Enhancing Collimation Efficiency at LHC: Impedance Reduction Strategies

Impedance Reduction for (LHC) Collimators Alessio Mereghetti, on behalf for the LHC Collimation Team and the LHC Impedance Team A.Mereghetti, MCBI 2019

Outline A.Mereghetti, MCBI 2019 • Introduction • Contribution to Impedance from Current System • The HL-LHC Challenge: • Present baseline and expected performance • Benchmark measurements • Other options • Conclusions

Collimation Systems Superconducting machines Accidental failures Radiation containment Radiation containment Superconducting machines Reduction of background Diagnostics A.Mereghetti, MCBI 2019 • The collimation system is fundamental for the safe operation of a superconducting collider for high energy physics like the LHC: • Halo-cleaning vs quench limits; • Passive machine protection; • Concentration of losses / activation in controlled areas; • Ease of maintenance by avoiding many distributed high-radiation areas; • Reduction of total doses to accelerator equipment; • Provide local protection to equipment exposed to high doses (e.g. warm magnets); • Cleaning of physics debris; • Avoid magnet quenches close to the high-luminosity experiments; • Optimize background in the experiments; • Minimize impact of halo losses on quality of experimental data; • Beam tail / halo scraping as beam diagnostics; • Control and probe the transverse or longitudinal shape of the beam; …while keeping impedance at a sustainable level!

The LHC Collimation System The LHC is equipped with a sophisticated collimation system for a safe and clean operation: • IR3: off-momentum cleaning system; • E.g. cleaning of uncaptured beam at the beginning of the energy ramp; • IR7: betatroncleaning system; • E.g. machine protection against transverse instabilities; • IR2/IR8: injection protection devices; • E.g. local protection against injection kicker mis-firing; • IR6: extraction protection devices; • E.g. local protection against extraction kicker mis-firing; • Experimental IRs: • local protection of superconducting magnets against transverse losses (final focusing system) or collision debris; • Reduction of background to experimental apparati; A.Mereghetti, MCBI 2019

The LHC Collimation System (II) A.Mereghetti, MCBI 2019 • LHC collimators are made of two jaws kept parallel and centered around the circulating beam; • LHC collimation system organized in families: • Every family absorbs the unavoidable leakage from the upstream one; • Retractions / operational margins between families are essential to the optimal performance of the system;

Contribution of LHC Collimators to Impedance * @6.5TeV, eN=3.5mm Im() (2018, 6.5TeV) Re() (2018, 6.5TeV) • Pushing the LHC performance implies reducing b* at the high-luminosity experiments  machine aperture is reduced; • Collimators must be tightened to keep protected the machine aperture  settings must be carefully verified in order not to run into troubles with collimator impedance and beam stability; Courtesy of D. Amorim A.Mereghetti, MCBI 2019 • At top energy, the LHC collimators are the main contributors to the LHC impedance budget; • The largest contribution comes from IR7 primary and secondary collimators: • Jaws made of resistive material, Carbon-Fiber Composite (CFC); • Jaw opening among the smallest in the ring!

A Careful Approach to Operation • Careful approach starting with rather open collimator settings for the LHC Run 1 (2010-2013) and beginning of Run 2 (2015-2018); • Successive tightening of collimator settings based on simulations and beam studies / MDs, looking at both cleaning and beam stability; • Impedance of the present system ~ as expected, but in some cases up to a factor 1.5 discrepancy measurement vs model; • Discrepancy still under investigation + continuous effort in improving the LHC impedance model (see D. Amorim); Courtesy of R.Bruce A.Mereghetti, MCBI 2019

A Careful Approach to Operation (II) • The LHC currently operates with x2 more Landau octupole current than predicted by simulations (based on LHC impedance model); …thanks to a better control of the machine year after year; • All the possible interplays between the different phenomena leading to instability need to be analyzed in detail, e.g.: • Transverse damper to be included in beam stability analyses (also with beam-beam); • Landau Octupole with beam-beam effects (both long-range and head-on); • Destabilizing effect of linear coupling, transverse damper, noise… • A factor 2 margin between octupole current required by known instability sources and that operationally needed seems to be achievable also in future LHC configurations; • In particular, predicted octupole current should not exceed half of the maximum available; Improving understanding and control of LHC beam stability parameters Measured Predicted Prediction x2 Courtesy of X. Buffat Q’=~15; transverse damper gain: 50-100 turn damping A fraction of the factor 2 discrepancy on octupole current comes from accuracy of impedance model (factor 1.5 of previous slide), but there is still something missing… A.Mereghetti, MCBI 2019

The HL-LHC Challenge Increased beam brightness More than a factor 2 peak lumi compared to LHC A.Mereghetti, MCBI 2019 • The HL-LHC is an upgrade of the LHC aimed at achieving instantaneous luminosities a factor of five larger than the LHC nominal value; • the experiments would be able to enlarge their data sample by one order of magnitude compared with the LHC baseline program; • Essential parameters will include pushed b* (15cm vs 55cm), normalized emittance, and beam intensity (esp. bunch population); • In the context of the HL-LHC project, the collimation system will be upgraded to lower its impedance footprint;

