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AdV-TCS (Point Absorbers Mitigation)
spinicelli, mantovani, bersanetti, melo, ciardelli, nardecchia - 23:06 Tuesday 16 September 2025 (67723) Print this report
IPATSiA: measurements in LN3 with SR partially aligned

Today we carried on another IPATSiA shift. 
In the morning during the maintenance, we installed a new, more powerfull, chiller in order to solve the stability problem on the CO2 power we met last shifts. We ended the installation at 9:49UTC and switched ON the chiller and the laser, preparing the setup for the afternoon. With the new setup, the power is much more stable, with the temperature which remains into a variation of ~0.1⁰. Nevertheless, some small adjustments to the chiller parameters were still needed to avoid T drift which eventually caused some jumps on the power (see fig. 1).

We then started the study after reaching the LN3 state and further realigning the SR up to a DCP ~300Hz. In this way, we aimed at work in the cleanest possible state, with all the servo loop working on. Moreover, we removed the offset on SR_TX. 

The CO2 power stabilized at 236mW on the pickoff (with ~47mW reaching the mirror). 

At 14.35UTC  we took ~20' of  clean data in LN3 @300Hz DCP

Compared to the last position of the previous shift , I came back to the central position and moved down vertically by the same amount (up on the thermocamera, see fig. 2). 

At 15:00UTC we started shining the CO2-PA in the POSITION 1

The effect on the absorpion and BNS it's this time quite huge (see fig. 3). Unfortunately, the ITF unlocked at 15:34:52UTC, probably because the BS_TY loop diverged trying to compensate for the CMRF variation due to IPATSiA (tbc).

For this reason, in the next lock we decided to disable the servo immediately before the projection of the CO2.

At 16:45UTC: LN3 with SR@300Hz (directly) → 15’ clean data

At 17:25UTC, we started shining again, in the same POSITION 1

The effect of this point is clearly visible in Hrec (see fig. 4)

At 18:00UTC we stopped shining and moved to a new point, moving up (donw on the camera) from the central point, always by the same amount.

At 18:11:07UTC we started shining the CO2-PA in the POSITION 2. However, the ITF eventually unlocked few minutes later (18:16UTC).

At the next relock, I took some additional data in the same position, starting at 19:03 for 10' (this time with the BS servo ON again). 

At 19.13 I blocked the laser, took the ITF in the same configuration for 10' while unmounting IPATSiA to collect other clead data (at 19.18 the new chiller has been switched OFF).

I the put back the usual setting for both DoF of the SR and the BS_TY servo.

Further analysis will follow in the next days. 

 

 

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Comments to this report:
mwas - 8:21 Wednesday 17 September 2025 (67727) Print this report

Figure 1. The main effect when IPATSiA is shining in this configuration is a change in alignment. SDB1 turns in TY by 10urad, which normaly is the order of magnitude needed to add ~50% of misalignment losses in the OMC. The power in the arms decreases by 13%, which if nothing else was changing would result in a 6% reduction in DARM optical gain (it should scale as the square root of the power in the arms when the B1 power is kept constant by the DARM loop). However the optical gain decreases by 25%, which would be consistent with losses in the OMC increasing by ~35%. The power on B1p also increases by a factor 2, and the 6MHz and 56MHz sidebands on B1p become misbalanced.

Figure 2. Zooming in on the transition, the decrease in arm power has a significantly shorter time scale than the SDB1 alignment. So that change is not a consequence of the SDB1/OMC misalignment. The change in optical gain as measured by Hrec has a time scale that is in between the two. Which would indicate that there is some other degree of freedom appart from OMC alignment that is changing working point.

Figure 3. Last week IPATSiA did not have an impact on SDB1/OMC alignment, but it did have impact on the offset on B1p 56MHz I, the RF error signal for DARM. Those measurements were done in LN2 and not in LN3 with DCP at 300Hz. 

Figure 4. The image on B1p when the power increases looks like a TEM00 mode, not an order 1 misalignment mode. So it is more likely that there is an offset in a longitudinal signal that changes (for example in MICH).

An important question is if it is the effect on the different position of IPATSiA on the mirror, or if it is effect of the LN3 with 300Hz DCP configuration that we don't know well. 

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