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Machine Protection Issues for LHCb

Machine Protection Issues for LHCb. J örg Wenninger AB-OP-SPS Co-chair LHC Machine Protection Working Group. Introduction Circulating beam failures Injection and dump failures Summary. The Machine Protection challenge. A factor 2 in magnetic field A factor 7 in beam energy

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Machine Protection Issues for LHCb

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  1. Machine Protection Issues for LHCb JörgWenninger AB-OP-SPS Co-chair LHC Machine Protection Working Group • Introduction • Circulating beam failures • Injection and dump failures • Summary LHCb - J. Wenninger

  2. The Machine Protection challenge • A factor 2 in magnetic field • A factor 7 in beam energy • A factor 200 in stored energy LHCb - J. Wenninger

  3. Machine Protection Organization Since 2001 the Machine Protection Working Group (MPWG) deals with all machine protection related issues of the LHC and the SPS. All MP sub-system experts are members of the MPWG. >> For any question or worry about machine protection please ask for advice from the MPWG !! The MPWG will now be reorganized into a new ‘body’ responsible for the operational aspects of MP at the LHC. LHCb - J. Wenninger

  4. Machine Protection @ LHC • Protection of the machine will play a prime role in operation of the LHC: • 360 MJ stored energy versus ~10 mJ/cm3to quench a magnet • ~200 kJ beam may damage accelerator components (magnets) • We already quenched a dipole with a direct impact of 3.5109 p at 450 GeV… • To ensure that the LHC can be operate safely : • Detailed studies of failure scenarios were used to assess if the machine is adequately protected. This effort is still ongoing… • The LHC is equipped with an unprecedented number of active and passive protection elements. • LHC machine protection was tailored for the machine, but always keeping in mind that it must also protect the experiments ! LHCb - J. Wenninger

  5. LHC Machine Protection Systems • The LHC machine protection system is subdivided into: LHCb - J. Wenninger

  6. MP Commissioning Status • Following the various LHC schedule changes, the dedicated ~1-2 month FULL machine checkout period has vanished : • Replaced by ‘piece-wise’ checkout in parallel with Hardware Commissioning. • >> Complicates integration tests of sub-systems. • >> Complicates commissioning of some machine protection sub-systems. • Commissioning of the MP system is progressing rapidly. • >>All MP inputs required for low intensity operation will be ready for Sept. 10th. • Parts of the MP system can only be commissioned with beam: collimators, absorbers, beam instruments for MP… • >> Beam related MP commissioning will follow the overall LHC commissioning. LHCb - J. Wenninger

  7. Failure categories In the event a failureor unacceptable beam lifetime, the beammust bedumpedimmediately and safely into thebeam dump block. Two main classes for failures: • Passive protection • - Failure prevention (high reliability systems). • Intercept beam with collimators and absorber blocks. • Active protection systems have no time to react ! Beam loss over a single turn during injection, beam dump or any other fast ‘kick’. Active Protection - Failure detection (by beam and/or equipment monitoring) with fast reaction time (< 1 ms). - Fire beam dumping system Beam loss over multiple turns due to many types of failures. Fastest failures >= ~ 10 turns LHCb - J. Wenninger

  8. Circulating Beam Failures- Multiple Turns LHCb - J. Wenninger

  9. Timescales for Failures Time to ‘impact’ Operational ‘mistakes’ 10000 turns = 0.89 s 1000 turns Quenches 100 turns Normal conducting magnet powering failures 10 turns 1 turn = 89 ms Kicker magnets (injection, dump) LHCb - J. Wenninger

  10. Main active protection devices (*) Fast Magnet Current change Monitor LHCb - J. Wenninger

  11. Collimation system – passive protection • The LHC is equipped with a multi-stage halo cleaning (collimation) system to protect the magnets from beam induced quenches. • Halo particles are first scattered by the primary collimator (closest to the beam). The scattered particles (the secondary halo) are absorbed by secondary collimators, or scattered to form the tertiary halo. •  the collimators have an key role for protection as they define the aperture. •  the ‘distance to beam’ (retraction) hierarchy must always be respected. •  contrary to TEVATRON and HERA the LHC cannot be operated without collimatorsas soon as ~few permill of the nominal beam intensity is stored ! Experiment Protection devices Primary collimator Secondary collimators Tertiary collimators Triplet magnets Absorbers Tertiary halo hadronic showers Primary halo particle Secondary halo + hadronic showers Beam LHCb - J. Wenninger

  12. Timescales Time 10000 turns = 0.89 s Operational ‘mistakes’ 1000 turns 100 turns Quench protection Quenches Powering interlocks 10 turns NC magnet powering failures FMCM 1 turn = 89 ms Kicker magnets BPMs BLMs Absorbers LHCb - J. Wenninger

