Danny, Dan
9MHz RIN line over locks
We put a line at 70.123Hz in the 9MHz for a few locks to see how it was behaving over time. When it was one we had a few short locks and two longer ones so far. It seems there are two states in which the coupling finds itself, one that peaks around "8" and another "6". This may coincide with going to LOWNOISE_ESD_ETMX (Guardian state 514) - the jump up in RIN coupling for red/green/orange seems to happen at roughly the same time after we go up to state 514. During the two long locks it's clear there's some long time constant to the RIN coupling, it takes around 4000s after going to 30W for this to settle. So depending on the thermal state we took the RIN measurements previously this might be a reason we got confused.
ITMY Mask
Last night we tried out the ITMY mask. The initial plan was to just apply the mask with the IFO unlocked to see how the induced OPD compared to what we expected. Then as we had the IFO to ourselves we decided to just go for it and try it out in a full lock to see what happened. Edit: this test was just to get the CO2 mask on at the expected power without causing a lockloss, the expected implementation requires changing ring heaters as well which is something to try another day.
Initially the OPD doesn't look quite like we expected. What isn't obvious is the crescent moon like shape seen in alog 46976 (Top right figure). This could be because the HWS probe beam isn't illuminating the full area so we just see a small section of it. I took the 30W point absorber OPD and added it to the offline mask OPD to get a rough idea of what might be the total effect, from this it reduces the overall optical depth and the larger spatial frequency heating from the absorbers.
From initial inspection the alignment of the minimum is not too far off from the point absorbers, we might want to try shifting the mask slightly in future. However it looked reasonably well aligned enough to try in a full lock.
We powered up to the 30W state but didn't go to low noise ASC. We then put the mask in and stepped up in CO2Y power 100mW, 200mw, 400mW in 1000s intervals. We compare this with the 30W lock we had yesterday with the 9MHz RIN line on where we also didn't go to a low noise state. Looking at the RIN coupling we can see an improvement as the mask is introduced, then when we switched it off it goes back to as it was before. I injected a line that was 10x larger than the previous day by accident so the amplitude is rescaled and the line is less noisy. In hindsight we should have left it on a bit longer to see how it affected the steady state after 4000s, however we wanted the thermal state to return to normal for the night shift commissioning. From the data it looks as if the RIN coupling levels off between 3000-4000s. It looks to be at roughly the same level as the steady state case without the mask. Perhaps there is another dominating coupling effect at that stage where the mask no longer helps.
RF90/RF18/PRG/HWS traces with and without the mask.
Comparing with and without the mask we can see:
- RF18 looks slightly higher, RF90 slightly lower
- PRG maybe a little lower
- ITMY spherical power drops 155uD to 148uD
Things to try next:
- Alignment of the mask. The mask OPD needs ~-0.5cm in x to align it better with the point absorbers. Assuming the CO2 telescope images the mask with a magnification of 23 on to the compensation plate, that means we need to move the mask ~200um
- Could try adjusting CO2X or RH Y to change differential lensing and see if we see similar drops in RIN coupling, RF18, etc. Perhaps we just need to tune the current TCS actuators better.
- Try mask after 4000s and at nominal low noise to see if it actually improves the sensitivity and drops below the steady state RIN we see
Adding a plot of power levels of the two locks with mask and without. PRG and arm power remain lower with CO2 ITMY mask on, but POP18 is higher.
The other night we put the mask on once the IFO had thermalized alog 47097. The effect of the mask looked to be leveling off. However when applying the mask at a later date we also saw an improvement in the coupling.
Here is the 9 MHz RIN line amplitude at the start of the lock, when we switched the mask on, and when we switched it off. Overall saw ~30% reduction in coupling.
Attached is a breakdown of the mask test with a thermalised IFO at 30W. We switched on the mask for two hours. Plotted is the OPD changes between several points:
- OPD change during power up before any mask is applied
- OPD change with the mask switched on
- The change in the OPD due to the mask (Difference between 1 and 2)
- The steady state OPD of the mask after 2 hours
What's confusing us is that we now see an OPD change that is different to when we applied the mask separately. i.e. case 4 does not look like this.
The magnitude of the optical depth change is completely different too. Case 4 has an OPD change of 40nm, whereas applying the mask out of lock gave us ~140nm. I can't think of why this would be the case, perhaps the mask induces a change in the beam which introduces a different OPD, so some non-linear effect is in play. If so, it will be difficult to predict what mask shape to actually use.
It does however have a crescent like shape similar to aidans model Aidan's model (top right image here), although that may be a coincidence.
The reference for the "140nm measurement" of the CO2Y mask thermal lens was not taken at a cold state but rather with 0.85W of CENTRAL heating on. This yields a strong positive lens. If this positive lens is taken as a reference (or zero) point and then central heating is turned off, we will see a strong negative lens in the measurement.
Probably best to repeat the calibration of the CO2Y mask from a genuine cold state.
So the "140nm OPD measurement" is looking at the difference between a reference state of "0.85W central + 0.0W custom mask" and "0W central + 0.45W custom mask".
Alexei, Dan
Pulled the data from OMC_DCPD_SUM_OUT_DQ corresponding to the injections of frequency and intensity noise lines as outlined in alog 47097.
The first black line is when the CO2 was switched on the second black line is when it got switched off. Looks like the mask increases intensity noise coupling but doesn't do much of anything to the frequency noise.
The ringing towards the end is likely some instability as there is a lock loss about 10 minutes after the data ends.
