craig.cahillane@LIGO.ORG - posted 16:00, Wednesday 04 June 2025 - last comment - 17:39, Wednesday 04 June 2025(84796)
IM4 and MC2 TRANS now
Looking at IM4 and MC2 trans, we see that the amount of power on MC2_TRANS and IM4_TRANS has decreased by around 1%. The plot shows a time in March 12, 2025, with white dashed lines where those values are now on June 04, 2025. The IM4_TRANS calibration was fixed after the HAM1 work in alog 82260. The PRG is around 3% higher now, going from 52.2 to 53.9. The PRG calculation already incorporates this apparent 1% fall in IM4_TRANS. Matt Todd checked the PRG at 2W in alog 84668. As far as alignment, MC2_TRANS is servoed to zero by the input PZT using IMC ASC DOF3, so that is "unchanged". IM4_TRANS has drifted only a small amount, from 0.1 to 0.2 in PITCH, and -0.04 to -0.10 in YAW. This is all loosely consistent with the small changes in input pointing we've seen in the ISS array pointing seen in alog 84728. Keita noticed that the SEG3 on the ISS QPD is totally saturated at -32768 cts due to the misalignment onto that PD, so it becomes even harder to trust the ISS QPD reference. We have not yet closed the ISS secondloop as a part of the locking sequence, so we don't know if the beam jitter coupling to DARM problem will get worse when we close this loop. I reckon there is a very high likelihood that it will. Keita had more suggestions, including 1) injecting into the IMC and checking coherence between IMC WFS and the ISS array and DARM (Mr. Todd has already started this analysis alog 84775). 2) moving the input pointing into the array in PITCH and YAW and checking the PHASE between each individual array diode, maybe via line injection, to see if there is any cancellation happening, or if some diodes are more susceptible to misalignments 3) far future: change around plugs for INNER and OUTER so that the INNER diodes are the most stable, best-aligned ones. This could improve our overall beam jitter coupling, if we think it's coupling via the intensity loop.
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Trending back to a recent 2W lock, our PRG is a bit too high, nearing 60 when Matt has recently measured it to be 55.6. I added a gain correction factor to the PRG bank of 0.93 to account for this, in FM4. This makes our current 60 W locked PRG about 50!
I checked out the individual diodes on the array to see if they have different responses to beam jitter. It seems like Keita's intuition was correct. ISS array PDs 2, 3, 5, and 7 have significantly higher intensity noise at 3.5 Hz, 2.0 Hz, 1.1 Hz, and 0.15 Hz. These frequencies correspond to some HSTS suspension resonances for YAW in particular. Most likely is we are clipping somewhere on the path to the ISS array for only a few of these diodes. There is also some improvement in the diode noise at around 330 Hz, 380 Hz, and 960 Hz. PDs 1, 4, 6, and 8 are better performing. Out of those, we still see some beam jitter, just significantly reduced. PDs 1, 4, 6, and 8 are better for both low and high frequency motion, which is consistent with clipping. Jennie Wright will post a picture of the ISS array itself, but she believes the diodes are arrayed like5 6 7 8 - - 1 2 3 4 - -when looking from the back of the array where the cables are coming out. The underlined diodes are the "bad" diodes. Seems to not be totally sensible as an overall DC YAW adjustment, which is consistent with our attempts to align onto this array. To reproduce this PSD plot, runquick_psd H1:PSL-ISS_SECONDLOOP_PD?_OUT_DQ -t1 1433112446 -d 3600 -b 0.01on a CDS machine.
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