1. LHC collimation MDs, crystals and halo control
LHC Study Working Group day
September 2nd, 2014
CERN, Geneva, Switzerland
LHC collimation MDs, crystals and halo control
S. Redaelli, R. Assmann, F. Burkart, R. Bruce, M. Cauchi, D. Deboy, P. Hermes, L. Lari, A. Marsili, D. Mirarchi, E. Quaranta, V. Previtali, A. Rossi, B. Salvachua, G. Valentino, D. Wollmann
Acknowledgements: OP, ADT, MD, BI-BLM teams, UA9 collaboration; FNAL team; included also results from LHC aperture team.

2. (Some) collimation people

3. Collimation quench tests
Collimation MD notes
Collimation quench tests:CERN-ATS-Note MD S. Redaelli et al. Collimator losses in the DS of IR7 and quench test at 3.5 TeV
CERN-ATS-Note MD G. Bellodi et al. Pb ions collimator losses in IR7 DS and quench test at 3.5 Z TeVCERN-ATS-Note MD B. Salvachua et al. Collimation quench test with 4 TeV proton beams
Machine performance: Collimation hierarchy for beta* and impedanceCERN-ATS-Note MD A. Rossi et al. Summary of MD on nominal collimator settingsCERN-ATS-Note MD R. Bruce et al. Tight collimator settings with β∗ = 1.0 mCERN-ATS-Note MD S. Redaelli et al. End-of-fill study on collimator tight settings CERN-ATS-Note MD R. Bruce et al. Tests of tight collimator settings in the LHCCERN-ATS-Note MD B. Salvachua et al. Results on nominal collimator settings MD at 4 TeV
Operations optimization: Collimation alignment optimizationCERN-ATS-Note MD G. Valentino et al. Improving LHC Collimator Setup Efficiency at 3.5 TeVCERN-ATS-Note MD G. Valentino et al. Beam-Based Collimator Alignment MD
Detailed/Advanced collimation studies:CERN-ATS-Note MD G. Valentino et al. Halo Scraping, Diffusion and Repopulation MDCERN-ATS-Note MD D. Wollmann et al. Beam distribution measurements in the LHCCERN-ATS-Note MD A. Rossi et al. IR3 collimation combined betatron and momentum cleaningCERN-ACC-NOTE D. Mirarchi et al. Study on off-momentum tail scraping in the LHC
Miscellaneous
CERN-ATS-Note-2014-xyz MD A. Marsili et al. Scans of the Physics Debris Absorbers (TCL) at the 4 TeV LHCCERN-ATS-Note MD W. Höfle et al. Emittance blow-up and loss maps using the ADT as exciter
IPAC2013 (MOPWO038) E. Quaranta et al. Cleaning efficiency during the ramp (continuos loss maps)
IPAC2013 (MOPWO046) L. Lari et al. Losses on collimators during asynchronous beam dumps
Machine performance: LHC aperture for beta* reachCERN-ATS-Note MD S. Redaelli et al. IR1 and IR5 aperture at 3.5 TeV CERN-ATS-Note MD M. Giovannozzi et al. IR2 aperture measurements at 3.5 TeVCERN-ATS-Note MD P. Hermes et al. IR8 Aperture Measurements at injection energy CERN-ACC-NOTE P. Hermes et al. IR2 Aperture Measurements at 4 TeV
Collimation quench tests
Pushing hierarchy
End-of-fill
Improved alignment
End-of-fill
Advanced studies
End-of-fill
We were very happy to exploit end-of-fill studies after physics runs (e.g. TCL scans) and are eager to jump in during “floating MD’s”...
Aperture and beta*

4. Collimator support to MD teams
Most MD’s required changing the standard collimation configuration –> substantial participation from coll team!
Collimator are useful diagnostic tools!
Beam diagnostics - instrument’s calibration vs. beam intensity through collimator scans
Participation to all quench tests including preparation (e.g. ADT preparation)
Impedance measurements
Optics changes and measurements
Dynamic aperture MDs
New optics commissioning: high-beta, ATS, ...
Special TCT configuration for all beam-beam MD’s

5. Highlight results of 2011-13 MDs Run 2 proposals - commissioning
Outline
Introduction
Highlight results of MDs
Run 2 proposals - commissioning
Run 2 proposals - MDs
Conclusions

6. Collimation quench tests
Goals: Understand better quench limits at the limiting locations (DS) for cleaning Define operation-optimized BLM thresholds
Idea: Maximize primary beam losses with full IR7 collimation in place to approach (exceed) the quench limit of dispersion suppressor (DS) magnets.
This probes empirically the isolated locations that limit the collimation cleaning performance.
2011: protons and ions at 3.5 TeV Z : protons only at 4 TeV.
MD notes and international publications
CERN-ATS-Note MD
CERN-ATS-Note MDCERN-ATS-Note MD
Achieved – safely – unprecedented beam losses: about 3 times the Tevatron beam on the TCP’s in ~10 s... and we did not quench!

