Follow up on previous tests (72106)
First I injected noise on SR2_M1_DAMP_P and SR2_M1_DAMP_L to measure the transfer function to SRCL. The result shows that the shape is different and the ratio is not constant in frequency. Therefore we probably can't cancel the coupling of SR2_DAMP_P to SRCL by rebalancing the driving matrix. Although I haven't thought carefully if there is some loop correction I need to do for those transfer functions. I measured and plotted the DAMP_*_OUT to SRCL_OUT. transfer functions. It might still be worth trying to change the P driving matrix while monitoring a P line to minimize the coupling to SRCL.
Then I reduced the damping gains for SR2 and SR3 even further. We are now running with SR2_M1_DAMP_*_GAIN = -0.1 (was -0.5 for all but P that was -0.2 since I reduced it yesterday). Also SR3_M1_DAMP_*_GAIN = -0.2 (was -1). This has improved a lot the SRCL motion and also improved DARM RMS. It looks like it also improved the range.
Tony has accepted this new configuration in SDF.
Detailed log below for future reference.
Time with SR2 P gain at -0.2 (but before that too)
from PDT: 2023-08-10 08:52:40.466492 PDT
UTC: 2023-08-10 15:52:40.466492 UTC
GPS: 1375717978.466492
to PDT: 2023-08-10 09:00:06.986101 PDT
UTC: 2023-08-10 16:00:06.986101 UTC
GPS: 1375718424.986101
H1:SUS-SR2_M1_DAMP_P_EXC butter("BandPass",4,1,10) ampl 2
from PDT: 2023-08-10 09:07:18.701326 PDT
UTC: 2023-08-10 16:07:18.701326 UTC
GPS: 1375718856.701326
to PDT: 2023-08-10 09:10:48.310499 PDT
UTC: 2023-08-10 16:10:48.310499 UTC
GPS: 1375719066.310499
H1:SUS-SR2_M1_DAMP_L_EXC butter("BandPass",4,1,10) ampl 0.2
from PDT: 2023-08-10 09:13:48.039178 PDT
UTC: 2023-08-10 16:13:48.039178 UTC
GPS: 1375719246.039178
to PDT: 2023-08-10 09:17:08.657970 PDT
UTC: 2023-08-10 16:17:08.657970 UTC
GPS: 1375719446.657970
All SR2 damping at -0.2, all SR3 damping at -0.5
start PDT: 2023-08-10 09:31:47.701973 PDT
UTC: 2023-08-10 16:31:47.701973 UTC
GPS: 1375720325.701973
to PDT: 2023-08-10 09:37:34.801318 PDT
UTC: 2023-08-10 16:37:34.801318 UTC
GPS: 1375720672.801318
All SR2 damping at -0.2, all SR3 damping at -0.2
start PDT: 2023-08-10 09:38:42.830657 PDT
UTC: 2023-08-10 16:38:42.830657 UTC
GPS: 1375720740.830657
to PDT: 2023-08-10 09:43:58.578103 PDT
UTC: 2023-08-10 16:43:58.578103 UTC
GPS: 1375721056.578103
All SR2 damping at -0.1, all SR3 damping at -0.2
start PDT: 2023-08-10 09:45:38.009515 PDT
UTC: 2023-08-10 16:45:38.009515 UTC
GPS: 1375721156.009515
If our overall goal is to remove peaks from DARM that dominate the RMS, reducing these damping gains is not the best way to acheive that. SR2 L damping gain was reduced by a factor of 5 in this alog, and a resulting 2.8 Hz peak is now being injected into DARM from SRCL. This 2.8 Hz peak corresponds to a 2.8 Hz SR2 L resonance. There is no length control on SR2, so the only way to suppress any length motion of SR2 is via the top stage damping loops. The same can be said for SR3, whose gains were reduced by 80%. It may be that we are reducing sensor noise injected into SRCL from 3-6 Hz by reducing these gains, hence the improvement Gabriele has noticed.
Comparing a DARM spectrum before and after this change to the damping gains, you can see that the reduction in the damping gain did reduce DARM and SRCL above 3 Hz, but also created a new peak in DARM and SRCL at 2.8 Hz. I also plotted spectra of all dofs of SR2 and SR3 before and after the damping gain change showing that some suspension resonances are no longer being suppressed. All reference traces are from a lock on Aug 9 before these damping gains were reduced and the live traces are from this current lock. The final plot shows a transfer function measurement of SR2 L taken by Jeff and me in Oct 2022.
Since we fell out of lock, I took the opportunity to make SR2 and SR3 damping gain adjustments. I have split the difference on the gain reductions in Gabriele's alog. I increased all the SR2 damping gains from -0.1 to -0.2 (nominal is -0.5). I increased the SR3 damping gains from -0.2 to -0.5 (nominal is -1).
This is guardian controlled in LOWNOISE_ASC, because we need to acquire lock with higher damping gains.
Once we are back in lock, I will check the presence of the 2.8 Hz peak in DARM and determine how much different the DARM RMS is from this change.
There will be SDF diffs in observe for all SR2 and SR3 damping dofs. They can be accepted.
SR2 and SR3 damping gains changes that Elenna made have been accepted
The DARM RMS increases by about 8% with these new slightly higher gains. These gains are a factor of 2/2.5 greater than Gabriele's reduction. The 2.8 Hz peak in DARM is down by 21%.
This is a somewhat difficult determination to make, given all the nonstationary noise from 20-50 Hz, but it appears the DARM sensitivity is slightly improved from 20-40 Hz with a slightly higher SR2 gain. I randomly selected several times from the past few locks with the SR2 gains set to -0.1 and recent data from the last 24 hours where SR2 gains were set to -0.2. There is a small improvement in the data with all SR2 damping gains = -0.2 and SR3 damping gains= -0.5.
I think we need to do additional tests to determine exactly how SR2 and SR3 motion limit SRCL and DARM so we can make more targeted improvements to both. My unconfirmed conclusion from this small set of data is that while we may be able to reduce reinjected sensor noise above 3 Hz with a damping gain reduction, we will also limit DARM if there is too much motion from SR2 and SR3.
