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AdV-DET (Commissioning)
bersanetti, ruggi, gouaty, bonnand, frolov, kasprzack - 19:43 Friday 22 September 2017 (39554) Print this report
Comment to Investigations on OMC angular noise coupling and best SDB1 angular set points (39537)
We wanted to investigate if the 270 Hz and nearby noise peaks are from the OMC beam jitter.
The first plot compares the best reference from last week (when the 270 peaks were very low) to the current typical noise floor and to the two bench misalignments (TX, TY). In one of the misaligned state (TY), the 270 peak height increased as if it was driven by the beam jitter. But we didn't observe the same increase for the other misalignments - TX or TY.
The second plot shows the response to excitations from 100 to 400 Hz. A different set of peaks was excited while the height of 270 peaks remains unaffected. This would mean that we are not only looking at OMC beam jitter coupling, but also at some other noise coupling that gets affected by the global alignment (by moving SDB1, we are also moving the beam on SDB2, which has QPDs used for common arm alignment).

Furthermore, the DARM coherence with PRCL at 270 Hz is high according to BRUCO (see third plot). This makes us suspect that the peak is actually from frequency noise coupling, and it gets modulated by the global alignment.

The 270 peak was persistently in the DARM spectrum starting from September 15. Prior to this date, there were several days when the best spectrum of the day didn't have the peak. Coincidentally, the injection lab AC status changed on September 14.

During the same shift, we tried to see if SPRB alignment had an effect on the peaks, but scanning the full SPRB_B4_112 and LSC_B4_DC ranges didn't modulated the 270 peak.
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