CERN-ACC-Note-2016-0010

13 January 2016Gianluca.Valentino@cern.ch

Diffusion and Halo Population Measurements with

Collimator Scans at 6.5 TeV

G. Valentino, R. Bruce, S. Redaelli, R. Rossi, B. Salvachua, A. Valloni,J. Wagner CERN BE-ABP, Geneva, Switzerland

G. Stancari, FNAL, Batavia, Illinois, USA

Keywords: beam diffusion, collimator scan, halo population

Summary

Beam halo measurements at 6.5 TeV in the LHC were conducted via collimator scrapings inan MD carried out on the 4th November 2015. From the time evolution of the beam losses ina collimator scan, it is possible to extract information on the halo diffusion and population.Six scans were performed with two collimators in the vertical and horizontal planes in B1 andB2 respectively. The scans were done with squeezed, separated beams, with colliding beamsand once again with separated beams but with a gentle continuous transverse blow-up withthe ADT (transverse damper). The results obtained were compared with those from similarscrapings performed in an MD in 2012 at 4 TeV.

1 Introduction

The time evolution of beam losses during a collimator scan gives information on halodiffusion, halo population, emittance growth, beam lifetime and collimation efficiencyas a function of the particle transverse amplitude. This study is also useful to checkcalculations of the dynamic aperture, and augments the experimental understanding ofpresent and future collimation systems: how fast particles get to the collimators, howmany of them etc. For this purpose, an MD was performed in 2012 at 4 TeV to measurethe diffusion rates and halo population [1], based on a technique [2, 3] already used atthe Tevatron. An MD request was made for 2015 to redo the measurements at higherenergy (6.5 TeV) and obtain a better approximation of the conditions at the nominalLHC energy of 7 TeV.

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2 MD Programme

The MD started off with squeezed (β∗ = 80 cm), non-colliding beams at an energy of6.5 TeV. One nominal bunch per beam was used, with a starting intensity slightly lessthan nominal. The first step was to perform a beam-based alignment of the horizontaland vertical IR7 primary collimators to determine the beam centers at each collimator.The IR7 primary collimators were then retracted from their nominal settings of 5.5 σ to ahalf gap of 7 σ, in order to have a larger scan range. The 1 σ beam size is determined fromthe nominal beam emittance of 3.5 µm and local, nominal beta functions at the individualcollimators. An overview of the settings of the collimators not used for scraping duringthe MD are shown in Table 1. The settings for these collimators are the same as used innormal operation, except for the skew IR7 TCPs.

Table 1: The settings of the collimators not used for scraping throughout the MD(grouped by families), for both scraping configurations.

Collimator Family Half Gap [σ]TCP IR3 15.0TCSG IR3 18.0TCLA IR3 20.0TCP.B IR7 7.0TCSG IR7 8.0TCLA IR7 14.0TCSP IR6 9.1TCDQ IR6 9.1TCTP IR1/5 13.7 / 13.7TCTP IR2/8 37 / 15

TCL out

In the study, the left jaws of the TCP.D6L7.B1 (vertical plane) and the TCP.C6R7.B2(horizontal plane) collimators were moved in steps of 5 µm to 20 µm. The collimatorswere selected from different beams to be able to perform the scrapings in parallel withoutinducing cross-talk in the BLM signals. The jaws were moved in as soon as the beamlosses from the previous step had decayed back to a steady-state (approximately every10 to 40 seconds).

The jaws were left for a few minutes in the beam after they had reached their finalinward position, to allow the losses to stabilize. Subsequently, the jaws were moved outin steps of 20 µm to 100 µm, with the next step being taken when a steady-state lossrate was observed. The beams were then brought into collisions, and the procedure wasrepeated. The beams were then separated again, and the scrapings were repeated witha gentle ADT blow-up (white noise excitation) running during both inward and outwardsteps, first for B2 horizontal, and then for B1 vertical. A gain of 0.001 with excitationamplitude of 0.2 was used for B2 horizontal, while a gain of 0.04 with excitation amplitudeof 0.05 was used for B1 vertical. This measurement is useful in order to understand thedynamics of how particles hit the collimators during loss maps.

The initial and final jaw positions for each scraping in terms of the nominal beamsize are provided in Table 2. Finally, a full scraping was performed with 20 µm steps

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Figure 1: The collimator positions and beam intensities as a function of time during theMD (t[0] = 04.11.2015 16:43:00).

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Figure 2: The collimator positions and the associated BLM signals as a function of timeduring the MD (t[0] = 04.11.2015 16:43:00).

