Phase 2 Collimation Concepts

1. Phase 2 Collimation Concepts R. Assmann, CERN, AB/ABP Acknowledgements to the colleagues in the LHC Collimation Working Group which worked out and presented most of the results shown here: http://www.cern.ch/lhc-collimation Acknowledgements also to the colleagues at SLAC and in other US labs (LARP), working on a phase 2 design in collaboration with us: http://www-project.slac.stanford.edu/ilc/larp/ MAC, December 8th, 2006

2. The LHC Challenge

3. Why Phase 2? • Looking at the required extrapolation (2-3 orders of magnitude), it is evident that predictions for the new LHC regime must be taken with caution. • Give normal experience, unexpected features in the new LHC regime should be expected we must be able to react! • The phase 1 design was done with significant time pressure: About three years from start of the new system design to signature of production contract  conventional design with advanced features. • Conflicting design routes were encountered  choice of maximum robustness with compromises in cleaning efficiency and impedance. • I proposed early on a phase 2 system as integral part of the LHC collimation system 30 locations implemented into the LHC layout and fully equipped for fast upgrade (cables, water, supports installed before startup). • Detailed performance estimates only after decision for phase 2 upgrade preparation. • Phase 2 collimators will allow implementation of advanced, more powerful and risky designs (hybrid collimators).

4. Uncertainties I • There are significant uncertainties in our predictions. • Loss rates in normal operation: • We allow for up to 0.1% of beam lost per second for up to 10 seconds (0.2 h beam lifetime). • Expect these losses during b squeeze, while bringing beams into collision, beam tuning (tune), … • Parameter strongly supported by international experts in external collimation review in 2004 (experience from HERA, TEVATRON, RHIC, SSC design, SNS design). • Can be better or worse. Judgement depends on the person looking at this. • Abnormal losses: • We allow for up 0.3 % of 7 TeV beam lost on a collimator (single-turn) without damage (nominal dump error: single-module pre-fire). • Frequency of these errors unknown (assume at least once per year in LHC). • Population of beam halo close to collimators unknown: 1% of beam in the halo corresponds to twice the full TEVATRON beam! • General uncertainties from limited knowledge of halo beam dynamics.

5. Uncertainties II • Quench limits: • Uncertainties in the quench level of SC magnets can reach a factor 2 easily. • Nuclear physics: • The nuclear physics processes in the CFC collimator jaws can have up to a factor 2 uncertainties at 7 TeV. • Modeling of energy deposition can be affected by the limitations in the modeled geometry by up to a factor 2. • Impedance: • LHC resistive wall impedance will be dominated by the collimator-induced impedance contributions. • Only tolerable with the predicted “inductive bypass” at low frequencies, which gains up to factor 100 compared to the classical thick wall theory. Never proven experimentally. • Collimator lifetime with strong radiation: • High dose rates in the collimator jaws and other collimator parts (10-100 MGy/year). • Designed for robustness against radiation damage. However, lifetime unknown.

6. Collimator RW Impedance

7. System Design “Phase 1” Momentum Cleaning Betatron Cleaning 8 different types of collimators plus masks and absorbers. In total 160 ring and 28 transfer line locations for LHC collimators and absorbers. Phase 4 (ultimate upgrade), absor-bers, … not further discussed here. Phased approach: Phase 1 112 collimators 40% of nom intensity Phase 2 30 collimators nominal + ultimate(after each TCSG)

8. Review of Phase 1 Limitations and Potential Solutions • Inspite of uncertainties a lot of effort is spent to predict LHC performance with collimation as well as possible (detailed presentation last MAC). • Major possible limitations and possible solutions are reviewed in the following slides: A) Impedance. B) Cleaning efficiency. C) Operational ease: Not discussed in the following. If limitations in set-up of system are encountered (e.g. tuning forever): Implement advanced concepts, for example BPM buttons incorporated in collimator jaws  easy centering.

