J. Oberling, R. Short, J. Driggers

Summary

I'll get a more detailed alog in tomorrow, right now I am exhausted so a summary it is.

We swaped our in-service PMC, SN 007, with our spare, SN 004, to correct what appeared to be a very slow build up of loss in the PMC (time scale of several months).  The swap itself went well with no issues (some very slight yaw alignment required to lock the new PMC), but we ran into a problem upon PMC recovery.  The new PMC was presenting the exact same issue as the old PMC, in that we were measuring a high visibility but a markedly lower power throughput.  We could find no cause for this: control loop looked fine, PMC optics were clean (had 1 optic that had to be cleaned pre-install, but all looked great afterwards), our locked voltage on the locking PD was well above the PD's dark voltage.  After much investigation and deliberation, we decided to continue recovering the new PMC so we could get the IFO back up and running.  We carefully upped the power incident on the PMC in stages until we had reached full power (measured with our water cooled power meter at 128.1 W).  At this point we were measuring a visibility of 89.6% and a power throughput of only 82% (105.1 W transmitted with the ISS OFF).  Our locking PD locked voltage is roughly -0.061 V and its unlocked voltage is roughly -0.453 V; the dark voltage on this PD is -0.015 V.  The UGF is ~1.1 kHz with a phase margin of ~53.9 deg.

We then found we could not lock the FSS RefCav, and upon inspecting that path found the alignment to be off.  So the PMC's mounting scheme did not exactly maintain the post-PMC alignment.  Already being almost 3 hours past the end of maintenance we did a quick recovery of the FSS while Jenne and Sheila worked to recover the IMC and the beam alignment on ISCT1 (since the post-PMC alignment shifted slightly this changed the beam alignment into the IMC and on ISCT1, which are both many meters downstream of the PMC).  The FSS will have to be tuned up next week, as we were only able to get a RefCav TPD of just over 0.7 V.  We forgot to check the ALS/SQZ pickoff before leaving the enclosure, but found it was reading too low once we were out; as a quick fix Sheila lowered the thresholds for locking both ALS and SQZ, and I will go back into the enclosure in the morning to correct the beam alignment into the pickoff.

There is still optimization to do on this new PMC, but it's up and running for now.  We will watch the PMC Refl signal over the next week to see if it exhibits the same behavior; if it does then the problem likely lies elsewhere, we'll see what happens over the next week.  Regardless, we will have to go into the enclosure next week to do some PMC optimization, re-tune the FSS path, and see if we can figure out what the root issue here is.  More to come (including a more detailed alog of today's work).