[Evan, Kevin, Dan, Elenna]

We have been, for a very long time, operating with large offsets in the REFL WFS 45 yaw signals. This stayed the same even when we moved CHARD Y sensing to WFS 45 yaw and PRC2 off POPX onto REFL. The recent locklosses we experienced when powering up beyond 60W have all been characterized by a rapid loss of alignment in yaw and a sharp increase in the REFL signal. Moving PRC2 to POP and changing DC loop gains has not helped. We concluded the issue was that some DC coupled loop could not hold onto the yaw alignment, and that the source of the problem must be related to this massive 45 yaw misalignment.

One interesting characteristic: REFL WFS pitch signals do not see any misalignment. The pitch loops are sensed as follows:

• CHARD P uses a positive combination of all four REFL WFS I signals
• PRC2 P uses a positive combination of REFL WFS I 9 A and B
• INP1 P uses  +REFL WFS 45 I A and - REFL WFS 45 I B

I have highlighted this last bullet point because this was the large distinction. INP1 yaw uses a combination of REFL 9 signals (+A and -B). This is weird! There is also no particular reason why this should be, as INP1 yaw shows up fairly equivalently in magnitude in all four REFL WFS signals. My crazy idea was this: we should emulate the pitch sensing in the yaw, and see if we still have alignment problems.

At 2W, I tested a +45 A, -45 B combo for INP1 (1.6 and -1.6 respectively). This worked in both magnitude and phase (I tested with the usual 8 Hz injection). I returned PRC2 P to just 9 sensing; this was sensing that had been used before and tested extensively at high power. We did not change CHARD Y, which is sensed on negative 9 with positive 45, all four sensors.

We powered up to 60W successfully with this combination. After moving to lownoise ASC (RPC now off), we powered up further to 70W (no instabilities at all, evidence we can abandon RPC for power up). The REFL WFS 45 signals stayed centered around zero, and the REFL 9 signals drifted to small offsets, but still crossed zero. Even more importantly: the CHARD Y optical gain stayed constant over the first hour of lock. Long readers of the ASC alog will know this has been an issue for this particular loop stability.

This sensing has definitely brought us to new IFO alignment, but I am going to claim that this IFO alignment is better. Whether or not we choose to operate at higher power, staying with this new sensing and alignment is a must. The REFL 9 signals have drifted in a small amount away from zero. We can try to help this alignment shift further by changing the REFL 9 combination used by CHARD Y to the difference of 9s, so we can control that degree of freedom.

The first screenshot is the REFL WFS signals over this lock at 70W. I put the +/- combo of REFL 9 signals into DC7, showing that if we try to control this to zero, we will likely improve the REFL 9 drift.

Obviously, this alignment change will change the calibration. Once we determined we were stable, we took several calibration measurements to check:

• The broadband PCAL to DARM transfer function showed us we were off by 10% around 100 Hz. The blue reference in the second attachment shows the calibration before the alignment change and power up. The brown trace shows the calibration after the alignment change and power up. The red trace shows the calibration after we applied a calibration fudge to the sensing function. In the screenshot, I also included the calibration summary page, and the calibration fudge (CS DARM ERR gain changed from 1 to 1.04)
• The third screenshot is a swept sine PCAL to darm transfer function. I'm not sure why the phase is so weird. Evan says this is some DTT error.
• The fourth screenshot is a DARM OLG at lower frequency to see how the spring has changed. We are still operating with the -175 SRCL offset. The spring is slightly less than an earlier measurement today at 60W and old alignment

All these calibration traces are saved in the CalSVN.

The IFO is very stable so far at 70W. We chose to stop here as a proof of concept. We would like to see how long we can hold this particular lock. We think that the alignment change is very beneficial and might solve many of our power up problems. If this is stable, we can continue to push the power up and see if 75 or 80 W is possible.

Parameters at about 2 hrs 70W lock:

• 70 W from PSL
• 66.5 W at PRM
• PRG of 47
• Arm power of 410 kW

This is exciting!