These modulation are caused by the high amplitude of the DIFFp tx and ty lines used for the UGF servos. The choice of these amplitudes is due to the 'necessity' seen in the past of surviving the initial transient of CARM null in which diffp gains change a lot. Reducing of a factor 2 the amplitudes improves the fluctations on the fringe, keeping enough coherence for the servos, fig.1.

We can simply decide to reduce the amplitude in the automation once we reach the steady state (or checking if we still need such high values of the lines amplitude in the initial transient).

Figure 1 and 2 zoom on the lines at 6.6Hz and 10.4Hz. They are extremely narrow (less than 1mHz), and at a round number of decihertz, so I don't believe to be a mechanical resonance, and they must be some kind of intentional dither or modulation.

ITF found locked at CARM_NULL_1F in COMMISSIONING mode.
Activity on TCS tuning / CARM NULL locking (Bersanetti Mantovani, Corubolo, Lumaca) in progress.

It went on till 19:40 UTC.

ITF left locked in LOCKED_ARMS_IR.

Today we performed various DAS common steps in terms of variation of ITM RoC (changing the ITM RoCs of equal amounts) to increase the SB power in CARM_NULL_1F. The initial configuration of CO2 actuators is described by the following table:

08:24 UTC STEP 1 

NI DAS IN and OUT +12%

WI DAS OUT +6% (DAS IN OFF)

10:30 UTC STEP 2 

NI DAS IN and OUT +12%

WI DAS OUT +8%

Both steps led to a clear increase in SB power, whereas other signals (such as arm power or B1p) degraded linearly, seemingly unrelated to the DAS steps (see Fi.g. 1).

Following these two steps, at approximately 12:10 UTC, after more than 5 hours in CARM NULL, the ITF unlocked due to a glitch on WI. 

The subsequent lock was achieved relatively quickly. By around 12:25 UTC, the ITF was back in CARM_NULL_1F.

Another unlock occurred around 14:12 UTC due to a second glitch, this time on NI, although it was less clear than the previous one. 

We agreed to perform an automatic lock acquisition, letting the ITF realign by itself; however, this took too long (entry 69831). On the second attempt, manual realignment was performed, but the SB power in CITF were low. We then agreed to also compensate for the two morning DAS steps on the CHs.

15:13 UTC STEP 3

NI CH +17%

WI CH +25%

This step increased the SB powers in CITF and facilitated the CARM_NULL_1F acquisition (CARM_NULL_1F around 15:53 UTC).

Then, we noticed that the steps 1 and 2 didn’t induce a pure common RoC variation, as our OPL simulation was using outdated intensity maps. However, the overall RoC differential component was small compared to the RoC common one.

After implementing the updated intensity maps, we performed an additional DAS step, restoring a pure common RoC variation (around 8 m of total variation of RoC, considering all the three DAS steps). 

16:32 UTC STEP 4 

NI DAS IN and OUT +4%

WI DAS OUT +6%

This final step further increased the SB power, though by a smaller amount. 

Figure 2 and 3 illustrates the overall behavior of the signals throughout the day:

• SB power increased: 12 MHz increased from 0.03 mW to 0.041 mW; 112 MHz increased from 0.02 mW to 0.026 mW.

• Arm power decreased: following a trend not directly correlated with the DAS steps.

• Dark fringe power increased: becoming brighter in a manner not directly correlated with the DAS steps.

• U-LSB unbalance: the unbalance at the start of CARM NULL progressively grew over time (before slowly recovering balance during CARM NULL).

• DCP increased: the DCP initially followed the first two steps of DAS power, improving its value.

The origin of the overall trend (decreasing arm power and increasing dark fringe power) remains unclear.

Below, the table summarizes the final configuration of the CO2 actuators.

A consequence of working with the decentered beam in CARM Null (3mm in NI_Y direction, for now done by hand) is that most of the alignment needs to follow and re-adjust.

One smaller consequence is the fact that arm cavities and injection bench are slightly more misaligned once we unlock and try the next acquisition. This is not critical in terms of statistical success of the relock of the arms, but the dithering control at that stage may need more time than usual to converge, so we increased the timer in LOCKING_ARMS_IR from 60 s to 120 s.

One larger consequence is the PR mirror resulting misaligned in TX when the next DRMI lock acquisition happens (Figure): we made one test to leave the acquisition alone and see how much time the automatic macro needs before we can see triggers again, and this is around 13 minutes, clearly too long for an unattended lock acquistion.

