Details of vent of October 2016
Overview
After the vent of the summer of 2016, we did not check the centering of the AS beam on all of the OMs steering this beam out of the vacuum. After pumping down, it was realized that the AS beam was getting clipped. The first hint was the appearance of the AS spot on the control room monitors. Subsequently, we checked that the clipping was indeed in vacuum and not on some in-air steering optics on the AS table.
As a last ditch effort to see if we could mitigate the issue without venting, we tried applying a DC voltage to the PZT mounted OMs to see if that fixed the clipping. Turns out that either the clipping was upstream of the first PZT mounted OM (OM3 in the BS/PRM chamber) or it did not have sufficient actuation range. Either way, we decided that a vent was required.
Major objectives of this vent:
- Center the AS beam on all the in-vacuum OMs
- Identify the source of the many green ghost beams that are visible on the PSL table
Vent details
Only the ITMY/SRM, BS/PRM, and OMC chambers were opened for this vent. The rough sequence of steps followed for each of the problems is listed below.
AS beam clipping
Inspection by IR card showed that the centering on the very first OM in the ITMY/SRM chamber was poor. So it was decided to start from scratch. The steps followed are essentially those listed in the post-vent closeup checklist
- First, the centering on SRM was verified using the SOS tower slide-on iris.
- Where possible, the beam was centered on a given OM by using the preceding OM mount to steer the beam appropriately.
- For OM1, this was not possible (as the preceding mirror is the SRM). So the mirror + post assembly was moved to center the beam
- In the OMC chamber, we realized that a beam-splitter was sending some fraction of light to the OMC path, even though we are not (and have not been for a while) using the OMC. It was decided to rotate OM5 in-situ to steer 100% of the AS beam directly to OM6, by-passing the OMC completely.
While working in the vertex chamber trying to center and steer the AS beam out, we were working with the Oplev loops on. It is very easy to interrupt the Oplev optical path with the IR card while tracing the beam - so the Oplev should be disabled for such work!
Green scatter on PSL table
- This issue was narrowed down to possible sub-optimal AR coating of the window from the OMC chamber to the PSL table
- In particular, the reflection from the window was getting scattered back from some of the optic mounts in the OMC chamber
- The fix was to install a black glass plate in the OMC chamber to catch the reflection from the window. The small amount of reflected light from the black glass beam dump has been directed onto the chamber wall.
Summary of in-vacuum optical layout changes
- OM1 was moved in the ITMY/SRM chamber
- OM5 was rotated approximately 90 degrees CW (as viewed from above) in the OMC chamber
- A clean razor blade dump, which was required for the OMC beam path, was removed from the OMC chamber
- A black glass beam dump was installed in the OMC chamber
- ITMY/SRM, BS/PRM, and OMC tables were checked for levelness before close-up. Some counterbalance weights were moved in the ITMY/SRM chamber. Some new counterweights were installed on the north-east edge of the OMC table.
Links to relevant elogs
To be updated...
Pictures from this vent have been uploaded to the 40m Google Photos page.
Optical layout has to be updated on the 40m CAD diagram...
This vent was completed in 7 days