CERN-ACC-NOTE-2015-0043

16th December 2015

Arkadiusz.Gorzawski@cern.ch

Collide and Squeeze MD

A. Gorzawski, D. Mirarchi, G. Papotti, T. Pieloni, B. Salvachua, J. Wenninger

Keywords: collide and squeeze, b* leveling, orbit, beam–beam, operation

Summary

Colliding the beams during the squeeze to profit from Landau damping due to head–on beam-beam and β∗ levellingare two operational modes that may have to be used in a not so distant future at the LHC. This MD aimed atimproving the process control during the squeeze with much improved handling of reference for the orbit feedbacksystem and at evaluating instruments and techniques to maintain the beam in collisions with active feedback ona good observable.

Contents

1 Introduction and MD overview 2

2 Machine settings for collide and squeeze 22.1 Collide and squeeze settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Orbit references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3 Collimators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Main MD phases 83.1 Collide and Squeeze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2 Beam offset tracking and corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Beam emittance evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.4 Beam scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.5 Beam stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Conclusion and outlook 13

5 Acknowledgments 14

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1 Introduction and MD overview

This MD took place on 29.08.2015. The MD started on 29.08.2015 at 19:00 and finished on 30.08.2015at 05:00. Three machine cycles to 6.5 TeV were executed during this MD, corresponding to fills 4292,4294 and 4295. In the first and the last fill, the beams were brought to the end of the squeeze withcollisions in IR1, IR5 and IR8 (see Fig.1). The second fill was lost after the first 2 steps of the squeezeon a collimator interlock because of a missing mask in the BIS.

Figure 1: Overview of the luminosity and β∗ evolution during the MD. Special tests wereperformed at the end of the first and the third cycle. Complete cycles to the end of thesqueeze were performed during the first and the third highlighted period.

2 Machine settings for collide and squeeze

2.1 Collide and squeeze settings

The settings for this MD were based on the standard squeeze. Only the separation bumps in IR1, IR5and IR8 were collapsed all along the squeeze to provide collisions. During the MD luminosity knobcorrections were added to ensure that the beams were colliding head-on (HO). The beams were driventhrough the standard cycle with chromaticity of around +5 up to the end of the tune change beamprocess. At the end of that process, the separation bumps were collapsed to enter the squeeze withcolliding beams.

2.2 Orbit references

One of the important limitations of the β∗ levelling tests performed during Run 1 [1] was the qualityof the orbit control during the process. Conditions were significantly improved during this MD. First ofall the orbit setup for the squeeze was much cleaner in 2015 than in 2012. Secondly the luminosity scanknobs were now made available to be used as overlay shapes for the orbit feedback (OFB) references.

For the dynamic phases of the LHC cycle the reference orbit is build with a base orbit, that isidentical along the cycle, and with overlay shapes for the various bumps (crossing angles, separation atthe IPs, ULO location etc). For this MD the luminosity optimization knobs (’lumi knobs’) were addedin order to reflect accurately the orbit shape after optimization of the luminosity. The aim was to obtaina better, easier to manage and much more reproducible orbit control for β∗ levelling. The evolution of

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the lumi knob shapes along the standard squeeze is shown in Fig. 2 for two representative examples. InIR1 and IR5 the shape does almost not change. In IR8 the shape does change more significantly becausethe optics has to perform the transition from the injection to the collision configuration. To simplifythe preparation for this MD, only a limited lumi knob shapes were prepared. For IR1 and IR5 the knobshape at β∗ of 4 m was selected as representative and was used for all optics points. For the typicaltrims that do not exceed 0.3 mm per beam, the approximation is good to a few micrometers near Q4which is well in the noise and errors of the BPM system. For IR8 two shapes corresponding to β∗ of 9 mand 4 m were prepared.

During the first phase of the MD, intermediate stops were made at every matched optics point.Luminosity scans were performed in IR1, IR5 and IR8. The lumi knob corrections were incorporatedinto the PC functions and added as overlays to the reference orbits. This worked well, when the OFBwas switched on with the new reference orbit including the corrections for the luminosity optimizations,the luminosity remained stable at or very close to the peak. Once the corrections were defined afterthe first pass, the established reference orbits were used to drive the OFB through the squeeze duringsubsequent passes. Fig. 3 gives an example of a reference orbit.