7. Collimation quench test results
1.06 MW on TCP’s
1 kW at Q8
Achieved at the third attempt after ADT excitation setting up (14/02/2013)
Successfully achieved Peak losses of ~1MW on TCP (2x system design)!
Achieved 3.4 times the assumed quench limit at 4.0 TeV without quenching!
Very important set of data that triggered an in-depth analysis of quench behaviour in the DS for collimation losses collimation review.

8. Collimator faster alignment
Goals: Improve algorithms for BLM-based beam-based alignment of collimators. Test software/algorithms before deploying them for standard commissioning.
Various ingredients for improving: - Test of faster acquisitions (BLM/motors); - Parallel closure of collimators; - Improved spike recognition; - Improved cross-talk recognition; - Orbit interpolation at collimators; - Automatic settings handling.
Main ref.: PhD work by G. Valentino
Setup time per collimator ( )
Reduced number of dumps from human mistakes compared to fully-manual alignment.
All users (MD + commissioning) profited from this important achievement!
More that 550 individual collimator alignments in !!

9. Collimator faster alignment - BPM’s
Next challenge:
Faster alignment with new BPM collimators!
Measured beam position within jaws [mm]
Achieved collimator alignment to 10 um resolution in less than 20 seconds with 20mm full gap!
Important validation during SPS MD’s. Now working on a tests-stand to prepare for the LHC. But clearly this will require studies at the LHC.
Faster alignment with “old” collimator still important, as only <20% of the Run 2 collimation system will mount BPM-jaws! More MD’s to come...

10. Tighter collimator settings
Goals: Understand the “hierarchy limit” in IR7 ➞ push further β* performance! Optimize collimator settings and number of required alignments per year. Understand the collimation impedance models and stability limits.
Idea: tighten IR7 collimation hierarchy (with/without re-alignment) to address the operational limits.
How tight can we go without lengthy re-alignments?
Are losses acceptable?
Benchmark of impedance models.
Setting table - Relaxed vs tight.
2011: Demonstrated the feasibility of tight settings with a single BB alignment per year!
Demonstrated improved cleaning.
2012: tried “nominal” collimator settings.

11. Tighter settings - impedance
N. Mounet et al.
Effect of secondary collimator gaps on tune shift → benchmark of impedance models
Nominal settings in 2012: possible hierarchy issue requires more studies.

12. Beam halo: diffusion and population
Phys. Rev. ST Accel. Beams 16,
Goals: Measure beam population and diffusion speed in different conditions Collect experimental data for extrapolations to higher energies
Idea: tail populations and diffusion rates can be inferred from loss behaviour during scans IN and OUT with collimators.
We definitely need to repeat this at higher energy!!
Candidate for end-of-fill?
Otherwise, the LHC lacks halo monitoring...
First measurements, proving that collimator scans technique is suitable.
⇒ models for impact parameters on TCP’s. Inputs to halo control simulations.

13. Continuous loss maps during ramp
Goals: Establish loss map technique for dynamics machine phases (ramp, squeeze) Benchmark complex simulation tools at different energies / settings
Very good agreement - note the 6 orders of magnitude on y scale!
Measurement challenges:
Controlled losses of individual bunches at selected energies.
Balance losses: good cleaning accuracy versus risk of dumping.
E = 450 GeV
Time [hh:mm]
Beam energy / intensity
E = 4000 GeV
Intensity B1
Intensity B2
Collimator settings in [mm]
Beam intensity and energy
Controlled loss maps achieved! Plan to develop this further, e.g. for squeeze, in 2015 in collaboration with ADT team!

14. Passive abort gap cleaning in IR3
Goals: Characterization of the off-momentum tail population (25 ns) Can the abort gap be cleaned passively with the momentum collimators?
Idea: Scan with TCP collimators in IR3.
Particles outside the bucket lose energy due to synchrotron radiation and might be cleaned in the momentum cleaning insertion for appropriate collimator settings.
Comparison with AG populations achieved with active ADT cleaning: comparable levels!
ADT: faster for cleaning. Passive cleaning would be in place all the time!
Promising, and needs more studies before possible deployment.

15. LHC aperture with collimator scans
Discussed in details at the LBOC of last Tuesday!
Goals: Determine IR aperture in physics conditions for optimum β* reach (Other important aperture measurements are part of the commissioning)
First aperture measurement at 3.5TeV showing the “platinum” mine !
All details at last LBOC meeting...
Since 2011, aperture measurements at top energy with squeezed beams are the basis for the β* reach of the LHC.

