Jennie W, Sheila
Sheila wanted me to look at how good our optical gain is doing since the burn on the OFI that roughly happened (we think) on the 22nd April.
Before this happened we made a measurement of the OMC alignment using dithers on the OMC ASC degrees of freedom. We got a set of new alignment offsets for the OMC QPDs that would have increased our optical gain but did not implement these at the time.
After the OFI burn we remeasured these alignment dithers and found a similar set of offsets that would imrpove our optical gain. Thus I have assumed that we would have achieved this optical gain increase before the OFI burn if we had implemented the offset changes then.
Below is the sequence of events then a table stating our actual or inferred optical gain and the date on which it was measured.
Optical gain before vent as tracked by kappa C: 2024/01/10 22:36:26 UTC is 1.0117 +/- 0.0039
Optical gain after vent: 2024/04/14 03:54:38 UTC is 1.0158 +/- 0.0028, optical gain if we had improved OMC alignment = 1.0158 + 0.0189 = 1.0347
SR3 yaw position and SR2 yaw and pitch positions were changed on the 24th April ( starting 17:18:15 UTC time) to gain some of our throughput back.
The OMC QPD offsets were changed on 1st May (18:26:26 UTC time) to improve our optical gain - this improved kappa c by 0.0189.
Optical gain after spot on OFI moved due to OFI damage: 2024-05-23 06:35:25 UTC 1.0051 +/- 0.0035
SR3 pitch and yaw positions and the SR2 pitch and yaw positions were changed on 28th May (starting at 19:14:34 UTC time).
SR3 and SR2 positions moved back to pre-28th values on 30th May (20:51:03 UTC time).
So we still should be able to gain around 0.0296 ~ 3% optical gain increase, provided we stay at the spot on the OFI we had post 24th April:
SR2 Yaw slider = 2068 uradians
SR2 Pitch slider = -3 uradians
SR3 Yaw slider = 120 uradians
SR3 Pitch slider = 438 uradians
| Date | kappa_C | Optical Gain [mA/pm] | Comments |
|---|---|---|---|
| 10th January | 1.0117 | 8.46 | O4a |
| 14th April | 1.0347 | 8.66 | Post vent |
| 23rd May | 1.0051 | 8.41 | Post OFI 'burn' |
I'm not really sure why, but our optical gain is particularly good right now. And, it's still increasing even though we've been locked for 12+ hours.
The other times in this plot where the optical gain is this high is about April 5th (well before the OFI incident), and May 30th.
Actually, this *might* be related to the AS72 offsets we've got in the guardian now. Next time we're commissioning, we should re-measure the SR2 and SRM spot positions.
Jennie W, Sheila, Louis
I recalculated the optical gain for pre-vent as I had mixed up the time in PDT with the UTC time for this measurement, it was actually from on the 11th January 2024.
Also the value I was using for OMC-DCPD_SUM_OUT/LSC-DARM_IN1 in mA/counts changes over time, and the optical gain reference value in counts/m also changes between before the vent, April, and now.
Louis wrote a script that grabs the correct front-end calibration (when this is updated the kappa C reference is updated) and the measured OMC-DCPD_SUM_OUTPUT to the DARM loop error point.
Instructions for running the code can be found here.
The code calculates current optical gain = kappa C * reference optical gain * 1e-12 / (DARM_IN1 divided by OMC-DCPD_SUM)
[mA/pm] = [counts/m] * [m/pm] / [counts/mA]
All the kappa Cs in the table below and all the optical gains were calculated by Louis's script except for the 14th April.
I calculated the optical gain on the 14th April assuming as in the entry above that we would have got a (~0.0189) increase in kappa C if we had previously (before the 14th April) implemented the OMC alignment offsets we in fact implemented post OFI burn, on 1st May.
I went to these reference times and measured coupled cavity pole, f_cc and P_circ the arm cavity power.
I also checked the OMC Offset and OMC-DCPD_SUM for these times (which shouldn't change).
| Date | kappa_C | f_c | Optical Gain [mA/pm] | P_Circ X/Y | OMC Offset | OMC-DCPD_SUM | Comments |
|---|---|---|---|---|---|---|---|
|
06:36:26 UTC |
1.0122 | 441 +/- 7 | 8.33 | 368 kW +/- 0.6kW | 10.9405 +/- 0.0002 | 40mA -/+ 0.005 | O4a, time actually 11/01/2024 06:36:26 UTC |
|
03:54:54 UTC |
1.0257 | 391 +/- 7 | 8.70 | 375 +/- 0.8 kW | 10.9402 +/ - 0.0002 | 40mA -/+ 0.003mA | Post vent |
|
06:35:41 UTC |
1.0044 | 440 +/- 6 | 8.52 | 382 kW +/-0.4 kW/ 384 kW +/- 0.4 kW | 10.9378 +/- 0.00002 | 40mA +/- 0.006 mA | Post OFI 'burn' |
|
09:43:04 UTC |
1.0171 | 436 +/- 7 kW | 8.62 | 379 kW +/- 0.6 kW/ 380 kW +/- 0.5 kW | 10.9378 +/- 0.00002 | 40mA +/- 0.004 mA | Post HAM6 pressure spike |
In summary we have (8.62/8.70) *100 = 99.1% of the optical gain that we could have achieved before the OFI burn, and our current optical gain is (8.62/8.33)*100 = 103.5 % of that before the vent.
We do not appear to be doing worse in optical gain since the vacuum spikes last week.