2.3 Collimators

During the MD the TCTPs in IR1, IR5 and IR8 were aligned around the ’colliding orbit’ using theembedded BPMs. The gaps were set to the opening corresponding to the smallest β∗. Along the squeezethe TCTPs were not moved, but the reading of the TCTP BPMs and the losses at the TCTPs weremonitored closely. No problems were encountered. Tables 1 and 2 as well as Fig. 4 show the simulatedevolution of the collimator center during all the 12 squeeze steps. During the MD the measured orbitshift in the TCTPs was at the level of 800µm which corresponds to less then a 1σ in the TCTP for thesmallest β∗ of 80 cm. The orbit changes measured by the BPMs embedded in the TCTPs are shown inFig. 5.

Step TCTPH TCTPV TCTPH TCTPV TCTPH TCTPV4L1.B1 4L1.B1 4L5.B1 4L5.B1 4L8.B1 4L8.B1[µm] [µm] [µm] [µm] [µm] [µm]

1 238 1388 -2110 0 3671 -742 218 1238 -1797 0 3644 -2263 206 1145 -1601 0 3610 -3564 198 1082 -1466 0 3573 -4655 196 1065 -1431 0 3537 -5546 194 1051 -1401 0 3505 -6247 191 1027 -1350 0 3478 -6778 191 1027 -1350 0 3467 -6979 191 1027 -1350 0 3463 -70510 191 1027 -1350 0 3458 -71211 191 1027 -1350 0 3458 -71212 191 1027 -1350 0 3458 -712

Table 1: The calculated (collimator) center for Beam 1 for each of the squeeze steps.Values were obtained with a MADX simulation.

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(a)

(b)

Figure 2: (a): shape of the luminosity knob for the horizontal plane of B1 in IP1 for thematched point optics in the squeeze. The shape is almost identical at all points. (b): shapeof the luminosity knob for the vertical plane of B1 in IP8 for the matched point opticsin the squeeze. In IR8 the optics changes are more significant and the shape does changesignificantly.

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(a)

(b)

Figure 3: (a): Example of the reference orbit for β∗ of 2 m with the base orbit and theoverlay shapes. (b): display of the overlay shapes for B1 with the crossing angle bumps andthe lumi knobs.

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Step TCTPH TCTPV TCTPH TCTPV TCTPH TCTPV4R1.B2 4R1.B2 4R5.B2 4R5.B2 4R8.B2 4R8.B2[µm] [µm] [µm] [µm] [µm] [µm]

1 -237 1361 -2097 0 3615 682 -218 1210 -1782 0 3588 2213 -206 1117 -1585 0 3554 3524 -197 1053 -1450 0 3517 4615 -195 1036 -1415 0 3481 5516 -193 1022 -1384 0 3449 6227 -190 998 -1333 0 3422 6758 -190 998 -1333 0 3411 6959 -190 998 -1333 0 3407 70310 -190 998 -1333 0 3403 71011 -190 998 -1333 0 3403 71012 -190 998 -1333 0 3403 710

Table 2: The calculated (collimator) center for Beam 2 for each of the squeeze steps..Values were obtained with a MADX simulation.

(a) TCTPH B1 (b) TCTPH B2

(c) TCTPV B1 (d) TCTPV B2

Figure 4: The simulated evolution of the collimator (centers) for both planes in IP1, IP5and IP8 during the squeeze. The exact values can be found in Tables: 1 and 2.

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Figure 5: Evolution of the horizontal orbit at the TCTs in IP1 and IP5. The largest orbitshift for beam 2 corresponds to 800µm on the right side of IP5, for beam 1 the largest driftof 500µm is observed on the left side of IP5. The drift in IP1 is barely noticeable and doesnot exceed 100µm. Most of the drifts have stabilized when β∗ of 3 m was reached.

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3 Main MD phases

3.1 Collide and Squeeze

Two machine cycles in fills 4292 and 4295 were performed with the full β∗ squeeze maintaining all threepoints (IP1, IP5 and IP8) in collision as shown in Fig. 6.

(a) 4292 cycle (b) 4295 cycle

Figure 6: The evolution of the luminosity and of β∗ in the colliding IPs. Fig. (a) showsthe continuous luminosity recorded during 4292 fill for IP1 and IP5. For IP8 the luminosityis reconstructed from data that was provided at higher sampling rate. Fig. (b) shows onlythe IP1 and IP5 for fill 4295 data with a long stop at β∗ of 4 m due to a settings issue.

The first cycle was used to establish all reference orbits (see Section 2.2). During this cycle collisionswere maintained in all IPs (within 1.5σ) except for one step in IP1 (from 700 cm to 400 cm). That pointcoincides with an important local orbit correction that had to be made around IP1 at the time of thesqueeze commissioning in April 2015.