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every 7 seconds with the TCP.C6R7.B2 starting from 4 σ to dump the beam. Thecollimator positions and beam intensities during the MD are shown in Fig. 1, while the12.5 Hz signals of the BLMs at the collimator locations and corresponding jaw positionsare shown in Fig. 2. A zoom of the BLM signals and jaw positions at the innermostpositions is shown in Fig. 3.

Table 2: The initial and final collimator jaw nominal half gaps in units of σ for thedifferent scrapings. The beam centers used in the calculations were determined in beam-based alignments during the MD.

Collimator Separated Colliding Separated BeamsBeams Beams with ADT blow-up

TCP.D6L7.B1 (initial) 7.3 σ 8.1 σ 5.1 σTCP.D6L7.B1 (final) 2.3 σ 1.4 σ 3.0 σTCP.C6R7.B2 (initial) 7.3 σ 8.2 σ 4.2 σTCP.C6R7.B2 (final) 2.8 σ 2.8 σ 2.3 σ

3 MD Results

3.1 Evolution of beam emittance

The evolution of the beam emittances in both beams and planes, as measured both fromthe synchrotron-light telescopes and the wire scanners, are shown in Fig. 4. A summaryof the emittances measured using wire-scans at the start of each scraping is shown inTable 3 for the beams and planes of interest. There is little change in the emittancefrom separated to colliding beams, however the emittance growth during the third setof measurements with separated beams but with the ADT blow-up running is apparent.The wire scan measurements were taken with both normal gains and with gains set in away to saturate the core measurement, but amplify the measurement of the tails. Onlythe normal measurements are shown in the plots; the saturated measurements will beanalysed at a later stage. It will also be possible to determine the slope of the diffusioncoefficient in the beam core from these data, as in [4], to be compared with the measureddiffusion coefficients in the halo.

Table 3: Normalized, 1 σ wire-scan emittances for B1 V and B2 H at the start of eachmeasurement.

Measurement ǫB1

V [µm] ǫB2

H [µm]Separated beams 1.81 2.18Colliding beams 1.65 2.19

Separated beams ADT on 2.23 3.03

3.2 Parametric fits of diffusion model

The model described in [4] was used to fit the inward and outward steps separately.Examples are shown in Fig. 5. Figure 6 shows the diffusion coefficients as a function of

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the action, calculated as J = x2/(2β) for all 6 cases: horizontal and vertical, each withseparated, colliding and separated (with ADT blow-up) beams. As expected, the diffusionis higher for colliding beams than it is for separated beams. The diffusion coefficientsappear to be similar for both B1 vertical and B2 horizontal, for the same conditions. Thiscontrasts with what was observed in the measurements at 4 TeV, in which with separatedbeams, the diffusion in the B2 horizontal plane was higher than that in the B1 verticalplane. In addition, at 4 TeV for the horizontal plane there was little difference betweenthe separated and colliding cases, while in the vertical plane, collisions had enhanceddiffusion by about 2 orders of magnitude. The explanation put forward for the 4 TeVmeasurements was due to the larger emittances observed for the horizontal plane in B1,which could have enhanced the diffusion in the vertical plane with sufficient coupling.However, in the 6.5 TeV measurements, the emittances are similar for both planes andbeams.

The diffusion coefficients measured with a gentle ADT blow-up with separated beamsdo not appear to vary with action as is the case for the other measurements withoutthe ADT. Further analysis taking into account the strength of the ADT excitation is

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Figure 3: A zoom of the collimator positions and the associated BLM signals showingboth inward and outward steps.

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Figure 4: Synchrotron-light and wire-scan emittances during the MD.

needed before any conclusions can be derived. A systematic effect also observed in thescraping measurements done at 4 TeV is the larger diffusion rates obtained from outwardsteps. They are not understood, but are probably due to the fact that after scraping adifferent beam population is being sampled. The difference in diffusion speed in the threeconditions can also be appreciated from a plot of BLM signals as a function of time forthe same collimator at the same jaw position following an outward step in Fig. 7.

3.3 Full scraping

In order to measure the full beam profile, and dump B2 in preparation for the off-momentum end-of-fill MD [5], a full scraping with the TCP.C6R7.B2 was performed,the first such measurement at 6.5 TeV. A linear fit was applied to the intensity dropand corresponding BLM signal peak for each step (see Fig. 8) to obtain a calibrationfactor of 7.772× 10−12 Gy/p. This compares well to the calibration factor of 5.5× 10−12

Gy/p obtained from FLUKA simulations [6]. The intensity lost during the scraping asa function of collimator position was fit with a double-gaussian (see Fig. 9), and a fitcoefficient of σ = 0.247 mm was obtained, which is close to the nominal σ of 0.275 mmfor a 3.5 µm emittance. The fit coefficients are shown in Table 4. A single-gaussian fitwould not have accounted properly for the tails in the distribution, as has been alreadyobserved in previous scrapings at lower energies [7].

