Using the low frequency line injections from this morning, we can estimate if the OMC alignment at the time was good. Note that at the time of the test, not all OMC QPD offsets were on.
The first attached plot shows a time series of the OMC QPD signals, and BLRMS in a few bands, computed on OMC-DCPD_SUM. The most interesting is the 410-411 Hz band, which track the amplitude of a DARM calibration line, the one used to compute the optical gain KAPPA_C. We see a clear large modulation of the optical gain with the OMC alignment, and the zero position for all QPD is not the one giving the highest KAPPA_C.
The lines were injected at slighlty different frequencies to map as well as possible the entire 4d space of OMC QPD A and B PIT and YAW. The second plot shows the BLRMS at the DARM calibration line as a function of each QPD vavlue, so four slices in this 4d space. The third plot shows two-dimensional slices, but this is somehow harder to interpreter.
Looking at the second plot, it looks like the optical gain would be improved by moving the QPD values to
QPD_A_PIT 0.1
QPD_A_YAW 0.1
QPD_B_PIT 0.05
QPD_B_YAW -0.07
to maximize the optical gain. Iterative adjustments might be needed. Also, we could inject lines ona single DOF at a time to finetune the offsets
During the test the offsets were on for A and off for B. From the estimates above, we can guess what's the delta needed to move toward a higher optical gain, and compute new offsets
| Offsets during test | Delta from measurement | New suggested offsets | |
|---|---|---|---|
| QPD A PIT | -0.15 | -0.1 | -0.25 |
| QPD A YAW | 0.2 | -0.1 | 0.1 |
| QPD B PIT | 0.0 | -0.05 | -0.05 |
| QPD B YAW | 0.0 | 0.07 | 0.07 |
These new offsets are better, specifically we have achieved a 2.4% increase in kappa_c relative to the end of O4a (see Louis's alog). Great work team! The OMC suspension is not saturating, so all is well.
These QPD offsets have been SDFed on both observe and safe, see screenshots.
Comparison of DARM with new OMC alignment and previous one (this morning), with No SQZ + OM2 cold condition.
(Blank line is the reference of one of O4a - Dec 20, 2023 + No SQZ, cold OM2)
Noise went down compared to this morning's alignment in low frequency range.
Started another set of low frequency lines at 15:39 LT
