Vacuum System Failures at HERA

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Presentation transcript:

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) NEG pump heating circuit – shortcut to ground! vacuum interlock electronics failure (dangerous!) collimator damage by beam (hard to diagnose)

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)

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!

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

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!

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

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

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

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

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)

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

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

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

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)