We tried to solve the lock acquisition issues by tuning the CH powers.

The nominal starting values are listed below.

I manually unlocked the ITF at 13:29 UTC and set it to CITF.

The steps taken are summarized below:

1. At 13:47 UTC in CITF, the NI CH power was decreased by 10%:
    Initial value: 0.648 W
    New value: 0.648 × (1 – 0.10) = 0.583 W
    → Negative impact on sidebands and fringe; two lock acquisitions failed

2. At 14:48 UTC in CITF, the NI CH power was increased by 20% with respect to Step 1:
    →  ITF unlocked twice

3. The WI CH has been increased by 10 mW (48 mW → 58 mW)
    →  ITF unlock

4. The WI CH has been increased by further  10 mW  (58 mW → 68 mW)      

           → ITF unlock

Despite the various changes, the interferometer still unlocks in the same way.

In agreement with the control room, I kept the values from the last step.

A comparison of B1p, B4_DC, B4_12 MHz, and B4_112 MHz using the initial CH power values and the updated ones — corresponding to step 4 for WI and step 2 for NI — is shown in the attached figure.

All quantities increased coherently by approximately 7%.

Due to WI issue an unlock occurred, the top stage guardian opened.

Inspecting the data (fig.1), we noticed data corrupted on the corrections signals.

Once we reached CARM NULL 1f, we noticed that the DCP monitor was measuring 40Hz. We knew that the usual values are between 380 and 400Hz, and even if this probe is not used until LN3, at that stage it is used to "misalign" the SR, in particular we misalign it until the DCP reaches 175Hz. However, with the DCP monitor pointing 40Hz, the SR alignment loop wasn't working properly.

So we took a measurement of the OLTF of DARM while in CARM NULL 1f (09.13.38 UTC, 120s) and we adjusted the calibration factor accordingly in the ini file (from 32 to 275). Then we adjusted also the calibration in LN2 based on the same measurement (since there are no changes in the optical configuration in between), going from 148 to 1468. Finally we also compensated the calibration in LN3, and we made a last measurement to make sure that the misalinged position was indeed around 175Hz as requested (10.28.41 UTC, 120s). The range seemed recovered so we let the interferometer in this configuration.

Yesterday we modified the notch fitler to face the reintroduction of noise at the WE violin modes frequencies. In order to improve the response we relaxed the Q ratio of the pole/sero structures. (fig.1,2)

The results was to loose an amount of phase at the DARM HF line frequency (491 Hz), see fig.3, and at the UGF of DARM (fig.4).

The results didn't affect the performance of the loop itself, but as a drowback this morning we had to retune the monitor of the DCP, which is based the loop for the alignment of SR.

After the realignment of the West Arm we tried the lock acquisition. Besides some issue with PSTAB, we unlocked due to the WE violin modes during the transition to LN2, fig.1. 

We looked at the notches in the control filter and we noticed some miscentering of the notches wrt the excited modes. The DARM correction was substaining the noise at those frequencies so we sligthly modified the notch filter in order to limit the modes noise reintroduction (see Fig.2 for comparison)

Most likely the modes will naturally damp with their own tau (6 hours ca).

We leave the ITF in CARM null 1f, with the PSTAB loop open.

I was called by Ettore to check some data after the opening of the WI top stage controls of tonight (loops properly closed by him). While I was checking the data I started to look also at the failed locking attempts of tonight. There was one attempt which died at ACQUIRE LN2 due to the BStx PRtx 1.2 Hz oscillations.

At the next lock, while the BS was closing in FBW the oscillation arised again so (since the ITF was in troubleshooting) I manually put in drift control before a too high divergence of the oscillation. After a while, reaching LN3, I restored the FBW control. Now we are in nominal configuration. I didin't modify anything in the automation.