  13. ‘Global’ failures • Almost all perturbations due to failures (typically magnet powering, an injection or dump failure) affect the entire machine. • To first order the amplitude of the perturbation at any place in the ring is proportional to the beam size s and to a phase factor, i.e. : • Amplitude ~ scos(phase) • >>> The parameter that defines if an element is likely to be hit is not the physical distance to the beam, but the distance in units of beam size. • >>> Since at the LHC the collimators must ALWAYS define the aperture, one of the collimators is always the first elements that is hit by the beam: •  Dump triggered by BLMs at or downstream of collimator(s). LHCb - J. Wenninger

  14. IR8 physical aperture LHCb - J. Wenninger

  15. Beam aperture • The standard LHC beam aperture model includes mechanical tolerances, alignment errors, orbit tolerances, magnetic errors etc. • >> The net aperture after subtraction of all clearances is normalized by the beam size and expressed in ‘n1’ units, n1 being the primary collimator setting (in beam s) required to ensure that the aperture is beyond the beam halo of the secondary collimators. • Example : for a SC magnet where n1=10, it is possible to set the primary collimators to 10s before risking a quench of the magnet due to halo particles (assuming an properly setup collimation system !!). LHCb - J. Wenninger

  16. IR8 normalized aperture at injection • ARCs are the aperture limit (n1 ~7s). • The triplet magnets are slightly behind the ARC. • The primary collimator opening must be set ~5-6 s to protect the ‘cold’ arc aperture. • When it is retracted VELO is well protected … by the triplets ! Velo @ 30 mm Triplets IP8 Arcs LHCb - J. Wenninger

  17. Beam sizes • During the energy ramp, the beam size shrinks but the optics is not changed, and n1 scales with E. • During the betatron squeeze, the beam size decreases at the IP and increases in the triplet magnets, reducing the aperture near the IR. • >> This is due to the fact that phase space is conserved, a smaller size means more divergence and therefore a larger size in the next quadrupole (triplets). LHCb - J. Wenninger

  18. IR8 normalized aperture at 7 TeV Velo @ 5 mm, perfectly centered • At 5 mm from the beam, VELO remains behind the triplet aperture. • For b* = 2 m, the triplets define the apertures in IR8. Tertiary collimators are necessary to protect the triplet magnets (quench from halo and failures). For global perturbations of the beam parameters, VELO is in the shadow of machine elements (Arc magnets, triplets). LHCb - J. Wenninger

  19. Measured Apertures • So far the measurements show that mechanical apertures of the 2 tested sectors (23 & 78) are as designed. IR7 IR8 LHCb - J. Wenninger

  20. ‘Orbit bumps’ • Local orbit bumps are commonly used in rings for : • Crossing angles. • Separation of the beam at the IP. • Spectrometer bumps ! • Etc… • Issues with bumps: • Large localexcursions : ‘break’ global protection mechanisms. • Mostly an issue at injection: the vacuum chamber can be reached ~ everywhere. • Protection/mitigation: • Even the fastest bumps take few seconds (to minutes) to bring the beam to the aperture, which leaves time for loss monitors to react and dump the beam. • >> Fast loss detection system is also recommended for all exps. • At injection energy, current limits will be set for the orbit correctors to limit the range of bumps. The experiments vacuum chambers / VELO must be out of range of bumps. x Corrector magnet s LHCb - J. Wenninger

  21. Comments on ‘stable beams’ • A common misconceptions is to believe that during the ‘Stable beams’ phase which corresponds to experiments data taking, the beam is … stable !!! • During the ~12 hours of colliding beams, one observes : • Thermal effects on warm magnets that affect beam parameters. • Orbit drifts (~ 1s equivalent over 12 hours at LHC) due to ground motion and tides. • Beam parameters drifts due to the intensity decay. • Etc… • Changes are either corrected manually or by automated feedback (orbit drift). • The distinction between a ‘dangerous’ and a beneficial change one is usually only given by… the amplitude: • >> Operational errors can happen during stable beams, but they very slowcompared to the most severe failures. LHCb - J. Wenninger

  22. Kicker Failures LHCb - J. Wenninger

  23. Overview injection/extraction • Same principle for injection and extraction: • “kicker” magnets: fast rise time,muchless than one turn, large (~ mrad) angles • Septa magnets: two apertures with different magnetic fields • Injection: • Beam 1: IR2 • Beam 2: IR8 • Extraction: • Both beams in IR 6 • Injection and extraction lead to single turn failures !! LHCb - J. Wenninger

  24. Injection into an empty ring • Injection can be risky when a ring is empty, since one must be sure that all magnet settings are adequate to avoid immediate loss. • The LHC injection schema, implemented by hardware signals, is based on the following principle: • >> Injection logic: • - If beam is present (> ~ 2x109 p), any beam intensity may be injected. • - If no beam is detected, the maximum intensity that can be injected is • > 1010 p for regular operation – present setting. • > 1011 p in dedicated periods of machine studies (as required). • This scheme is already implemented and tested. Injection of high intensity is only allowed if some beam is already present in a ring: this ensures that conditions for injection of high intensity are sufficiently good to avoid immediate loss of the beam over the first turns. LHCb - J. Wenninger

