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Vacuum System Failures at HERA

Vacuum System Failures at HERA. M.Seidel, DESY OLAV Workshop, CERN, April 12, 2005. Categorization of Failures. Non-Interrupting Failures getter pump power supply failure cable failure; radiation damage pump address or location error (software)

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Vacuum System Failures at HERA

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  1. Vacuum System Failures at HERA M.Seidel, DESY OLAV Workshop, CERN, April 12, 2005

  2. Categorization of Failures Non-Interrupting Failures • getter pump power supply failure • cable failure; radiation damage • pump address or location error (software) • NEG pump heating circuit – shortcut to ground! • vacuum interlock electronics failure (dangerous!) • collimator damage by beam (hard to diagnose)

  3. Categorization (2) Interrupting Failures (Leaks!) • beam induced leaks by heating/thermal shock • leaks by electrical failure in feedthroughts • water-vacuum leaks at brazing joint • RF window breaks by heat load (after long term metallization) • fatigue failure at bellows (wire scanner, collimator) (not discussed here → systematic problems)

  4. Example non-interrupting failure – Proton Collimator Damage Tungsten Collimator Jaw, 40cm long melting point: 3422 °C ! p-beam: 920GeV, 100mA damage was not realized during operation; mechanism not clear Result: reduced collimation efficiency; hard to diagnose!

  5. Interrupting Failure: p-Loss induced leak in VAT sealed flange connection • specially shaped vacuum chamber; VAT sealed; located in half quad in IR • after uncontrolled beam loss (dump fired too late): • high activation 2mSv/hour • leak at seal >10-6 mbar l/sec

  6. Model for p-Loss induced leak Newton’s law: τ loss time approx. 2ms! yield strength of seal exceeded by compression force for beam energy deposition >4% @65mA stored  very well possible!

  7. Leaks from asymmetric heating of Conflat flange connections • leak occurs when heat load at flange is switched off suddenly • laboratory tests show this can happen for ΔT=40°C !! • hard SR penetrates chamber or absorber and heats flange from outside typical signature lab-experiments asymetric heating

  8. Collapse of steel chamber from SR heating in 2004 keyhole chamber that collapsed, right side of IR schematic layout HERA IR upstream absorber - gap in side-channel opened up for background reasons

  9. collapsed chamber • chamber collapsed from heating by mis-steered SR fan • Temperature more than 1000°C • orbit feedback failed inside view

  10. New interlock – flat T-sensors • between chamber and magnet poles: PT 100 film sensors; thickness 0.4mm • 5 sensors each side; • distance 30cm • already 3 beam dumps by this interlock in 2005/04

  11. High Current Electrical Feed-Throughs • Problem: NEG heating circuits or Ti sublimation pump filaments are operated at relatively high currents – 30-50Amps • high electrical power can be dissipated at shortcuts or transitional resistance (connectors)

  12. NEG Pump Feed-Through • internal shortcut produced an arc resulting in a macroscopic leak • happened during activation!!!

  13. Water-Vacuum Leak in Brazing Connection • tuning cylinder RF cavity; internally water cooled tuner and cavity

  14. Another Tuning Cylinder... same type of device: massive cracks developed theory: bubble of air reduced cooling at top of cylinder

  15. Summary • most serious trouble happened with heating by particle beam or SR → build your system passively safe if possible; any orbit that circulates will occur! • high current electrical feed-throughs are problematic; especially with a large number of components • power supplies and electronics fail most often; however, this does not interrupt operation (mostly)

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