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Protection Against Accidental Beam Losses at the LHC

Protection Against Accidental Beam Losses at the LHC. J. Wenninger CERN AB Department / Operations Group. On behalf of the LHC Machine Protection Working Group. Fast beam losses at the LHC Powering failures Redundant interlocking strategy Summary. The LHC challenge.

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Protection Against Accidental Beam Losses at the LHC

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  1. Protection Against Accidental Beam Losses at the LHC J. Wenninger CERN AB Department / Operations Group On behalf of the LHC Machine Protection Working Group • Fast beam losses at the LHC • Powering failures • Redundant interlocking strategy • Summary PAC 2005 / J. Wenninger - CERN

  2. The LHC challenge • A factor 2 in magnetic field • A factor 7 in beam energy • A factor 200 in stored energy The LHC beam dump block is the only element that can withstand the impact of a full beam without damage. PAC 2005 / J. Wenninger - CERN

  3. Time constants for beam losses time Very slow beam losses (lifetime 0.2 hours or more) Cleaning system to limit beam losses around the ring min … hours Very fast beam losses (some turns to some milliseconds) Fast beam losses (5 ms – several seconds) Slow beam losses (several seconds – 0.2 hours) At all times collimators limit the aperture – particles lost on collimators Hardware surveillance and beam monitoring, failure detection and beam extraction onto the beam dump block ms … sec accidental beam losses Ultra fast beam losses • Single turn failures at injection • Single turn failures at extraction • Single turn failures with stored beams Hardware surveillance and passive protection with beam absorbers s

  4. Powering failures The Magnet powering system will account for a considerable fraction of beam dump requests due to magnet quenches, power converter failures, mains failures, etc.. The most critical failures concern circuits with normal conducting magnets, for example the separation dipoles in the high luminosity interaction regions. Such powering failures lead to • Global orbit drifts, at a speed of up to  1 mm/ms in some locations. Orbit drifts maybe detected anywhere around the ring. • Beam losses when the beams start touching the aperture(s). Since the LHC must always be operated with aperture-defining collimators, losses will almost always appear in the vicinity of the collimators. PAC 2005 / J. Wenninger - CERN

  5. One of the fastest mechanism for multi-turn beam losses Failure of a NC separation dipole (D1) [pessimistic time constants, 7 TeV] Damage to collimators possible after ~ 30 turns / 3 ms Fraction of protons touching collimator Assumes Gaussian beam profiles 1.5 ms damage level ~1012 protons detection ~109 protons [turns] orbit [m] 0.7 mm orbit shift PAC 2005 / J. Wenninger - CERN [turns] V.Kain

  6. Beam loss monitoring • So far the Beam Loss Monitoring (BLM) System was the main (and only) beam monitoring system for detection of fast powering failures : • The LHC BLM system : • Approximately 3600 monitors • 6 BLMs on each quadrupole • 1 BLM near each of the ~ 240 collimator jaws • Response times : • 1 turn (89 ms) for critical locations (collimators, low-beta,..) • 2.5 ms at other locations In case of a powering failure with global perturbations, • BLMs at aperture limiting collimators see the loss first. Critical condition: the collimators must really define the aperture ! • BLM reaction time depends on the shape of the halo and thresholds Halo is sensitive to machine details (non-linearity, beam-beam…) Operational thresholds are critical for quench and damage prevention PAC 2005 / J. Wenninger - CERN

  7. Quench protection Diode becomes conductive > 80ms Heaters efficient >> Current in dipole circuit Voltage > threshold Quench alarm issued Energy extraction Beam loss 1-2 ms 6..9 ms 10-200 ms 10 ms Quench develops in the magnet Energy extraction switch opening & arc extinction Completion of beam dump • For slow losses / quenches :  the beam will be dumped before the field is affected ! • For very fast quenches as observed at the Tevatron (few ms) : • the field could change before the beam dump is activated by quench protection. • we must rely on BLM system. We will re-evaluate the quench speed and mechanism in the LHC dipoles for ‘massive’ beam loss to take into account Tevatron experience. PAC 2005 / J. Wenninger - CERN

  8. Magnet current decay monitoring • A device that could detect very fast current changes - ~ 0.1% / millisecond - in electrical circuits with short time constants would provide efficient protection against powering failures. • Tests were performed at CERN using Hall probes (fixed to bus-bar) and simple voltage detection devices – but no operational device was built. • In parallel an operational device was developed at DESY for HERA. This device was tested at CERN in March/April 2005. The tests were so successful that we intend to take over the DESY device. • We foresee to install 30 devices in the LHC and in transfer lines to the LHC. • Such a system provides efficient protection against failures of the most critical electrical circuits. PAC 2005 / J. Wenninger - CERN

  9. Current decay detection tests • Measurement of current changes. • Based onmeasurements of the magnet voltagerather than a DCCT readout. • RT estimate of the current changebased on a model of the circuit impedance. Achievable thresholds are as low as few 0.01% / ms, depending on PC ripple. Magnet Voltage Example of measurements during a shut down of a NC separation dipole (D1) Hall-Probe RT current estimate  used for interlocking DCCT Readout 4 ms Courtesy M. Werner, DESY M. Zerlauth, CERN PAC 2005 / J. Wenninger - CERN

  10. Beam position interlocking 4 interlock BPMs will be installed per beam/ring : • Grouped in 2 redundant BPM pairs. • Phase advance between BPM pairs is 90° to cover all betatron phases. • Large betatron function of 600 m (LHC ARC ~ 200 m)  sensitivity. • Detection of fast changes with respect to closed orbit : • Thresholds of ~ 1 mm in 1-50 ms. • Reaction time ~ 1 turn – configurable. This system will provide independent interlocking wrt BLMs : • No dependence on collimator positions or halo distributions. • The reaction times are adequate to protect collimators against beam loss caused by fast powering failures. PAC 2005 / J. Wenninger - CERN

  11. Fast beam current monitoring Estimated damage levels at the LHC (fast beam loss) : 450 GeV ~ 2 × 1012 protons (metal) 7 TeV ~ 1010 protons (metal), ~ 1012 protons (C collimators) The detection of losses with a sensitivity 1011 protons / ms would provide significant protection at all energies !! Experiment on damage limits at SPS, RPPE018 • R & D on fast current loss detection with such tolerances, based on various Beam Current Transformer design, is starting. • Aim is to install such a device sometimes after the LHC start-up. PAC 2005 / J. Wenninger - CERN

  12. Conclusion • Initially • Protection of the LHC against beam losses was relying mostly on the beam loss monitoring system. • The quench protection system provided a second line of defense. • We are now planning to add : • A device to detect fast magnet current changes in critical electrical circuits. • A fast beam position interlock system.  Both system provide redundancy for the BLM system. • Work on a BCT for the detection of fast losses is starting. PAC 2005 / J. Wenninger - CERN

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