  25. Injection layout • Beam is coming in from the TI2 (IR2) and TI8 (IR8) transfer lines (both ~ 3km). • Injection in 2 steps: • Septum magnet (MSI) provides a horizontal deflection to bring the injected beam parallel to the circulating beam. • The injection kicker magnet (MKI) provides a vertical deflection to bring the injection beam on the orbit of the circulating beam. • TDI and TCDD absorbers protect against damage from MKI failures. MKI MSI Transfer Line TI8 TDI TCDD LHCb - J. Wenninger

  26. Injection kicker failures • Injection: • TDI absorber opening ~ +-7-8 beam sigma – interlocks injection. • TDI protects against kicker misfiring (for example erratic discharges  kick the circulating beam), up to nominal intensity. • TDI is critical for multi-bunch injection/operation. • >> MKI misfiring could lead to high rates from showers in LHCb, but not to direct impact. TDI absorber MKI nominal beam axis +- (7-8) s 4 m • Ramp and collisions: • TDI is opened during the ramp. • MKI interlocking only allows operation at 450 +- 2 GeV. • >> MKI is locked off above 452 GeV (+ will be switched off). LHCb - J. Wenninger

  27. Beam Dumping System Septum magnet deflecting the extracted beam MSD Beam 1 H-V kicker for painting the beam Beam Dump Block Q5L Q4L 15 kicker magnets MKD about 700 m Q4R about 500 m Q5R Beam 2 LHCb - J. Wenninger

  28. Requirements for a ‘clean’ dump • The extraction kicker rise time • must coincide with the 3 ms long • particle free abort gap. • The abort gap must be free of • particles: measurement & cleaning • (if needed) • The kicker setting must match the beam energy (‘Energy Tracking’). Energy tracking errors are among the worst failures for the LHC, the system is therefore highly redundant. Abort gap LHCb - J. Wenninger

  29. Dump sweep: asynchronous dump • When the kicker rise does not coincide with the abort gap (‘asynchronous dump’) the bunched are swept out over all amplitudes. Protection against DAMAGE due to such events (expected ~ 1 / year): • Moveable graphite absorber TCDQ (7 m), plus normal collimator (TCS) downstream from the kicker. • Tertiary collimators protect the triplets (mainly IR5) against beam leaking out of absorbers + coll. • >>> IR8 is rather well protected, since beam 1 must pass the collimation section in IR7 before reaching IR8. LHCb - J. Wenninger

  30. Summary : failures that may affect LHCb / I • During injection large settings errors of dipoles/correctors can lead to direct beam impact in the vicinity of or into LHCb. Protection/mitigation : • Surveillance of magnet currents. • Limit on the intensity that may be injected into an empty ring. • During injection showers from beams hitting the TDI (or also the transfer line dumps) can reach LHCb – has already happened. • With circulating beamLHCb/VELO is protected from direct impact due to global failures by collimators and by the triplets, but could be affected by showers from triplets. • Local orbit bumps could bring the beam very close to LHCb/VELO at injection (most likely impact on triplets). • Bumps will be limited in amplitude by surveillance of corrector currents. LHCb - J. Wenninger

  31. Summary : failures that may affect LHCb / II • Early collisions & data taking at 450 GeV come with a number of solvable MP issues for the experiments. • Bumps must be limited in amplitude by surveillance of corrector currents. • For collisions at 450 GeV a special locking procedure for the injection kicker will have to be put in place. The TDI must remain in place (background??). • For LHCb the risk/possibility of moving VELO towards the beam for collisions at 450 GeV must be evaluated. • There is an issue for the interlocking of movable devices since it does NOT allow moving detectors towards the beam at 450 GeV. LHCb - J. Wenninger

  32. Settings errors Velo @ 30 mm • A special case of failures at injection are (large) setting errors of separation dipoles (D1/D2) or orbit correctors close an IR: • Beam may be deflected directly into the detectors. • Detailed study for LHCb by R. Appleby (TS/LEA) – will be published soon. • Cures/mitigation : • Limit intensity injected into an empty ring (see previous slides). • Current surveillance of the critical magnets (by Software Interlock System) – in place. V plane Velo @ 30 mm H plane LHCb - J. Wenninger

  33. Injection kicker setup • Injection kicker OFF, TDI closed : beam hits upper edge of TDI. • Injection kicker ON, TDI closed : beam should be ~ on nominal beam axis. TDI absorber MKI nominal beam axis 1 Min. gap = 3 mm Max. offset = +-3 mm 2 4 m LHCb - J. Wenninger

  34. Showers into LHCb • Weekend 22-24.08, sector 78 test: • After a period of few hours without beam, when the first beam came back, it was displaced vertically and hit near the TDI gap: • >> showers/beam leaked into LHCb – beam cut by LHCb. • Problem was ‘cured’ by moving the gap from +3 mm to -3 mm… • Due to control system issues we were not yet able to set the jaws with an angle to close the gap. TDI absorber MKI nominal beam axis 4 m Min. gap = 3 mm Max. offset = +-3 mm LHCb - J. Wenninger

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