Today, we performed a few iterations of weld annealing with the goal of reducing the differential pitch offset of the PUM-to-ETM from the initial welded value of ~3mRad. After 2 iterations, we had the pitch down to ~1.6 mRad. As we were preparing to break for lunch, I noticed that the PUM prism crack (referenced in aLog 41111) seemed to have grown from what I recalled. During the break, I looked at Betsy's pictures that were taken shortly after removal from chamber which confirmed my suspicions that the crack had increased in length. We decided this warranted a discussion with several of the decision making staff (Dennis, Peter, Calum, Fred, and Garilynn). Together we concluded that it was probably fine to carry on since the crack only seemed to lengthen when exposed to heat from welding (the initial cause of the crack and the cause of this case of extension) and that this side of the PUM would not need to see any more heat for the annealing that was continuing. (Note that the copper prism shield that was designed to protect this from happening again after the first instance WAS installed, but apparently enough radiative heat was present to affect the crack anyways.) We continued on with another iteration of annealing which resulted in a final differential pitch of ~350 µRad. This concludes the final post-O2 monolithic welding session. Check.
Attached are the pictures of the crack from today along with the 2013 and extraction PDF of pictures for comparison.
Note, we did discuss various options such as adding or wicking epoxy or silicate bonding solution into the crack. However, looking closer at the various viscosity properties of each and a previous attempt at this at LHO, we decided it was likely not to help the situation. We also are not convinced this will be a problem for us. We will however mobilize to prepare a spare PUM for the next time around.
Final alignment numbers for the ETMx fiber weld. All measurements were done with the ETMx suspended; the PUM and UIM were both locked. All numbers assume the reader is looking in the -X direction (i.e. at the AR surface of the ETMx); this is opposite the notation used in the alignment notebook, which is done from the perspective of the alignment equipment (i.e. looking at the HR surface of the ETMx). All measurements (except the pre-welding roll) were done after yesterday's correction of the differential pitch; before the correction the differential pitch was 3.13 mrad down. In addition, I have also included before and after welding numbers for the roll of both the PUM and ETMx, as there was a change in roll somewhere during the welding process that needs to be documented.
We're not entirely sure why the roll changed during the welding process, especially as much as it did. The going theory is that when the ETMx was hung it pulled on the PUM, causing the observed roll issue. Maybe a small mechanical misalignment between the PUM and ETMx (my guess is possibly a horizontal position offset between the 2 masses; this is something we do not measure with the IAS alignment equipment, so I have no numbers to back this up) caused a shift of the ETMx relative to the PUM during the hang which pulled on the PUM and caused the roll change (this is just a guess on my part). The differential pitch correction work did not have an effect on the roll of either mass, as the roll measured after the correction was identical to that measured before the correction. I don't recall seeing an issue like this with any of our previous welds at LHO, either during this post-O2 vent work or during aLIGO install. It should also be noted that this roll error is a contributor to the difference in measured fiber stretch between the left and right side of the monolithic.
The serial numbers of the fibers used and their location in the monolithic are as follows:
I'd like to thank Chris Sioke for his help in lugging the alignment equipment to the end station Monday morning, and Stephen Appert for his help in getting everything set up. You both made things go quicker than if I were doing it all myself.