An attempt to produce another polynomial reconstruction of the sensitivity has been done. The first problem to face is the presence of many glitches, reducing the length of the available good segments of data. We used a trick which seems acceptable: from a long period of continuous lock in the same condition, a list of segments has been selected, putting  a threshold on BNS range and considering only the segments long enough. On those data, an amplitude spectral density has been computed, making the average of the averages.

This operation has been done for the data acquired on Jul 4, starting from 16:40 UTC, for 70000 seconds. The BNS range was high thanks to the offset on SR_TX. Eleven segments with BNS range > 55.8 Mpc has been selected for a total time of 2156 s, obtaining an averaged range of 56.5 Mpc.

The floor of the spectrum has been reconstructed by a quadratic sum of simple contributions

• 1/f^4      3.2e-23 @ 35 Hz   (control noise)
• 1/f^3      0.29e-23 @ 60 Hz (control noise) a small adjustment of the low frequency part, dominated by the previous term
• 1/f^2.5   30e-23 @ 10 Hz (pendulum thermal noise)
• 1/f^0.5   0.55e-23 @ 100 Hz (coating thermal noise)
• 1/CavPOLE   0.55e-23 @ 0 Hz (readout noise)
• 1/(CavPOLE*f^0.5)    0.6e-23 @ 100 Hz (mystery noise);   the amplitude regards the 1/f^0.5 part, before the cavity pole.

The list of contribution is a bit arbitrary and doesn't want to be a real noise budget: the only check that it could be a good model comes from the quality of the final reconstruction, but other models could be equally good or better.

The final reconstruction is the output of a semi-automatic iterative process, which returns the 6 amplitudes shown in the list. It is a search of a minimum error, but not on 6 free parameters: the two regarding the thermal noise are fixed, coincident with an expected value. A check on those two values has been done, observing that the quality of the fit starts to degradate if too different vaues are choosen. A full automatic search was unsatisfactory, because too dipendent on the starting values.

A note on the mystery noise slope: putting it as a parameter of the search, the output was nomally closer to -0.5 than -0.66. Using it as a fixed number, the results were normally more accurate with -0.5, than -0.66. A fixed number -0.5 has been considered the best choice: using slopes as parameters gives not well converging results and seems not a good way to proceed, if the aim of the study is the evaluation of contribution amplitude in different conditions. As a personal opinion, -0.5 slope is preferable also in case of equivalent fitting accuracy, because it has more probability of being associated to a fundamental process.

The optimal values of the parametric amplitudes have been obtained after a double iteration. The first has been applied to all the dots in the interval 15-2000 Hz. Using the first result, several dots has been excluded: all the ones too far from the fit. The aim of this selection was to exclude peaks, bumps and lines from the curve and keep only the floor. A second search has been done on the selected data.

Fig 1 shows the reconstruction for 56.5 Mpc curve.

Fig 2 shows shows the accordance between the selected data and the fit. The yellow dots are residual values for all the data, re-including the dots excluded after the first iteration.

Fig 3 shows a similar analysis performed on data acquired with SR_TX aligned, starting from Jul 3 20:05, 45000 s. THe threshold on BNS has been put at 53.5 Mpc. The averaged range is 54.2 Mpc. The new amplitude of the mystery noise is 0.68e-23; the one with the adjustment of SR_TX was 0.60e-23. The output was slightly different also for other two components: the 1/f^3 and the readout noise. For both, the amplitude is about 6 % lower than the previous case. At high frequency the variation could be explained by an increase of the optical gain; the variation at low frequency could be not significant because regards a small component of uncertain origin.

Fig 4: the same analysis has been applied to data in LN2, SR_TY aligned and mystery noise dominant in the data (Jul 3 15:45). Also for those data, the slope -0.5 seems to me preferable, even if the quality of the fit is less good (see also fig 5).

Fig 6: the recontruction done for the curve at 56.5 Mpc is used to show how a curve without mystery noise would be. The BNS range would reach 69 Mpc and there would be still a lot of noise to remove, before reaching the supposed limit of fundamental noise for this configuration, at 89 Mpc.