16. Highlight results of 2011-13 MDs Run 2 proposals - commissioning
Outline
Introduction
Highlight results of MDs
Run 2 proposals - commissioning
Run 2 proposals - MDs - Requirements from Run1 experience - Active halo control and beam halo - Crystal collimation
Conclusions

17. Plans for beam commissioning
Discussed and presented at the Evian2014 workshop
Detailed commissioning of new collimators with BPMs - Commissioning of BPM acquisition, monitoring functions and software - Comparisons with standard BLM methods at injection and top energy - Quantification of non-linear signal components versus gap - Validation of alignment algorithms - Effect on signal from bunch intensity and bunch spacing
Measurements of collimation impedance for different settings - Check impedance with different setting conditions (reduced set of TCSG’s) - Verification of models (vs. octupole currents and Q’) as feedback for β*
Revised/improved loss maps for 6.5 TeV beams - Checks with ADT for “unsafe” beams - Explore “gentle” beam losses with 25 ns trains - New methods for controlled off-momentum loss maps - “Continuous” loss maps during ramp and squeeze
Validation of beam losses / impedance for new TCL layouts - Verify effectiveness of 2 new TCLs in cells 4 and 6 of IR1/5
Protection settings of collimators (joint with dump team), if ATS

18. MD proposal based on Run 1
Collimation quench tests with proton and ion beams at 6.5 TeV - Crucial to address experimentally limitations from collimation losses.
Tighter collimation hierarchy - Do we have machine stability (or sufficiently fast alignment in IR7) to allow for nominal collimator settings? - Clearly, coupled with understanding of impedance limits.
Even faster collimator alignment with BLMs - Improved algorithms. Understand loss spikes at ~ 7 TeV - We are commissioning a 50 Hz movement of motors (8 Hz in 2012!)
Validate the proposed passive abort gap cleaning in IR3 - Will require end-of-fill at larger energy.
Advanced studies with BPM collimators (with BI) - Use quadrupolar components to determine beam size? - Phase advance (adjacent collimators) to compute beta-beating? - Precise setup of collimator jaw angle and comparison with BLM alignment.
Need to repeat halo population scans and diffusion at 6.5 TeV
We need to continue on the fronts that we profited from in Run1!
Some MD’s can be combined (hierarchy with impedance, BPM->BI, setting for MP)

19. Crystal collimation experiment
Primary
Secondaries
Absorbers
Beam
Standard collimation
Promises of crystal collimation: 1. Improved DS cleaning in channeling; 2. Reduce impedance: less secondary collimators and larger gaps; 3. Much improved cleaning for ion beams. ?
Beam
Absorber
Crystal
???
Crystal-based collimation
Uncertainties on the extrapolation to unknown energy territories and operational challenges call for solid experimental validation before this technology can be relied upon for future designs.
Goals: Verify experimental setup and demonstrate channeling at 450 GeV Demonstrate cleaning improvements compared to present collimation at top energy (low intensity only allowed in 2015 by HW interlock) Demonstrate that crystal collimation can cope with dynamics machine changes like ramp and squeeze
Clearly, very ambitious program...

20. Two goniometers for hor. and vert. crystals installed in Apr. 2014
Crystal layout in IR7
Two goniometers for hor. and vert. crystals installed in Apr. 2014
Using very advanced goniometer designs. Pre-requisite: MDs at the SPS with same hardware -> LHC MD’s after SPS ok.
Cour. A. Masi
Cleaning improvement can be demonstrated (on paper) by using only existing collimators!

21. Halo population and control
Goals: Test two active excitation mechanisms: Tune ripple to drive resonances - Narrow-band excitation with ADT
Couple of illustrative examples taken randomly from the LHC elogbook...
Ramp + Squeeze + Adjust
Physics
25h
Ramp
Squeeze
Adjust
Injection
10 h
Motivation: behaviour of loss spikes?
2011
2012

22. MD plans for halo control
Idea: using de-tuning with amplitude, one can in theory use - sidebands in tune spectum excited by quadrupole current ripples - narrow bands excited by the ADT to resonantly excite the beam halo particles without perturbing the beam core.
There is a general consensus that these methods require a solid validation for the LHC case before being relied upon. Issue: core blow-up, bunch footprints. Ultimate solution is the hollow e-lenses, but cannot be used before Run 3.
Working with warm magnet and ADT teams to prepared the MDs...

23. Conclusions
The collimation MD results during the LHC Run 1 were reviewed. Only a selection of highlight results could be addressed.
Some collimation studies sit between MD and OP developments Showed several examples of MDs with immediate outcome on LHC performance: Faster alignment, tighter settings, quench tests We identified a set of measurements that we will try to perform early on. But clearly several important studies still remain part of the MD period.
A first list of MD requests for 2015 was presented. A lot of interesting studies are ahead! Important to continue important work on hierarchy, impedance, alignment, and exploit now hardware to its maximum (e.g. BPM feature).
Outstanding new studies for MD: Crystal collimation tests and active halo control methods (Halo control might be pushed earlier on in case of problems at high energy)
The proposed list of MDs must be reviewed and prioritized!