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Collimators: Operations - Baseline Assumptions

Collimators: Operations - Baseline Assumptions. Types of losses Beams Operational cycle & role of collimators Lifetime limits Collimator efficiency Operational constraints on beam parameters Annual losses LHC commissioning - phased approach.

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Collimators: Operations - Baseline Assumptions

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  1. Collimators:Operations - Baseline Assumptions • Types of losses • Beams • Operational cycle & role of collimators • Lifetime limits • Collimator efficiency • Operational constraints on beam parameters • Annual losses • LHC commissioning - phased approach As indicated the collimators have to protect the machine and experiments while we’re spraying beam around at all stages of operations LHC collimator review

  2. Types of loss • Abnormal (Fast & Ultra fast loss) • Equipment malfunction etc. • Short lifetimes • Operator error • Beam instabilities • Parameter control challenges (persistent currents etc.) • Stable • Transverse • Beam gas • Nonlinearities • Long range beam-beam • Electron cloud • IBS • Collisions • Longitudinal • Touschek • RF • IBS Other: e.g. electron-capture by pair production LHC collimator review

  3. Short Lifetime/Stable conditions Required Beam Intensity Operational cycle Permitted beam loss Acceptable Lifetimes Collimator efficiency Operational tolerances Beam Instrumentation Coming later… Abnormal losses Collimator design Transfer Line collimation Other protection devices Machine Protection LHC collimator review

  4. Here to Protect • 1. Damage: • Dangers clear and well enumerated. • 2. Quenches • For example, local transient loss of 4 × 107 protons at 7 TeV Nominal beam energy → One British aircraft carrier at 11 knots One girl in a Porsche at 1600 mph LHC collimator review

  5. Beams LHC collimator review

  6. Nominal cycle LHC collimator review

  7. Injection – 450 GeV • Pilot & Intermediate beam to check & adjust beam parameters, position collimators etc. • 12 SPS batches per ring, 1 batch up to 288 bunches • Big beams, lower dynamic aperture • Protection of cold aperture in arcs • Collimators to protect during: • Injection process (injection oscillations etc.) • Accidents: kicker misfires, timing errors • Inevitable lifetime dips LHC collimator review

  8. Ramp & Squeeze • Start ramp - out of bucket flash: • ~5% total beam primarily onto the momentum collimators • Start ramp - snapback: • Tune, chromaticity, momentum, orbit, -beating. Lifetime. • Ramp: • Collimators stay (more-or-less) where they are. Beam emittance shrinks. Still protecting arc cold aperture. • Scraping at end of ramp? • Squeeze: • Aperture limit now becomes inner triplet [IR1 & 5]. Collimators need to move in before/during the squeeze to protect the insertion quadrupoles. • Tune, chromaticity, orbit, -beating. Lifetime. LHC collimator review

  9. Beam lifetimes 7 TeV - Physics The contributions for collisions have to be doubled up to get an estimate for an intensity lifetime of around 17.8 hours. NB figures preliminary • Plus: Lifetime dips, background optimisation, abort gap LHC collimator review

  10. Emittance growth rates Plus random power supply noise, ground motion, RF noise, electron cloud, nonlinearities . Small contribution to beam lifetime at 7 TeV especially given the presence of synchrotron radiation damping LHC collimator review

  11. Minimum beam lifetimes LHC collimator review

  12. Allowable Intensity in the LHC 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) The nominal intensity of 3 × 1014 protons per beam requires a collimation inefficiency of 2 × 10-5 m-1. Injection has less strict requirements. LHC collimator review

  13. Operations • Limitations on the allowed minimal collimator gap: • The beam core must not be scraped by collimation, usually requiring collimator settings above 4-5 . • The collimator gap must be wide enough to avoid excessive impedance from the collimators and to maintain beam stability. • The two-stage functionality of the collimation system must be maintained during the whole operational cycle, e.g. the primary collimators must always remain primary and the secondary must always remain secondary collimators. Usually a relative offset of 1 nominal sigma is required, corresponding to about 200 µm at 7 TeV. Operational and mechanical tolerances are specified for this offset. LHC collimator review

  14. Operations - implications • Design aperture must be established • Max. -beating ≈ 20% • Max. orbit deviation ≈ 4 mm. • Transient changes in orbit and -beating under control (tune & orbit feedback, etc.) • Max. transient -beating ≈ 8% • Max. orbit shift ≈ 0.6  • Nominal beam loss rates established • Min. beam lifetime > 0.2 hours. Dump beam otherwise The settings n1, n2 and n3 of primary, secondary and tertiary collimators must be carefully adjusted in order to minimize the leakage rates of the cleaning insertions → tight demands on beam optics and stability. To go to significant intensity therefore: LHC collimator review

  15. Annual Doses • Take: assumed operational efficiency, number of days of operation, turn around → number of fills • For a fill, estimate: • Injection oscillation losses, lifetime at 450 GeV, scale to 7 TeV • Start ramp: out of bucket flash, snapback • Lifetime in ramp • Squeeze: lifetime, lifetime dips • Physics: lifetimes (plus lifetime evolution) - halo versus luminosity etc. • Dump • Plus some lost fills LHC collimator review

  16. LHC collimator review

  17. Phased commissioning • Initial commissioning: • Ending with Pilot physics: 43 on 43 with 3 - 4 x 1010 (if we’re lucky) • Year one[+] operation: • Lower beam intensity/luminosity: • Event pileup • Electron cloud • Phase 1 collimator impedance etc. • Equipment restrictions • Relaxed squeeze, lower intensities, 75 ns. bunch spacing Initial commissioning of phase 1 Use this period to stage commissioning of collimator systems & to optimise cleaning efficiency LHC collimator review

  18. Phased commissioning LHC collimator review R. Assmann, J.B. Jeanneret, E. Metral,

  19. Conclusions • Difficult beams, potential for quenches/damage high • Operational cycle will include challenges • effective collimation essential at all stages • Reasonable limits on lifetimes assumed • Tight limits on collimator settings • Tight limits on operational beam parameters to ensure required collimator efficiency • Annual dose estimates for IR3 & IR7 • Phased commissioning foreseen Acknowledgements… LHC collimator review

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