Yesterday morning, when the seismic condition started to be quite haevy, we noticed a very unstable dark fringe (fig 1). The fluctuation was entirely due to DIFFp_TX (fig 2), the spectrum of which was dominated by a large peak at 0.75 Hz (fig 3). The sensitivity of pitch to longitudinal motion is a well known problem: the peak at 0.75 Hz is the second resonance of the payload and its coupling to the angular motion is not strange, but yesterday it looked a bit too strong and it would be better to understand is everything is working properly at the level of suspension control.

So far, we didn't find anything broken, but we found that DIFFp_TX control filter has some margin of improvement concerning the gain at 0.75 Hz. We developed and implemented an improved control. We tested it this morning, during the first lock acquisitions after the maintenance. We have data in similar seismic condition (fig 4), confirming what expected from the model: there is a certain reduction of angular motion and a dark fringe a bit more stable (fig 5, fig 6). We will continue to keep the new control under observation during the next data taking.

This morning the interferometer was unlocking systematically in CARM NULL 3F. It seemed that the DIFFp TY loop was starting to oscillate as soon as we increased the gain (while still in drift control). We have commented this gain increase and the lock went well. In any case once the full bandwidth is increased the gain is adjusted automatically so this has no further impact on the DIFFp UGF.

While studying this we noticed that the flags that indicate the Full Bandwidth engagement were stucked at 2 since yesterday at noon. But this could not be real because while the flag has a value 2 the local controls are disengaged and we would have noticed. Diego noticed that the bridge between the DSPs and the DAQ that collects the data from the Gnames was not working. He restarted the SusDAQBridge process, as it was hanging again as a few weeks ago.

Some recent unlocks of the ITF in LN3 look like Fig.1, most recently this morning at UTC 07:25:16.

The cause seems to be very low coherence of the DIFFp_TY UGF monitor line, which disables the servo acting on the loop gain: eventually the loop enters its high frequency instability regime and starts oscillating at ~4 Hz, saturating B1 PDs. Shortly after, the ITF unlocks.

I have tested a substantial increase in the monitor line's amplitude from 0.75e-3 to 1.8e-3. The test happened online, the automation has not been changed. The coherence of the line rised to ~0.7 and the servo started operating again.

Fig.2, Fig.3 show the effects on DARM and on B1 PDs, I cannot spot any significant negative consequence on them of this adjustment.

If this is confirmed, the new line amplitude can be introduced in the automation and it will help prevent a few unlocks in the future.

we post the angular noise budget computed after a set of measurements performed this morning.

working conditions of the ITF: decentered position of the beam on the WE (12mm offset on vertical).

DIFFp TX (blue curve) shows higher coupling.

Today we investigated the issue experienced yesterday with the ID of the WE.

We performed a measument in order to recompute the calibration for the accelerometeres. We found out that ACC_H1 was the one miscalibrated, but the correct compensator was the one used before the last update. One possibility is that the accelerometer switches between two states. The reason of this behaviour is not clear. Some investigation is needed.

In the meantime, we reloaded the older compensation filter, and we modified the DSP card adding one switch for the two compensators in order to have the possibility to move between one 'state' and the other.

Today we started the recovery of the interferometer by working on the West Arm.

We could see from the beginning two (one and its reflection?) white spots on the WE camera, which we could not remember (see Figure, WARM unlocked but flashing).

We looked for flashes on both B8 and, after misaligning NI, B4, without finding any, even with the "snail". We then checked the old position of WE MAR LCs and they were quite different with respect to the current ones. We recovered those and the snail could find flashes quite soon, first on the camera markers signals, then on the photodiodes.

We could easily relock the West Arm (no change in demodulation phase or anything), but the locks did not last much, and we could see a very big oscillation on the MAR correction at about 80 mHz. Fabio pointed out that the Inertial Damping of the WE was not working as expected, probably since quite a while.

Valerio had a look at the ID loop, but in the end we kept it open (Position Control) and somebody will look at it tomorrow morning.

