference trajectoryX_measured - X_simulatedFigure 5: Corrected bare trajectory from the model with allerrors in and as measured in the TI 8 line.TRAJECTORY ANALYSISBeam time was allocated on 17 September 2008 torecord the TI 8 bare trajectory. The measured trajectoryexcursion showed amplitudes within the specification (±4mm) along the beam line, at the exception of the MQIF874and MQIF876 locations. The origin of the large trajectorydeviation was quickly identified as a radial displacement ofthe MQIF872 by dx= 2mm. TI 8 survey measurementsindeed confirmed that the entrance of MQIF872 was dis-placed by 2 mm and the exit by 1.4 mm. The strengthof the 14 correctors used with MICADO are plotted ver-sus the strength of the operational trajectory correctors inFig. 4. The strengths show a good agreement between thetwo cases. When all the measured beam line element errorsare added to the model and the bare trajectory is correctedtowards the end of the line using MCIAH872, the differ-ence between the resulting simulated bare trajectory andthe measured one is plotted in Fig. 5. The beginning of theline shows the largest discrepancies and the model will beimproved with the addition of magnetic corrections appliedduring operation in order to optimise the trajectory at this-4-3-2-1 0 1 2 0  1000  2000  3000  4000  5000  6000Dx [m]s [m]’Simulated dispersion with 2008 v2 optics and correctors on’’TI 8 dispersion measurements’’LHC dispersion measurements - August 08’Figure 6: Simulated horizontal dispersion after trajectorycorrection (14 correctors) and measured dispersion values.location. Along the beam line, at this stage, the agreementis considered satisfactory. These results allowed us to de-velop confidence in the beam line model with errors and totherefore exploit this model for further dispersion analysis.DISPERSION EFFECTS FROM ERRORSAND CORRECTOR STRENGTHSAdding all magnetic errors to the line model, togetherwith the trajectory corrector strengths used along the beamline to establish the trajectory, indicated that the misalign-ments and the trajectory correctors were, by themselves, re-producing the larger dispersion measured towards the endof the TI 8 line, Fig. 6. However the dispersion after the in-jection point still diverges from the revised model, althoughnow to a lesser extent. A ”dispersion-free” steering algo-rithm (thereafter, DFS) was implemented in the operationalsteering program [5]. It was applied to the measured TI 8and LHC sector 87 trajectory and dispersion. The resultsindicate that the large dispersion error in the LHC cannotbe corrected using DFS in TI 8, unless a very large trajec-tory oscillation of 6-10 mm is launched in TI 8 to producea ’compensating’ dispersion wave.SUMMARYThe model of the line has been refined with the additionof all known field and alignment errors, together with theactual corrector settings used for the measurements.For the trajectory, the measured alignment offsets repro-duce fairly well the measured ’bare’ trajectory in the line.Using MICADO to correct the trajectory, the corrector set-tings found in MAD-X and the actual operational settingsalso agree well. The beginning of the line shows some un-explained behaviours which need further clarification.The dispersion behaviour with all the errors included -with the trajectory correctors powered- shows the same am-plitude and phase of perturbation that was measured alongTI 8. There are still differences in the beating pattern in theLHC, but the magnet errors and corrector strengths in theLHC proper were not included, and these may explain partof the effects.It may be possible already to improve the beating withthe realignment of the TI 8 quadrupoles, especially in theradial plane. In addition, an algorithm for ’dispersion-free’steering was tested with MAD-X and showed encouragingresults. However, DFS alone can only correct a fraction ofthe dispersion error observed in the LHC.In conclusion it seems now as if the perturbation to thedispersion at the end of the TI 8 line is caused by the ac-cumulation of these small errors (alignment and steering)along TI 8 which have to be corrected by some strong pow-ering of corrector magnets. The main sources of dispersionat the end of the TI 8 line (MQIF876) seem to be nearerto the start of the line. The same model explains this andreproduces the measured trajectory. The larger amplitudeof the dispersion beating in the LHC, downstream of theTI 8 line, remains to be understood. Machine developmenttime has been requested in order to perform detailed mea-surements in 2009. The misalignment and magnetic errorsof the LHC ring elements seen by the injected beam will beadded in the model, together with the strength of the LHCcorrectors.The limited number of BPMs in the transfer lines madethe analysis more difficult, especially when trying to dis-entangle dispersive and trajectory effects. Therefore, 4 ad-ditional BPMs have been installed at the end of TI 8 inorder to have beam instruments at each quadrupoles in thisregion. Also, the acquisition system of all installed TI 8BPMs has been upgraded to allow dual plane measure-ments. The same improvements will be performed in TI 2during the 2010-2011 shutdown. Finally regular alignmentchecks / re-alignment campaigns will be planned.ACKNOWLEDGMENTMany colleagues contributed to these measurements andinvestigations, in particular O. Brüning, S. Fartoukh, M.Giovannozzi, L. Jensen, M. Jones, R. Jones, M. Lamontfrom CERN and R. Calaga from the USLARP collabo-ration. Finally the excellent support from the numerousteams involved in preparing and running the LHC is ac-knowledged.REFERENCES[1] V. Mertens et al., Beam commissioning of injection into theLHC, these proceedings.[2] I. Agapov et al., TI 8 transfer line optics studies, LHC Per-formance Note 004.[3] B. Goddard et al., Geometrical alignment and associatedbeam optics issues of transfer lines with horizontal and ver-tical deflection, LHC-Project-Report-719, 2004.[4] K. Fuchsberger et al., Coupling at injection into the LHCfrom reference frame rotation, LHC Performance Note 003.[5] J. Wenninger, Dispersion free steering for YASP and disper-sion correction for TI 8, LHC Performance Note 005.

Machine Studies During Beam Commissioning of the SPS-to-LHC Transfer Lines

Machine studies during beam commissioning of the SPS-to-LHC transfer lines

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