Impedance Reduction for (LHC) Collimators

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

Impedance Reduction for (LHC) Collimators