With the West Arm locked we checked the drift control on SWEB and slowly discharged its memory; also, we re-engaged the BPC loop for the West ALS. Then also the ALS could be locked without issue.

We also did the initial tuning of FmodErr, which was quite off, without problems.

At this point the automation was put back together so that later in the evening we could do some tests of the full lock acquisition. The reverted changes are the same ones of last time, with a few minor differences:

• SQZ_MAIN is still standalone, due to the ongoing activity on squeezing;
• the automatic 6 mm setpoint for WE_Y in CARM_NULL_1F has been disabled;
• the minor changes done to ALS_NARM for the ongoing OptChar measurements have been reverted, and they will be put back again next week when we'll continue such work;
• the bypass of the DOWN condition for the Etalon loops has been removed from NARM_LOCK and put back in ITF_LOCK, since now it is in charge again.

yesterday, due to the test I was doing during the shift, I disangaged the automatic BStx fbw transition in the ITF_LOCK metatron node. Once I finished the shift, I forgot to mention that the node should have been reloaded after one unlock. Indeed after the unlock of yesterday night, today the ITF was running with BStx in drift control.

I engaged the fbw by hand at around 14.48 UTC (adjusting mode for 1 minute).

Please at the next unlock reload ITF_LOCK metatron node.

today we performed some test for BS blending strategy in order to find one proper combination of parameters (blending filters and gain of the low frequency brench), to see if the interaction with PR tx loop could be reduced on the basis of what we understood in the past months.

None of the actions seemed to have a big impact on the interaction at 1.2 Hz. the most significant results are reported in fig.1 and fig.2 where the error signal spectra of PRtx and the correction spectra of BStx are shown respectively.

the legend speaks for itself:

• standard blending, higher LF gain (1 instead of 0.67). this didn't cause any improvement or major modification, besides an expected worsening of the correction (blue);
• the blending with higher crossing frequency (higher order with poles at 3.22 Hz), with higher LF gain, where the 1.2 Hz got damped, but an instability at around 2 Hz started to arise (red), together with a major worsening of the correction spectra. The situation was recovered when the gain was restored to its nominal value (yellow).
• the major effect on the loop was obtained switching to the older blending (first order, poles 1 Hz), where the 1.2 Hz oscillation improved a bit (purple), together with other region of the spectra. On the other side, the 400 mHz oscillation got excited. Indeed in the first place we moved to the 2Hz blending in order to get rid of this instability. Changing the gain didn't help as well but caused only the increase of the 1.2Hz.

in conclusion, working only on the blending didn't help to solve the problem.  the only knob left is to work on the control filters of the two loops.

we left the itf locked in LN3 with the standard working configuration of BStx.

fig.1 LSC budget march 8th vs DARM;

fig.2 LSC budget march 8th vs Hrec;

fig.3 LSC budget march 10th vs DARM;

fig.4 LSC budget march 10th vs Hrec;

clean data of march 8th were characterized by a noise bump up to 25 Hz.

march 10th measurements were more significative. CARM slow shows poor coherence wrt to hrec.

yesterday we increased the LF gain only of about a factor 1.3, which allowed to damp a bit the 1.2 Hz  interaction with PR.

from recent lines injections on PR and BS driving we noticed that the optimal ratio between the calibration weigths of the two BS branches (B1p and B4) might be wrong of a factor 3.

we could try to increase up to this value the gain of the LF branch to see if we can improve better the fbw response of the loop and reduce further the interaction.

It is also true that yesterday we were in the middle of a thermal transient, and this could have impacted a lot the optical gain of the 50 MHz signal (which depends on both the 6 and 56MHz optical gains), but anyhow this gain change is worth to be tried.

Profiting of a long lock (gps start 1424152564 for almost 36 hours) I ve compared the camera centering signals with the dithering signals and the B7 and B8, for the north and west signals respectively, QPD DC h/v error signals.

