S. Dwyer, J. Kissel

This completes the investigation of the broken beam dump found in HAM6 (see LHO aLOG 38918) -- the beam dump has been identified as the OMC REFL Beam Diverter Dump. 
This beam dump captures light *down stream* (toward the OMC) of the fast shutter.
The dump was broken in a relatively clean vertical fracture which lines up with the dump's set screw, and when reconstructed appears to show a small pock-mark from an apparent small laser blast.

While there's no way to prove why it broke, we have two main suspicions:
   - The black glass is secured to the dump's mount with metal set screws. It has been suggested that all such black glass should be secured with PEEK set screws. If not, these metal screws create un-due stress on the glass, especially if over-tightened.
   - During observation, it has become standard to leave the OMC REFL path's Beam Diverter CLOSED, i.e. blocking the path from hitting the OMC REFL QPDs and/or exiting HAM6 onto ISCT6. Thus, during some high-power lock loss in which both
      - the fast shutter protection failed, leaving the OM2 > OM3 > OMC > OMC REFL path exposed, and
      - the OMC was unlocked sending lots of OMC REFL light down the REFL path (instead of through the OMC, if it were locked).


Picture highlights and labeled drawings are attached as .jpgs, and a more complete collection of pictures are compressed as a .pdf.
Regarding the metal set screws on this dump: a survey of other similar beam dumps in the chamber indicate that *all* such dumps in HAM6 are secured with metal set screws (see 2017-10-11_HAM6_BumpDump_SetScrews.pdf).

Open question:
   - Was the beam diverter closed when the shutter failed and killed the OMC DCPDs in Aug 2016 (LHO aLOGs 28820 and 28842)?
   - If closed, did we inspect this path / dump when we went in to fix the DCPDs?
       This picture from the Corey's Resource Space collection show that the dump is at least intact then.