The IR7 Impedance Upgrade Nominal resistivity values of reference materials (source: IW2D) A.Mereghetti, MCBI 2019 • Backbone of HL-LHC impedance upgrade of IR7: change jaw material of collimator families impacting impedance the most; • Rich R&D program to identify suitable materials and collimator design, i.e. not only fulfilling impedance requests but also granting adequate beam cleaning and robustness against failures; • Changing CFC with materials with lower resistivity: • IR7 secondary collimators: Mo-coated MoGr; • IR7 hor&ver primary collimators: MoGr (thanks to consolidation project); • Staged implementation of the upgrade: • 4 TCSPMs + 2 TCPPMs (consolidation project) per beam installed in LS2 (2019-2020); • 5 TCSPMs per beam installed in LS3 (2023-2024);

The TCSPM Design Design of new primary collimators identical in key design choices, but no coating foreseen A.Mereghetti, MCBI 2019 • TCSPM design back-bone of IR7 impedance upgrade: • MoGr jaw – robust carbon-based material with electrical resistivity lower by 5 than CFC; • Mo-coating – metallic layer to further reduce collimator impedance; • In-jaw BPMs, for precise jaw alignment and monitoring of beam closed orbit (+interlocking?); • Tank BPM for monitoring the beam closed orbit on the non-cleaning plane; • 5th axis functionality; • Jaw allows to embark blocks of different materials, allowing to be used for different families; • Robustness of MoGr tested in HiRadMat against worst impact conditions on IR7 TGSGs (i.e. mis-kicking of a full HL-LHC batch injection into LHC); • Limited scratching of Mo surface and small enough to be compensated with 5th axis functionality;

Expectations for HL-LHC • The present LHC collimation system would not allow to keep the required octupole current below the max with the HL-LHC bright beams; • This include the factor 2 discrepancy between predictions and requirements for stable operation; • Full HL-LHC impedance upgrade of IR7 is fundamental to meet requirements; • Partial upgrade (foreseen for LS2) will provide more than half of the impedance reduction already in Run 3 (2020-2023), allowing to: • swallow the progressively brighter beams available in the LHC injectors; • Get acquainted with the new hardware; Courtesy of S. Antipov A.Mereghetti, MCBI 2019

Expectations for HL-LHC (II) HL-LHC collimators will anyway remain the dominant contributors to the machine impedance at flat top; H plane, 7 TeV, Ultimate HL-LHC, BCMS beams, d=100 turn-1, Q’=10, eN=1.7mm; Courtesy of D. Amorim Courtesy of S. Antipov A.Mereghetti, MCBI 2019

Impedance Measurements with HL-LHC Hardware • The finalisation of the TCSPM design required to verify with beam the beneficial effects of the material choice; • In early 2017, a prototype of TCSPM was installed for tests with beam: • Smallest beam s among the secondary collimators  ideal for impedance measurements; • Presence of a regular TCS in CFC in same slot  possibility to perform direct comparisons; • Three stripes of different materials, to assess effect of coating on impedance; A.Mereghetti, MCBI 2019

Impedance Measurements with HL-LHC Hardware (II) 30th Jun – 1st Jul 2017 TCSPM full gap / mm Tune TCSG full gap / mm Mo MoGr TiN Courtesy of D.Amorim A.Mereghetti, MCBI 2019 • Measurements carried out cycling the collimator gap and monitoring the tune signal; • Tune measurements obtained kicking the whole bunch and monitoring the damped oscillations; TCSPM stripe position

Impedance Measurements with HL-LHC Hardware (III) • Challenging tune-shift collimator measurements, with sensitivities at ~10-5; • Measurements in good agreement with predictions, apart from the case of Mo, where measurements are constantly x2 the expectations; 1.9 1011 p/bunch Courtesy of S.Antipov Courtesy of S.Antipov Mo resistivity from beam measurements seemed to be a factor ~5 higher than expected (i.e. ~250 n.m instead of ~50 n.m); A.Mereghetti, MCBI 2019

The Importance of the Microstructure of the Materials Courtesy of J. Guardia Courtesy of A.M. Hoffer Surface roughness of substrate affect Mo-coating because: • For a given amount of deposited Mo, a larger coated surface implies a lower thickness; • Column structure of Mo coating: smaller size of columns in case of lower thicknesses, increasing number of transitions crossed by electrons; Current supplier of Mo-coating attains values of conductivity close to nominal one; Effects of radiation damage on resistivity under investigation (C.Accettura); A.Mereghetti, MCBI 2019 Possible origin of discrepancy on Mo can be found in: Surface roughness of Mo coating; Surface roughness of MoGr substrate;

Other Options • Other options under study (non HL-LHC baseline): • New optics in IR7, optimizing b-functions at collimators to have larger gaps; • Asymmetric collimator settings: one jaw is kept at working point, the other one is set at a larger gap or even fully retracted; • Alternative collimation schemes may also be beneficial on impedance, e.g. the deployment of crystals or hollow electron lenses; Options targeting an impedance gain Options targeting beam cleaning; …impedance gain is a by-product! A.Mereghetti, MCBI 2019 • Estimates for HL-LHC take into account a factor 2 on octupole current – will it still be there in the HL-LHC era? • Origin of discrepancy is still not clear; • Scaling of effects of noise on stability of HL-LHC beams? • Plans to reduce noise from transverse damper and possibly power converters?

