Today I tuned up the FSS RefCav, as the TPD had been steadily falling over the last couple weeks. I started with a power budget of the FSS path (leaving the ISS ON and diffracting ~4%):
Clearly the single pass diffraction efficiency is well below its normal value of over 70%, so I proceeded to adjust the AOM alignment to improve it. Afterwards I adjusted the alignment of mirror M21 to improve the double pass diffraction efficiency. I then had to adjust the FSS EOM alignment to match the beam, it was almost clipping on the output aperture. The final power budget after all alignment tweaks (FSS In and AOM In were unchanged):
Try as I might, I could not find an AOM alignment with a higher diffraction efficiency (it usually hangs out in the mid-70% range). It might be time to start considering replacing the FSS AOM with a spare, as the peak single pass diffraction efficiency we've been able to achieve has been slowly falling over time.
Next I recovered the RefCav. Some alignment was needed using the RefCav input iris as a reference, and I realigned the RefCav Refl CCD camera to re-center the image. The RefCav locked without issue and I tweaked the alignment using the picomotor-controlled mirrors; the max TPD I could get while in the enclosure was 0.788 V, well below the maximums we've seen in the past. Next I aligned the RefCav Refl PD (since the beam alignment had shifted) and measured the RefCav visibility:
Visibility is also a little lower than usual, indicating there's some mode matching work to do (or maybe another indication it's time to change the FSS AOM?). Not much I could do about in the short amount of time left in the maintenance window, so I moved on. With the lower than normal TPD I checked the FSS TF. The UGF is currently ~320 kHz, which is a lower than the ~377 kHz we had at the end of the November NPRO swap. I left the FSS gains as they are (Common Gain of 14 dB, Fast Gain of 5 dB), but we could increase the Common Gain if we find we need more gain to compensate for the lower RefCav TPD. This complete the on-table RefCav alignment tweak.
By the time I got to the Control Room the TPD had already fallen slightly, likely due slight alignment changes as the enclosure returned to thermal equilibrium (this is normal). I tweaked the beam alignment from the Control Room and was able to get it back to ~0.775 V (this with the IMC locked). This completes LHO WP 12305.
Also, seeing the PSL cooling system flows not all the way back at their pre-chiller swap levels, I very slightly increased the chiller flow rate (by decreasing the amount of internal bypass); it was oscillating between 2.7 and 2.8 lpm, and is now a solid 2.8 lpm (as read at the chiller front panel). The flow rate for the Amplifiers returned to its usual 2.2 lpm, but the flow for the power meters and the return line are still slightly down from where they used to sit, by 0.1 lpm or less. We will monitor this and see how this effects the system (if at all, it's a very slight change).