We are devising some logic that acknowledges this drift due to the beam misalignment and remembers it at the next relock so that we can compensate it before we acquire the lock of the DRMI again.

B1p is currently flucutation between 200mW and 300mW with fast fluctuations (timescales much shorter than 1 second).

Figure 1. Looking at the spectrum there are 3 lines which are dominating it: at 6.6Hz, 8.5Hz and 10.4Hz. The high line at 8.5Hz makes sense, it is the NI TX line, and we are using the NI vertical drift control to mis-center the beam on the input mirrors. However for the other two I have no explanation.  The lines look quite narrow, so could be some kind of dither lines, or some resonances.

These lines are close to limiting to reach safety threshold that prevent opening the OMC slow shutter.

This morning I performed again the analysis after the shift of the input beam of the both WI and NI mirror images from the thermocamera (previous analysis can be found in #69785 and #69808, respectively). Here below are summarized the results:

WEST INPUT

Fig. 1 shows the thermocamera image of the WI. 

Fig. 2 shows the output of the matlab code showing a new PA (or at least more visible now) w.r.t the last analysis after the beam shift.

Here there is a table summarizing the positions and information of the PAs:

NORTH INPUT

Fig. 3 shows the thermocamera image of the NI. 

Fig. 4 shows the output of the MATLAB analysis, highlighting a PA that was not present in the images analyzed previously. The figure also shows a concentration of higher-temperature area near the center, with some peaks reaching approximately 0.3 °C.

Here there is a table summarizing the positions and information of the PAs:

Note: 

The thermal contrast associated with a Point Absorber is obtained from the differential temperature map between the interferometer Lockedn (in CARM_NULL) and DOWN states. Measurements performed on different days can therefore exhibit some variability, even when the same Point Absorber is analysed.

This variability can arise from several contributions. The thermal background observed on the mirror is not completely uniform and can contain spatial structures related to thermal radiation reflected by the HR surface. These structures can change with the environmental and thermal conditions of the interferometer. In addition, the absolute temperature of the mirror and its surroundings, the thermal state and stability of the infrared camera, the transmission and optical properties of the ZnSe viewport, and small changes in the camera–mirror geometry can affect the measured temperature distribution.

The differential analysis reduces the effect of global temperature variations by aligning the median temperature of the mirror in the LOCKED and DOWN states. However, it does not completely remove changes in the spatial structure of the thermal background. Furthermore, the Point Absorber temperature is currently estimated from the maximum value within a local search region, making the result sensitive to local fluctuations and residual registration errors between the two images.

Consequently, variations of the measured Point Absorber thermal contrast, for example from approximately 0.65 °C to 0.8 °C (in the WI PA 1 case) in measurements performed on different days, should be interpreted primarily as a combination of the intrinsic thermal signal and the reproducibility/uncertainty of the measurement procedure.

ITF found in COMMISSIONING Mode and CARM_NULL_1F State with Input beam mode matching activity ongoing.
All times are UTC.
16:10 - 16:46 INJ: realignment of the beam with the BPC FF quadrant (Gosselin, Pagabbe, #69825).
17:06 ITF relocked to CARM_NULL_1F at first attempt with no actions needed by the operator.
17:55 North Arm put back in the alignment position of yesterday (#69820).
17:57 INJ: End of activity on INJ to ITF mismatch measurement (#69824).
ITF kept CARM_NULL_1F up to the end of the shift. At 20:55 I manually unlocked the ITF and set it to LOCKED_ARMS_IR.
ITF left in COMMISSIONING Mode and LOCKED_ARMS_IR State.

Guard tours:
- 20:35 

This afternoon we performed a measurement of the current input beam mode mismatch toward the arms. 
Following the usual procedure of the kick of the end mirrors and using the matlab code detailed in #68646

An additional idea was to do a measurement with CH ON and OFF to see if there is any difference the difference. 

First measurement : 13:29:13 UTC CH switch off. 
Unfortunately injection unlocked when we put the ITF in down. 
Analyze could start only at 13:31:00 UTC 

Measured mismatch  for WA : 2.5 %

The data from NA where not usable, there was quite a lot of misalignment.
We realized that it was most probably due to the work done yesterday evening.  
Paolo brought back North arm to its nominal condition. 

Second meausrement : Carm_null lasted only 6 min. 

