J. Oberling, P. King, E. Merilh
Summary
We realigned the beam into the PMC and adjusted the position of the mode matching lenses to improved the power transmitted by the PMC. At the then end we had PMC TRANS = 23.3 W, PMC REFL = 1.9 W, and a visibility of 91.1%.
We then tested the TTFSS box that Peter had modified. We were able to get the FSS to lock with the modded box, but when we looked at the TF we noticed some ugliness between 200 kHz and 300 kHz. Peter has pictures of the TF and the data so he can post those. The UGF of the modified box was around 220 kHz, this with the common gain maxed at 30 dB. It was decided that to continue to work on the modded box, so we reinstalled the original TTFSS box and restored the common and fast gains to where they were before we swapped boxes.
Details
The PMC before we started working:
The FSS and ISS were turned off, PMC TRANS changed to 23.0 W. We first adjusted the beam alignment into the PMC using mirrors M06 and M07.
As can be seen the majority of the gain was in yaw. At this point we measured the RPD voltage in both the locked and unlocked state and calculated the visibility
Then the mode matching lenses L02 and L03 were tweaked in position to see if we could gain anything. Once this was done we had to tweak the alignment again using mirrors M06 and M07 and calculated the visibility.
We then tested the modified TTFFSS box. With the modified box the FSS was able to successfully lock. We then measured the TF of the FSS servo and noticed that between 200 kHz and 300 kHz the TF was fairly noisy. The UGF was at ~220 kHz with the FSS common gain slider maxed at 30 dB, but the noise caused the TF to cross unity a couple different times, with a final crossing around 280 kHz. Peter has the full TF data with pictures that he will post. We decided to pull the modified box out and reinstall the original. The common and fast gains were returned to their original values (20.5 dB and 22.2 dB respectively).
Finally we relocked the FSS and ISS. Upon ISS relock, the diffracted power went up to ~45%; the refsignal had to be changed to -2.56 to bring the diffracted power down to ~6%. While this is unusually high, the ISS seemed happy so we left it as is and will monitor it. The final PMC powers after FSS and ISS relock are:
ISS Follow-up
Just got a visit from Sheila, the ISS was oscillating. I found it bouncing between 10% and 0% diffracted power. What was happening was the ISS would decrease the diffracted power in response to the low refsignal (-2.56 V), and get too low and break the loop. This then puts the diffracted power at 10%, which is where it sits when the ISS is not locked. The autolocker would then re-engage the loop, and move the diffracted power down in response to the low refsignal of -2.56 V. Lather rinse and repeat. Solution was simple: raise the refsignal until the loop stabilizes. In this case I ended up setting the refsignal to -2.08 V. This gave a diffracted power of ~7% and it appears happy there for the moment.
Not sure why the ISS decided that a refsignal of -2.56 V was needed for ~6% diff power after our PMC work, then decided a refsignal of -2.08 V was needed for ~7% diff power a few hours later. This isn't the first time this has happened with the ISS. If anyone has any insight, let me know.