The reconstructed population from the BLM signal as a function of the collimatorposition in units of beam σ during the inward scan of the TCP.C6R7.B2 with separatedbeams is shown in Fig. 10. The Gy/p calibration factor of 7.772× 10−12 found in the fullscraping at the end of the MD was used to convert the BLM signal in Gy/s to p/s. AGaussian distribution is shown for comparison.

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(a) Inward step of TCP.D6L7.B1 during collisions (b) Outward step of TCP.D6

Figure 5: The BLM signals and resulting fit of the diffusion model during inward andoutward steps.

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007

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Figure 6: Diffusion coefficients as a function of action from the collimator scans.

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Figure 7: BLM signals versus time for the TCP.C6R7.B2 at Ji ∼ 0.0017, for an outwardsteps of 10 µm for separated beams with no ADT blow-up, and 20 µm for the other twocases.

Figure 8: Linear fit applied to the intensity drop and corresponding BLM signal peakfor each step during the full scraping.

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Figure 9: Double-gaussian fit to the intensity lost in the collimator scraping.

Table 4: Fit coefficients obtained from the double-gaussian fit to the scraping. The lasttwo columns show the beam center measured at the collimator just before the scraping,and the corresponding delta w.r.t. the fit.

I1 I2 σ1 [mm] σ2 [mm] µ [mm] µsetup [mm] ∆µ [mm]0.3219 0.6913 0.2466 0.2467 0.224 -0.05 0.274

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Figure 10: Reconstructed population during the inward scan of the TCP.C6R7.B2 withseparated beams and a Gaussian distribution shown for comparison.

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4 Conclusions

This note documents the results from an LHC beam halo scraping and diffusion studyperformed using collimators in various configurations at 6.5 TeV. The left jaws of severalIR7 primary collimators in the vertical and horizontal planes were moved in steps of5 µm to 20 µm towards the beam to obtain spikes in the BLM signal. The jaws werethen retracted in steps of 20 µm to 100 µm to observe halo repopulation effects. Thetests were done with squeezed separated and colliding beams, as well as with separatedbeams and with a gentle ADT blow-up of the beam. A diffusion model was fitted tolosses observed during the inward and outward jaw steps. In addition, a double-gaussianwas fitted to the intensity lost during a full scraping in B2 horizontal. The 1-σ beam sizewas found to be close to the nominal value.

Several tasks are left as future work, and may be reported in a further publication.These included analysis of the uncertainties in the diffusion fits, a comparison of thediffusion coefficients obtained with the core emittance growth, as well as a comparison ofthe coefficients with calibrated damper kicks.

5 Acknowledgements

We would like to thank the OP crew for their assistance during the MD study. Thanksalso to G. Trad for useful explanations on the logging and analysis of the BSRT data.

References

[1] G. Valentino et al., “Halo scraping, diffusion and repopulation MD”, CERN-ATS-Note-2012-074 MD, 2012.

[2] G. Stancari, G. Annala, T. R. Johnson, D. A. Still, A. Valishev. Measurements oftransverse beam diffusion rates in the Fermilab Tevatron collider. In Proceedings ofthe 2nd International Particle Accelerator Conference, San Sebastian, Spain (2011).

[3] G. Stancari. Diffusion model for the time evolution of particle loss rates in collimatorscans: a method for measuring stochastic transverse beam dynamics in circularaccelerators. FERMILAB-FN-0926-APC, arXiv:1109.5010v3

[4] G. Valentino et al., “Beam diffusion measurements using collimator scans in theLHC”, Phys. Rev. ST Accel. Beams, 16, 021003 (2013).

[5] H. Garcia, R. Bruce, B. Salvachua, “Off-momentum loss maps with one beam”,CERN-ACC-NOTE-2016-0011 (2016).

[6] E. Skordis, “Status of FLUKA simulations for collimator BLM thresholds”, presentedat the 6th BLM Thresholds WG meeting, 02.10.2015.

[7] F. Burkart et al., “Halo scrapings with collimators in the LHC”. In Proceedings ofthe 2nd International Particle Accelerator Conference, San Sebastian, Spain (2011).

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