9. (A) Collimator-Induced Impedance E. Metralet al  Limitation at about 40% of nominal intensity… (nominal b*, full octupoles) Important: Collimator impedance was measured in the SPS with LHC prototype collimator.

10. Potential Solutions Impedance Potential solutions are being considered: • Metallic secondary collimators (phase 2 design at SLAC and later CERN)  lower electrical resistivity of collimator “wall”. • Increased chromaticity stabilize beams. • Low noise transverse feedback at 7 TeV  stabilize beams. • Triplet upgrade with larger aperture. Collimator gaps at 7 TeV are a direct function of the triplet aperture and the b*  larger gaps for all collimators. • Crystal-based collimation with increased particle deflection  larger gaps for secondary collimators. • Non-linear collimation scheme  larger gaps for secondary collimators. All approaches must be looked at, even if they now seem challenging! LARP/SLAC takes first steps towards construction of phase 2 collimator.

11. (B) 7 TeV Proton Loss Prediction p losses ~ inefficiency G. Robert-Demolaize et al

12. Single-Diffractive Scattering Cross-section single-diffractive scattering: Comparison FLUKA – STRUCT – COLLTRACK/K2 • LHC p collimation system was optimized until fundamental limitation was met: • Some protons experience single-diffractive scattering in primary betatron collimators: large energy offset and small betatronic kick. • Betatron collimators generate off-momentum halo. • Most of newly off-momentum protons are lost in first place with high dispersion: downstream dispersion suppressor.

13. Zoom Dispersion Suppressor IR7 Collimation team and FLUKA team mW Heat load showers

14. Predicted Limits from Betatron Cleaning PhD G. Robert-Demolaize • Quite a good margin at injection energy: Higher intensities can be handled. • Limitations at 7 TeV to below nominal intensity. • Predictions are a function of minimum beam lifetime (peak loss of 0.1% in 1 s) and the quench limit of SC magnets. Significant uncertainties! • Prediction is for ideal cleaning performance. Showers not included (Q11)!

15. Can We Run at the Quench Limit? • BLM response depends on where the protons are lost! • Important shielding effect of materials. • Up to factor 10 different BLM response for different longitudinal locations! • BLM threshold must protect against quenches from all different loss locations. • Threshold at least factor 3 below quench limit (HERA factor 10). • We cannot run at quench limit! L. Ponce

16. Potential Solutions Efficiency Phase 1 collimation system was constrained for several reasons: • Only conventional collimator solutions that can be quickly built. • Warm magnets for IR3 and IR7 were final. • The cold regions were final and were not allowed to be touched. Solutions can be envisaged but involve major changes and /or risks. • Improved cleaning efficiency, impedance, operational handling with advanced phase 2 collimators: LARP/SLAC and CERN R&D effort. • Stronger dogleg bends (MBW) in cleaning insertions to catch off-momentum particles (changes beam separation). • New cleaning insertions with combined betatron and momentum cleaning. • Absorbers in SC dispersion suppressors downstream of cleaning insertions. Requires new SC magnets. • Advanced concept for primary collimators (e.g. crystals).

17. Collimation: LHC Intensity Limitations I

18. Collimation: LHC Intensity Limitations II

19. Basic Phase 2 Strategy • Uncertainties in extrapolation: • Take a decision on phase 2 only after 2 years of 7 TeV LHC operation. • Analyze carefully phase 1 performance and LHC behavior before final decision. • Advanced and risky phase 2 design: • Test phase 2 prototype collimators with beam before installation into the LHC. • Requirement for beam test stand at 450 GeV high intensity. • Test with LHC beam at 7 TeV during second year of 7 TeV operation • Potential performance limitations and high activation in collimation region: • Prepare for upgrade as early as possible (all infrastructure installed). • Win ~3 years by developing and building several prototype phase 2 collimators before detailed requirements are known. • Address risks and uncertainties by international effort of experts (LARP, FP7) and parallel development of 3 orthogonal designs (efficiency, impedance, operational ease: BP^‘s in jaws).