During the second fill (4295) the idea was to go through the squeeze with the references set in theprevious fill. However the first few steps were repeated again with some fine tuning for the referenceorbit. After reaching the 4m step it was realized that incorporation error was introduced for the IP1settings during the first fill that resulted into the large disagreement between the reference and the actualsettings leading to the partial separation and therefore luminosity drop. After a short investigation itwas decided to revert all the trims that had been done and to restart from scratch (at β∗ =4 m, seeFig. 6b). This eventually allowed to reconnect with the evolution established in fill 4292 and resultedinto a nice replay mode for the last β∗ steps.

3.2 Beam offset tracking and corrections

A large part of the offline analysis was devoted to the analysis of the new DOROS [2] system datainstalled on the Q1 BPMs of IP1 and IP5. The ultimate goal was to determine if tracking of the orbitinterpolation to the IP is good enough to be used as an input for a feedback on the beam offset duringthe β∗ steps. Fig. 7 presents the data for one of the β∗ steps. Fig. 7b shows the separation of the beamsat the IP tracked by DOROS during a β∗ change from 7 m to 4 m.

This analysis was performed for each step during both cycles. The result of the comparison can befound in Fig. 8. There is an overall agreement between the tracking of the beams by DOROS and theluminosity correction trims that were determined at each step.

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(a) Luminosity evolution during the step (b) Beam offset at the IP during the step

Figure 7: Evolution of the luminosity (a) and of the beam offset at IP5 (b) during theβ∗ step from 7 m to 4 m. Fig. (b) compares the DOROS with the result of the luminosityoptimization done after the step (•).

(a) IP1 during 4292 (b) IP5 during 4292

(c) IP1 during 4295 (d) IP5 during 4295

Figure 8: Comparison of the evolution of the beam offsets tracked by DOROS with theoffsets reconstructed from the luminosity optimization scans in IP1, (a) and (c), and in IP5,(b) and (d). The time scale refers to the time along the squeeze beam process. The greenarea corresponds to the ’safe’ 1σ separation limit, while the red band corresponds to themore critical region up to 1.5σ [1].

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The offset tracking comparison of the IP5 data (8b 8d) is biased by the fact that DOROS datafor that IP had to be manually post-processed. This processing was necessary to recover all the datachannels, but it was using data from one BPM electrode instead of two. This could only be done becausethe relevant information is the relative change of the position and not its absolute value.

3.3 Beam emittance evolution

Profiting from the small number of nominal bunches, a series of consecutive emittance measurementswere performed with the wire scanners along the squeezes. The results are presented together with theBSRT measurement in Fig. 9.

(a) Horizontal emittance 4292 (b) Vertical emittance 4292

(c) Horizontal emittance 4295 (d) Vertical emittance 4295

Figure 9: Evolution of the individual bunch emittances recorded by the the BSRT(dashed) and the wire scanner (points with lines) along the squeeze in fills 4292 and 4295for the horizontal ((a), (c)) and for the vertical ((b), (d)) plane. There is a disagreement inthe horizontal plane(a) between BSRT and wire scanner (part of BSRT data is outside thevisible range) due to the BSRT calibration at the time of the MD.

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3.4 Beam scans

The end of each cycle was used for a test of an alternative way of maintaining the beams in collision [3].This method consist of slowly rotating one of the beams around the other one while recording the lumi-nosity change. A potential offset or drift of the beam separation may be extracted from the luminositymodulation.

Scan settings

Since this was a first pilot test no real online orbit feedback was possible from the luminosity data.A dedicated beam process (BP) (ORBIT SCAN COLL RoundAndDrift, stored under the MD category), wasused for the tests. The BP included of a pre–defined beam movement for all the IPs using the LUMI-SCANknobs. The settings were generated for the nominal emittance of ε = 3.75µm and for a rotation radiusδ = 0.4σ with a period of 16 s. Fig. 10 shows the settings used for the tests, scaled here by the emittancein the machine :εB1 = 2.5µm, εB2 = 2µm.

(a) Beam 1 scan settings (b) Beam 2 scan settings

Figure 10: Beam 1 (a) and beam 2 (b) settings for a rotation scan with small superposedartificial drift (visible as a slight shift of the overall position). The settings were generatedto obtain the scan radius scan δ = 0.4σ for a nominal emittance . For the MD the effectivescan radius was δ ∼= 0.6σ due to the smaller emittance of ε = 2.5µm. The settings presentedhere correspond to β∗ = 80 cm for IP1 (or IP5).

Multiple tests were performed in IP1 and IP8 were ’high’ frequency luminosity data could be obtained.

• For IP1 the scans were analysed with 3 Hz luminosity data provided by the experiment and withthe DOROS system.

• For IP8 the scans were analysed with 10 Hz luminosity data provided by the experiment.

