Collimation Session “Beam Dynamics and Intensity Challenges”

CollimationSession “Beam Dynamics and Intensity Challenges” R. Assmann HHH 2004, CERN “Beam Dynamics in Future Hadron Colliders and Rapidly Cycling High-Intensity Synchrotrons”

Outline • Introduction to collimation • Challenges for collimation in the new LHC regime • Super-conducting environment and cleaning inefficiency • Triplet aperture, beta* and collimation gaps • Impedance from collimation • The LHC collimation system • Upgrade issues • Conclusion

Directions in accelerators New particle physics accelerators are always motivated by: • Higher energy and/or • Higher core intensity Technology choice often super-conducting: • Stronger fields Similar directions in other modern accelerators (neutron spallation sources, GSI future project, …)! Traditional focus: Understand beam dynamics of the beam core (optimize instantaneous luminosity). Locally protect detectors against beam halo (background). Additional focus: Understand beam dynamics of the beam halo and control its overall population.

Functions of collimation There will be unavoidable losses of particles in any accelerator (limited intensity lifetime). Then: Use collimators to intercept lost hadrons at well defined locations, such that no or minimal losses occur at other locations! • No beam loss-induced quenches of super-conducting magnets. • Minimized activation of accelerator components outside of collimation insertions. • Acceptable background from beam halo in experiments. • Limited passive machine protection (beam lost first at collimators). Collimation has become a central ingredient for the success of an accelerator!

Comparing stored beam energy Nominal LHC design: 3 × 1014 protons accelerated to 7 TeV/c circulating at 11 kHz in a SC ring At less than 1% of nominal intensity LHC enters new territory. Collimators must survive expected beam loss…

Comparing damage potential Transverse energy density is a measure of damage potential … … AND proportional to luminosity! In terms of damage potential, LHC advances the state of the art by 3 orders of magnitude!

We store 100-200 times more energy in LHC than done at HERA/TEVATRON!(we could melt 500 kg of Copper with beam) • Quench limits in LHC are stringent: 350 MJ stored beam energy versus 30 mJ/cm3 quench limit! (Quench every magnet ~1000 times if beam is lost in 1 turn and distributed over 27 km) • The spot size is 10 times smaller!(easily drill holes) • Extrapolation by three orders of magnitude in protection against quenches and damage! • Without collimation: Store and collide a few ‰ of nominal intensity! The collimation challenge of LHC LHC will rely on a sophisticated collimation system: Excellent cleaning efficiency: Capture > 99.9 % of high amplitude particles. Excellent robustness: At 7 TeV survive 8 bunches impact (≈ full Teva- tron beam) without damage. Excellent shadow on aperture: Operate with collimation gaps as small as 3 mm, leading to tight tolerances and strong impedance!

Principle of Beam Collimation Beam propagation Core Diffusion processes 1 nm/turn Primary halo (p) Secondary halo p p p Tertiary halo Impact parameter ≤ 1 mm p e Primary collimator p Secondary collimator Shower e Sensitive equipment Shower ... two stage cleaning ...

Super-Conducting LHC Environment Proton losses into cold aperture Local heat deposition Magnet can quench Illustration of LHC dipole in tunnel Control transient losses (10 turns) to ~1e-9 of nominal intensity (top)! Capture (clean) lost protons before they reach cold aperture! Required efficiency: ~ 99.9 %(assuming losses distribute over 50 m)

Inefficiency and Allowable Intensity (Luminosity) Quench threshold (7.6 ×106 p/m/s @ 7 TeV) Allowed intensity Cleaning inefficiency = Number of escaping p (>10s) Number of impacting p (6s) Beam lifetime (e.g. 0.2 h minimum) Dilution Length (50 m)

Secondary and Tertiary Beam Halo (zero dispersion) Secondary collimators Primarycollimators Strategy: Primary collimators are closest. Secondary collima-tors are next. Absorbers for protec-tion just outside se-condary halo before cold aperture. Relies on good knowledge and control of orbit and beta around the ring! Protection devices Cold aperture

Protection of aperture against halo/beam Expected physical aperture limits (freely available, a is half aperture) Aperture allowances: 3-4 mm for closed orbit, 4 mm for momentum offset, 1-2 mm for mechanical tolerances. Collimator setting (prim) required for triplet protection from 7 TeV secondary halo: ~ 0.15 ~ 0.6 Collimator gap must be 10 times smaller than available triplet aperture! Collimator settings usually defined in sigma with nominal emittance!

Collimating with small gaps • LHC beam will be physically quite close to collimator material and collimators are long (up to 1.2 m)! • Precision positioning • Risk of damage to collimators! • Beam electro-magnetic fields interact with the collimator material! Machine impedance increases while closing collimators. LHC will operate at the impedance limit with collimators closed!

Impedance versus collimation gap HFFS simulation by Tsutsui

Final result with reduced system UNSTABLE STABLE  Elias Metral

The phased approach • Conventional collimators even with advanced materials will not achieve LHC design goals. • Not enough time for R&D on advanced technologies. • Phased approach: • Build high-performance, maximum robustness collimators for LHC start-up in 2007 (phase 1)! • Gain time for advanced design (phase 2)! • Gain early experience on LHC commissioning as help for decision. • Broaden R&D for phase 2 technology (different solutions)

Building an LHC collimator (AB&TS department) Jaw clamping support with cooling Vacuum tank Completed jaw

Building an LHC collimator (AB&TS department) Beam passage for small collimator gap with RF contacts for guiding image currents Vacuum tank with two jaws installed

Beam tests with SPS beam Installation of two fully functional collimators into the SPS beam… Measurements during October 2004 to verify design. Series production after 2/2005.

Tuning the gap down to 1 mm and center it! Gap width Gap center With stored SPS beam!