20. CERN Activities • Our activities are delayed due to ongoing efforts to get the phase 1 system into place (limited resources must be put on highest priority). • However, even in a very limited situation we spend time on phase 2: • Phase 2 beam and cleaning simulation (PhD student part time on this). • Comparison of all possible upgrade options like advanced collimators, crystals, non-linear collimation, new local absorbers, … (new PhD student). • Phase 2 energy deposition studies (new PhD student). • Preparation of requests for CERN white paper and FP7 bid (European research program). • Support for SLAC work on LHC collimation phase 2: • Beam simulations. • Energy deposition studies. • Review of technical and engineering design. • Preparation of beam test stand for beam test of SLAC collimator for the LHC. • Monthly video meeting.

21. SLAC Collimator Design and Prototyping: Rotatable LHC Collimator Strong SLAC commitment and effort: Theoretical studies, mechanical design, prototyping. New full time mechanical engineer hired. Looking for SLAC post-doc on LHC collimation! Design with 2 rotatable Cu jaws First prototype with helical cooling circuit (SLAC workshop)

22. Collimator Infrastructure @ SLAC in Construction The “clean tent” Next steps @ SLAC: Assembly of prototype jaw. Heating (20 kW) and cooling test. Deformation tests. Cooling Heating and power Instrumentation Support table

23. Phase 2 Schedule (CERN Whitepaper) • 2005 Start of phase 2 collimator R&D at SLAC (LARP) with CERN support. • 2006/7 Start of phase 2 collimator R&D at CERN. • 2008 Completion of three phase 2 prototypes at CERN and SLAC.Prototype qualification in a 450 GeV beam test stand at CERN. • 2009 Installation of prototypes into LHC and tests with beam at 7 TeV.Decision on phase 2 design and production. • 2009 Production of 36 phase 2 collimators. • Installation of 30 phase 2 collimators during the 2010/11 shutdown. • 2011 Commissioning of the phase 2 collimation system.LHC ready for nominal and higher intensities. • Note: In case that the collimator production requires longer than one year the system can only be installed during the 2011/12 shutdown and commissioned during 2012.

24. Conclusion • LHC enters in a new regime: Collimation/cleaning is a major challenge in LHC. • Predictions are difficult and assumptions have uncertainties: peak loss rate, quench limit, imperfections, BLM thresholds, impedance, … • Only the machine will give us the real picture. Already learning from simulations: For example, benefit local absorbers in IR3/IR7 dispersion suppressors. • All performance studies indicate intensity limitations below nominal LHC intensity (basic physics processes). Reality is usual worse. • Therefore: Two orders of magnitude in performance have been gained, another factor ≥ 10 desirable to be prepared for LHC upgrades. • Further gains are difficult but possible: • CERN effort on phase 2 and other upgrade options. • LARP/SLAC effort on LHC collimation upgrade. • Preparation of FP7 program. • Start now to be ready for nominal LHC intensity in 2011 (pushy)!

25. Supporting Slides

26. Radiation Dose to Magnets M. Santana Leitner et al

27. MBW S. Ramberger

28. MQW S. Ramberger

29. Radiation Dose on Bake-Out System C. Rathjen

30. Heating Vacuum Chambers C. Rathjen

31. SC Link IR3

32. Dose to Optical Fibers I. Kurochkin

33. IR6 Problem

34. Imperfections reduce efficiency… 1 stage cleaning 2 stage cleaning Increase in inefficiency Production tolerance MAC Dec 2004 50% higher inefficiency Expect to loose factor 2-4 in efficiency from imperfections!

35. LHC Beam Loss Specification (Nominal Design) • The collimation system was designed based on the following assumptions on loss rates: 3.5 MJ (quench limit: 10 mJ/cm3) • Loss rates based on experience. Not too conservative: Peak loss at 7 TeV is 0.1% of beam in 1 s! • Supported by external review, taking into account Tevatron, HERA and RHIC experience! • Yearly loss rate in IR7: 2 × 1016 p at 7 TeV per year (nominal intensity).

36. Impedance and Chromaticity E. Metralet al