Third measurement : CH ON, ITF unlocked after 18 min of carm null but we managed to do the kicks anyway. 
Analyze started at 14:57:10 UTC over 200 s

Measured mismatch WA : 1.9 % not that clean.... maybe due to some misalignment when ITF got unlocked.
Measured mismatch NA : 2.8 %

Fourth measurement : CH ON, best measurement so far (figures) 
Analyze starts at 15:45:30 UTC over 200 s

Measured mismatch WA : 2.1 %
Measured mismatch NA : 2.8 % 

Giving the relatively low mismatch and the work to come on TCS we decided not to tune it. 

At the end of the shift (17:55 UTC), Manuel brought the NA back to the alignment found yesterday evening.
 

The error signal of the BPC TY was around +40 urad and reached +60 urad when the interferometer is in carm_null and the BPC uses the arms cavities as reference. This is due to the work of yesterday 69820, the laser beam was moved w.r.t. the ITMs.

With 60 urad we reached the limit of the linearity zone of the quadrant and were limiting the possibility to keep on shifting the beam towards the ITF. Today from 16:20 to 16:40 utc, we slightly tilted horizontally the EIB_BPC_M5 mirror in front of the FF QPD. Now the TY signal is at -20 urad. We changed the TY setpoint to -20 urad in the BPC DSP board.

The injection system relocked properly after the intervention.

To be noted the FF QPD of the BPC can not be taken as an absolute reference of the beam we sent towards the ITF. Or better, the absolute reference has been moved by -60 urad in TY.

ITF found in locked arms IR.

At 6:00 UTC ITF in Maintenance mode, below the list of the acticitied communicated in control room:

• cleaning of the experimental areas;
• check of the HYMOS (Fabozzi);
• vacuum activities;
• TCS thermo camera references (operator);

All the maintenance activities concluded at 11:00 UTC.

The ITF was relocked in CARM_NULL at 12:53 UTC and then started the planned afternoon activity of input beam mode matching.

Figure 1. Compares the last lock of yesterday to a radom lock during O4. Clearly, the power in the arms was 37% higher last year, as the input power was higher. What is interesting is that the power of the sidebands last year stops evolving after 25 minutes, while currently it has a decreasing trend for 1h30. 

The current TCS actuators (DAS, CHROCC) act only on large scale deformation, but not on small scales like point absorbers. A point absorber creates a small scale defect (the point part), and a large scale defect (the tails of the point), but the small scale defect forms on a time scale of minutes, while on the 1h30 time scale it is the tails of the point absorber and the uniform absorption that are evolving. This should mean that the changes in sideband power that happen more than 30 minute after the lock acquisition should be in the spatial scales that the current TCS actuators can act on. In other words, there should exist a tuning of these actuators that keeps the sideband magnitude at 0.035 or higher, with the same beam and mirror position as the last lock of yesterday evening.

The stability and the duration of the lock in CARM NULL 1f has been studied for different positions of the resonant beam on the ITMs. Four conditions will be analysed in this entry:

gps0=1474616480 --- standard working point. Lock acquired yesterday morning, before the beginning of the attivity. Unlock after 40 minutes, while the sidebands were low and the stability degradated.

gps1=1474622079 --- the lock has been acquired with WI shifted upwards by ~ 2mm. The same shift was tested on friday during a lock, but this time the shift was performed before the lock. During this lock, the shift was increased by 1 mm more.

gps2=1474633548 --- lock acquistion in the same condition as gps1 (WI 2mm up). During the lock, an offset has been applied to the dither signal NI_Y in loop, imposing a different alignment of the cavity optical axes. In this way, the spot on both the ITMs has been shifted downwards. The amount of this shift was much larger than the one performed by the physical displacement of WI (~ 6 mm). The effect on the sidebands was much more evident, but the carrier's power has dropped a bit, likely because of the mismatch between the input beam the the optical axes.

gps3=1474651865 --- the input beam has been shifted vertically before the lock, by a combination of PR translation and BPC-SIB1 rotation. We did only 3-4 mm, because doing more would have brought BCP sensing too close to the end of the range. The lock has been acquired in this misaligned condition, then the alignment has been recovered applying a corresponding offset to NI_Y (lower than the one operated in the previous lock). The carrier was higher than before, but not at its maximum level. The sidebands were lower than before, but the lock was stable enough, it lasted almost 2 hours and the unlock was due to a glitch of NI inverted pendulum.

In the attached figures the comparison of consecutive locks is shown: fig1 gps0 vs gps1; fig2 gps1 vs gps2; fig3 gps2 vs gps3. Carrier, sidebands and vertical position of the spot on WI (dither signal) are taken into account. The first plot in the grid shows the action performed during the lock.