DOROS tracking

Profiting from the DOROS system in IP1, it was possible to track (independently of the main test) witha high resolution the beam movement at the IP during the scans. Fig. 11 shows the horizontal andvertical beam position evolution during one of the tests.

A visible cross–talk between the two beams is still under investigation. The beam position shift couldpartially arise from the beam-beam kick (see Fig.12) that is introduced when the beams are separatedduring the scan. It is not due to long range beam-beam since there were only 2 nominal bunches in the

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machine. BE-BI experts pointed out that the origin of the effect is possibly in the DOROS electronicsor the BPM couplers. The noticeable asymmetry of this cross–talk is still under investigation.

(a) Beam 1 (b) Beam 2

Figure 11: Beam positions recorded with the DOROS system in IP1 during a rotatingscan. Figures (a) and (b) represent scans of beam 1 and respectively beam 2. The cross–talkbetween the beams (and its asymmetry) is still under investigation.

Figure 12: Example of the coherent beam–beam kick [4] for the scan configuration (0.4σand 0.6σ represented by the two vertical lines).The magnitude of the beam–beam effect,≈ 0.026 is less then measured cross–talk (Fig.11).

Luminosity modulation

The recorded high frequency luminosity and the luminosity analysis results are shown in Fig. 13. For thehigher frequency luminosity data (IP1 3 Hz data, Fig.13a) provided offline by ATLAS, the fit result andthe expected luminosity modulation agree rather well. The quality of this agreement is limited by theluminosity resolution coupled with the too modest drift that was overlaid on the beam position rotation.

In the second example (IP8 with 10 Hz data, Fig.13b) although the noise was dominating someluminosity modulation is visible and can be fitted. As for the case of IP1, the quality of the fit waslimited by the combination of luminosity resolution and modest drift.

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(a) Beam 1 scan, fill 4295, IP1 3Hz data (b) Beam 1 scan, fill 4292, IP8 10Hz data

Figure 13: Examples of the scan test results with beam 1 (a) in IP1 with 3 Hz data andbeam 1 (b) in IP8 with 10 Hz data. The red curve represents the fit to the experimentalpoints, the blue curve is the expected luminosity modulation.

3.5 Beam stability

At the end of fill 4292 the beams were separated, then the Landau octupoles and the ADT gain werelowered. The idea was to determine how robust the head-on collisions are. The following actions weretaken:

• ∼22:20 : the beams were separated by 1σ (ε = 2.5µm) for 10 minutes without change to any othersetting.

• ∼22:30 : the octupole strength was lowered in steps of 0.5 units (∼100 A). The current in theoctupoles reached 0 at around 22:45.

• ∼22:50 : the ADT was switched off.

The result of this test is shown in Fig. 14 in terms of signal on the BBQ system. Some beam activityis only visible after the ADT was switched off.

4 Conclusion and outlook

This MD demonstrated the feasibility of the simultaneous β∗ leveling in three collision points. The imple-mentation of an improved schema for a reference orbit handling eased the procedure and provided morerobust orbit and feedback handling. The tracking capabilities of the DOROS electronics was demon-strated. However, as the system is still under development some stability issues have been observed.The alternative method of the keeping beams in collision was partially validated, but was limited by thelow luminosity. A larger number of bunches (and therefore higher absolute luminosity) would providehigh rate luminosity data of better quality. This however would require additional level of preparationin terms of the machine protection.

The following points are foreseen the a follow up MD:

• TCTs alignment along the squeeze process,

• Beam scans with larger superposed drifts,

• Long term stability of the reference orbits.

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Figure 14: End of cycle test beam stability test. The magenta and blue curves correspondto orbit correctors involved in the beam separation knobs. The yellow and violet curvesrepresent the current in the octupoles. The beam spectrum of the BBQ (red, orange, green)only shows some activity once the ADT is switched off.

5 Acknowledgments

The MD team would like to thank the two LHC OP crews that were present on that evening and night:M.Solfaroli, R.Suykerbuyk and R.Giachino. Additional acknowledgments are sent to the experimentcontact persons: R.Jacobson and W.Kozanecki for providing the higher rate luminosity data.

References

[1] X. Buffat, et al. MD on squeeze with colliding beams, CERN-ATS-Note-2013-002 MD.

[2] A. Gorzawski, First experience with the Q1 DOROS BPMs, CERN LBOC nb.48.

[3] A. Gorzawski, Beam offset control during β∗ levelling, CERN LBOC nb.27.

[4] W. Kozanecki,T. Pieloni,J. Wenninger Observation of Beam-beam Deflections with LHC Orbit Data,CERN-ACC-NOTE-2013-0006.

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