Dan Brown, Cao, Kevin K., Elenna

I made a plot comparing the high frequency DARM spectrum around 10 kHz comparing our current 60W configuration to the configuration we had the last time we were at 60W, specifically April 6. These plots were both made well into each lock, so the thermal transients had settled. I believe they are an apples-to-apples comparison of the high frequency region for these two locks and IFO configurations. Three features stand out as different between the two spectra: the 2nd order HOM peaks are not in the same location, they are not the same height, and the overall DARM spectrum is lower in the blue trace (April), than the red trace (now). This indicates that the arm modes are not as well matched now as they were in April, and the frequency noise noise now is higher by a factor of 1.7 than it was in April.

If nothing else had changed in the IFO, we should have been able to successfully revert to the blue trace by powering down and reverting all other settings.

I think the reason we haven't been able to revert to the previous 60W state is related to the mysterious changes that occurred on and after May 19 when the HVAC caused many issues. Whatever changed in May caused us to ring up a PI, increased the jitter noise, and changed our mode matching. My hypothesis is that the alignment shifted in some way. Unfortunately none of the suspension, oplev or ISC signals indicate an alignment shift.

I asked Cao to take a look at the ITMX hartmann wavefront sensor. Cao made the following two plots: one showing the center of the beam position in x (yaw) and y (pitch), and the other showing a heat map of ITMX. The two times they compare are the two locks I compare in my DARM plot: the blue "old" trace from April 6, 60W, and the orange "new" trace showing now at 60W. The beam position has changed, and the heat map shows that we are heating the ITMX point absorber more than we did previously.

If the spot has moved closer to a point absorber, this would explain our increase in jitter. If the spots on the test masses are shifting, this could also affect PIs like the 80 kHz that may be spot dependent. A spot position change could also aggravate the point absorber to where the radius of curvature and substrate lensing would change enough that our mode matching solution via the ring heaters would no longer be sufficient. We only have evidence that the spot has changed on ITMX, but there is a chance that it could have changed on the ETMs as well, where we know the 80 kHz acoustic mode is located. Unfortunately, we don't seem to have any good signals that could track this on the ETMs, so we are flying blind.

Overall, one of our goals (especially in powering down) is to get the IFO mode matched well enough that we can reduce frequency noise and be able to achieve a good level of squeezing. I think we should consider moving the spots on the test masses, especially ITMX. Those who remember, you might be thinking "we already tried this and saw no effect!" However, we have a different IFO now, and I think it is worthwhile to try moving further away from our point absorbers.