The commissioning activity in CARM_NULL_1F went on through the afternoon until 19:00 UTC (Pinto, Ruggi, Spinicelli)

After ITF unlocked the last time at 19:17 UTC, I left it in LOCKED_ARMS_IR for the night

The broadband noise injection has been done also on NI MIR coils. The mechanical response of each coil (fig 1) is very similar to the one obtained for WI.

The mechanical response of WI mirror actuated by each coil has been measured at CARM NULL. A broadband noise has been injected, able to provide a measurement up to 800 Hz (fig 1). The results are quite similar and no unexpected structure is visible. The structures at 150 Hz and 200 Hz are due to notches in DARM control filter. At 491 Hz the measurement is bad because of the injected line. The conclusion is that the three magnets are correctly glued to WI mirror.

Figure 1, with the longer locks and fast relocks afterwards, it becomes apparent that the sideband gain in CITF is higher on the second relock after the mirrors where heated by the YAG. This is a relatively clear indication that the CH is delivering less power than the YAG that is absorbed by the mirrors. The 10% (0.4V) step on PR CHROCC, should be corresponding to 4m of change in effective PRC radius of curvature, and that should be also equivalent to about 4mW of YAG or CH absorption.

The green power in the west building has been increasing, while the green power in the west building has been decreasing. Both shows a day/night cycle, so probably this is due to change in the temperature in the building, changing the alignment into the infrared to green non linear crystal, or of some other parts of the green generation in the end buildings.

ITF found locked in CARM_NULL_1F in COMMISSIONING mode and morning acivity still ongoing.

planned activity:
Etalon controler change (Mantovani Bersanetti). It went on without major problems till 19:30 UTC.

ITF left locked in LOCKED_ARMS_IR.

Today we have relocked after the ALS issue has been solved.

The only parameter that has been adjusted was the gain of DARM (step 3/3 from 0.018 to 0.014)

After this the NI measurement has been performed (see TCS entry). After this work we had to reduce the gain of DIFFp ty @ Carm null from 14 to 10.

Then we have raised the WI mirror of 2mm, see Figure 1. Analysis to be done and shift to be continued on Monday.

After this the modifications on the Etalon loop have been performed. Namely the DC correction at mid dynamics and the removal of the initial slow controller. 

The behavior will be monitored this weekend.

During previous attempts to perform measurements with HWS-DET, a spurious beam was affecting the HWS reference after the unlock, making the measurement difficult to interpret, as a faint spot appeared on the HWS image (see Fig. 1).

After several attempts to identify the culprit, we tried keeping the NE aligned during the gentle unlock. This confirmed that the spurious beam was coming from the NE: in the CARM NULL 1F configuration, the beam reflected by the NE also reaches HWS-DET. By keeping the NE aligned during the gentle unlock, the problem disappeared.

The first measurement performed with this new configuration corresponds to the folder 20260925T1556. The HWS acquisition started at 13:56 UTC, after the ITF had been in CARM NULL for about 5 minutes. The ITF was then brought to gentle unlock while keeping the NE aligned.

To determine the position of the TM center in exactly the same alignment configuration, a second HWS-DET acquisition was performed after the gentle unlock, without changing the alignment of the mirrors. After approximately 30 minutes, a new reference image was acquired and the measurement was started (20260925T1635). At 14:40 UTC (WF 10), the NI CH was switched ON to identify the mirror center. The CH signature appeared approximately at the same position as in the NI single-bounce configuration (see Fig. 2).

Looking at the data acquired during the gentle unlock (20260925T1556), no clear feature is visible in the HWS maps if piston and tilt are not removed. After removing piston and tilt, however, a PA-like feature becomes visible in the lower part of the map (see Fig. 3).

In Fig. 3, the image on the left shows the PA-like feature about 5 minutes after the unlock (inside the black circle). The image on the right shows the HWS map acquired about 20 minutes later. At this time, a broader OPL variation due to the average absorption of the YAG beam is visible, overlapping with the localized PA feature.

Comparing its position with the mirror center identified in Fig. 2, the PA-like feature appears to be relatively close to the center of the mirror. However, its signature in the HWS map is rather faint, as it becomes visible only after removing piston and tilt.

The problem encountered yesterday and this morning with the ALS lock appears to have come from my misinterpretation of one of the ALS locking thresholds, combined with a slow drift in the green power reflected at the end buildings.

We adjusted the values in the ALS_WEB ini file accordingly, and the lock became stable again.