The TF and the coherence are shown in the attached plots, highligthing a good coherence below ~0.1Hz (apart from the WIx).

This study will be repeated once the signal from the cameras will be obtained with the new subregion (which detect only the beam).

An example of ASD between camera image signal and dithering signal for NE Y and WI Y is shown in figure 9, it is visible that the noise floor in much lower in the case of camera signal.

it is worth to be noted how the arm cavity are controlled

this morning we found the ITF locked with BStx in fbw, BSty in drift control. The BNS range was above 53 Mpc. We didn't want to affect the science data taking phase, so we left the ITF like this and we just uncommentend in ITF_LOCK.py the engagement of BS AA loop.

at the next unlock please reload ITF_LOCK metatron node.

looking at the injection of yesterday we could fine tune the gain of the loop by increasing it of about a factor 1.2 in order to have the right UGF and the right gain at LF. 

the engagement went fine.

fig.1: time domain response comparison of older filter VS new filter.

fig.2: spectra comparison. As expected, the filter gains more at the region of 2 and 3 Hz. In that region, also the phase margin has been a little been improved. There is a predicted worsening in some LF region, but the overall accuracy has not been touched.

we leave this control on line by default. we will evaluate the response also in better weather conditions, with less wind and sea activity.

Today I carried on the tests for the BS_TX loop using EDB_B1s_QD1_50MHz_Y_I that started a while back (entry 66104).

Giving a quick look at the spectra of the in-loop signal and the new one, back when the last test was done and today, no big difference can be spotted (Figures 1 and 2 respectively).

I first used the line at 3.1 Hz to verify the last measurement: line amplitude 2e-3 since 15:53:00 UTC onwards, while in LOW_NOISE_3. After finding the weight (with ASC_BS_TX_INPUT, not ASC_BS_TX otherwise the loop gain will cause a miscalibration) I attempted the handoff, failing twice (LOW_NOISE_3 and _2 respectively). The measurement is also not obvious given that the DIFFp_TX line nearby (3.3 Hz) muddles the reading.

A good attempt was done injecting the line (amplitude 5e-3) from 17:42:12 UTC to 17:56:40 UTC, while in LOW_NOISE_2; I found a ratio between the weighted in-loop ASC_BS_TX_INPUT and B1s_QD1_50MHz_Y_I equal to 0.015 (Figure 3).

At 18:01:33 UTC I made a successful handoff, while in LOW_NOISE_2, from the blending B1p_QD1_50MHz_V_I_LP/B4_QD1_6MHz_V_HP to B1s_QD1_50MHz_Y_I. Clean data from 18:01:40 UTC to 18:13:15 UTC.

At 18:13:17 UTC I injected again the same line, with the same amplitude; the line was turned off at 18:23:30 UTC, clean data afterwards.

In Figure 4 the in-loop signals with the usual one (in purple) and the B1s one (in blue).

At 19:01:53 UTC I moved on towards LOW_NOISE_3 with this configuration; clean data in LOW_NOISE_3 starting from 19:11:59 UTC.

I leave the interferometer in this configuration, which is not macroscopically detrimental to the sensitivity (Figure 5); at the next unlock the lock acquisition will bring back the usual loop configuration.

Today we had an unlock, at 12:52:13 UTC, as SRCL oscillated at ~2.25 Hz (Figure) causing then the saturation of B1 and, eventually, the unlock.

During the afternoon shift we put online a new filter, SRCL_ctrl5c, which should address this problem by removing some unnecessary structures and gaining more at low frequency. This is intended to be used instead of SRCL_ctrl5b in LOW_NOISE_3.

We haven't tested it yet not to mix things up with the ongoing shift, but it will be tested soon.

injections of last Feb 10th.

Fig.1-2: projections of SFFS I and Q sig to DARM and Hrec respectively.

Fig.3-4: TFs during the injections of SSFS vs DARM and Hrec respectively.

No major changes wrt to the previous set of measurements.