Collimator MDs #2 – (some) BBQ results Collimator cycled between • 51 mm and 3.86 mm (5h04) • 51 mm and 2.86 mm (5h35) • 51 mm and 2.46 mm (5h43) • 51 mm and 2.06 mm (5h50) • 51 mm and 1.86 mm (5h58) Direct Diode Detection Base-Band Q-Measurement

Collimator MDs #2 – (some) BBQ results Direct Diode Detection Base-Band Q-Measurement

Impedance expectation and measurement in SPS Preliminary Collective Effects Team (F. Zimmermann)

Upgrade issues • Higher intensity • Higher energy • Smaller beta*  Re-visit collimator constraints: 1) Robustness to expected beam impact 2) Induced impedance 3) Better cleaning efficiency

Increased stored energy Possible LHC upgrades in same regime as LHC:  Learn from LHC experience (how far from limit)!  Then extend by factor 2-3!  No showstopper!?

Beam impact on collimators for errors High energy error: Irregular dump For the LHC dump bunches over about 200 ns are swept onto the LHC collimator! LHC collimators for LHC (nom) close to limit! Superbunches  Lot’s of intensity in 200 ns! Tough!

For LHC upgrades beyond half nominal • Lower contribution to impedance with metallic collimator materials ( phase 2 collimation). • Improved efficiency with high-Z materials (advanced technology: repairable jaws  phase 2 collimation). • Better precision in jaw set-up, based on advanced halo models. • New technology for collimation: Crystals, non-linear collimation, ... • Larger collimation gaps with increased triplet aperture. • Improved machine stability with guaranteed long intensity lifetimes (peak loss rate in LHC: 1% in 10s)! • Eliminate error cases for collimation (modified injection and beam dump systems). • Improved optics for cleaning insertion with large beta* and more phase advance (longer insertions).

Conclusion • Collimation is a major issue in modern hadron accelerators. • Existing accelerators (TEVATRON, RHIC) are upgrading collimation. New accelerators (SNS, LHC) are spending significant resources on collimation. • In particular LHC collimation is in a new regime (2-3 orders of magnitude beyond experience)! • Collimation is expected to initially limit LHC performance to half of nominal intensity and larger than nominal beta*! • Upgrade program is an integral part of the LHC collimation system with a well-defined strategy and place-holders in the ring. • Collaboration on advanced collimation is being built (with BNL, FNAL, SLAC via US-LARP). Additional partners welcome! • Further LHC upgrades require careful studies and likely a new approach to collimation with advanced technology (renewable collimators, crystals, non-linear collimation, plasmas, ...).

The LHC “collimation mountain” Phase 2! Phase 1! CERN LARP 2003 2004 Collimate the LHC beam 2007

Scenario for worst case shock beam impact Danger of damage to accelerator components. In particular: Collimators close to beam! Equipment failures Equipment errors Operational errors Beam dump: Designed to extract beam within 2 turns. Pulse rise time of 3 ms (dump gap). Failure modes: - Total failure of dump or dump trigger (> 100 years) - Dump action non-synchronous with dump gap - Dump action from 1 of 15 modules, others retriggering after 1.0 ms. Difficult to predict Assume at leastonce per year!

Abnormal dump actions Kick [mrad] Downstream offset [s] TCDQ COLL One module pre-fire

Ensuring collimator survival At 7 TeV about 8 out of 3000 bunches can impact the collimator face (irregular dump): 1m Particle cascade and material heating Carbon collimator block Simulations indicate that graphite or fiber-reinforced graphite are the only material choices that would resist! Search for highest conductivity graphite is ongoing (lowest impedance)…

Minimum beam lifetimes LHC collimator review

Performance Efficiency: Phase 1: Efficiency reduced with respect to old solution! Phase 2: Potential of efficiency extended 2-3 times beyond old solution! These results used for design goals. Difficult to use for predicting quenches in the LHC cold aperture!

Loss Maps Around the Ring: Collision example Peaks in all triplets: Cure with tertiary collimators! Tertiary halo IR8: Initial optics with b* = 1 m

Final result with reduced system UNSTABLE STABLE  Elias Metral

Maximum Robustness Jaws for Phase 1 Driving criteria for material: Resistivity (7-25 mΩm)Short lead timesDesign work and prototyping under wayTS leads effort: A. BertarelliM. MayerS. Calatroni Visit of collimator Friday morning! 0.5 0.5

Design “phase 1” secondary collimators • More conventional design (next iteration on LEP concept) with advanced features. • Two graphite jaws, movable in angle and position, maximum robustness, concept of spare surface. • Full redundant read-out of gap at both ends, gap center, jaw positions. In addition temperature sensors and sensors for damage detection. • Thin coating for impedance reduction (coating destroyed in case of direct beam hit, graphite unaffected). • Mechanical “automatic” opening with motor failure (motor pressing against spring). • Quick plug-ins for electrical and water connections. Fast exchange flanges. Short installation and replacement time! Crucial for radiological reasons! • Three prototypes being constructed now. Surface flatness is a critical parameter. • Tests of prototypes with SPS beam after Aug 2004.

TECHNICAL DESIGN Collimator Cross-section (1/2) Jaw stroke +30/-5 mm Jaw (25x80x1200 mm) Support Bar Cooling Pipes Clamping springs Bellow Stepper Motor Return Spring A. Bertarelli – R. Perret CERN TS – MME Group

The machine layout IR7

External Review of collimation project – July 2004

Scope of the LHC collimation Two warm LHC insertions dedicated to cleaning: IR3  Momentum cleaning IR7  Betatron cleaning Building on collimation system design that started in 1992! Various collimators in experimental insertions IR1, IR2, IR5, IR8.  Four collimation systems: Momentum and betatron for two beams!

Collimation Session “Beam Dynamics and Intensity Challenges”