New IR7 Optics • Reduction of integrated losses of several tens of % wrt nominal LHC  gain on peak losses up to a factor 3! • Gain in octupole threshold up to ~25% (DELPHI); • Aperture at injection a concern  optics must be introduced during the energy ramp; Study not in the scope of HL-LHC A.Mereghetti, MCBI 2019 • New IR7 optics studied by R.Bruce and N.Mounet for obtaining larger b-functions, especially at primary collimators: • Larger collimator gaps imply lower impact on beam impedance; • Larger b-functions at primary collimators imply larger changes in normalized amplitude of protons scattered out;

Asymmetric Collimator Settings D. Kodjaandreev, A.Mereghetti B2, FT, HL-LHC v1p2, 7TeV, b*=48cm; Im(Z) DRY C2+NNNN C2+MHB2 Re(Z) Beam impacts at C2 single-sided primary collimators Gain in octupole current of some tens of A (estimation to be refined) B2H cleaning inefficiencies A.Mereghetti, MCBI 2019 • LHC collimators are two-jaws collimators with the beam passing at the middle; • Halo cleaning of the circulating beam is a multi-turn process; • In a multi-turn logics, the same cleaning effect from a single two-sided device can be achieved with a single-sided device; • IR7 collimators run as single-sided devices can degrade the cleaning performance (losses in the IR7 DS are essentially single pass), but can give an improvement in impedance; • Is there an optimum configuration of IR7 collimators with a limited loss in cleaning performance and a sizeable gain in impedance?

Electron Lens -Assisted Collimation • A hollow electron lens can be used to drive on purpose beam tails on collimation system; • Method for active halo control, i.e. deplete beam tails at specific moments during the LHC cycle; • It can be effective in case of: • Mitigate fast failures of crab cavities; • scraping of overpopulated tails that in case of beam jitterswould trigger unnecessary beam dumps; • Controlled scraping could be deployed also to tighten collimator settings during physics, following the decay of beam intensity during luminosity levelling; • Operation mode not studied yet… HL-LHC context Gain in impedance would come as by-product from the possibility of tightening the IR7 collimation system during the cycle, following the b* levelling! Courtesy of D. Mirarchi A.Mereghetti, MCBI 2019

Crystal-Assisted Collimation • Alternative cleaning method targeting improved ioncleaning; • Bent crystal is used to channel beam onto absorber; • Particles trapped in potential well between crystal planes; • Potentially better cleaning efficiency; • Lower probability of nuclear inelastic interactions of ions in crystals (once channeled) than in primary collimators; • Caveats: • Not considered for proton operation, but tests in Run 2 with low intensity beams were successful; • No reliable implementation for the absorber  IR7 layout should be re-thought; • Some secondary collimators and absorbers should anyway stay in for machine protection and phase space coverage; Courtesy of D. Mirarchi Gain in impedance comes as by-product from reduced number of collimators required! …impact of crystals on impedance still a big unknown A.Mereghetti, MCBI 2019

Conclusions A.Mereghetti, MCBI 2019 • The present LHC collimation system substantially contributes to the total LHC impedance budget; • Without upgrading the system, the brighter HL-LHC beams could not be stabilized with enough margin on octupole current; • In the present LHC, known sources of instability account for only half of the octupole current required to operationally stabilize the beam; • On-going effort (by impedance team, big thanks!) in improving numerical models and understanding the interplay between destabilizing processes; • Current baseline of the impedance upgrade of the IR7 collimation system is solid; • Based on a new design of primary and secondary collimators, where the jaw material will be exchanged for a low-impedance one, brining the expected octupole current required to stabilize the beam within acceptable values; • Secondary collimators will be coated, to further reduce effect on impedance; • Nevertheless, other options are under study, possibly bringing not only a gain in impedance, but also in cleaning performance;

Thanks for your attention A.Mereghetti, MCBI 2019

Back-up Slides A.Mereghetti, MCBI 2019

Crystal Collimation – Expected Performance Courtesy of D. Mirarchi A.Mereghetti, MCBI 2019

Tune-Shift Measurements - Procedure A.Mereghetti, MCBI 2019

Enhancing Collimation Efficiency at LHC: Impedance Reduction Strategies