Virgo Logbook

AdV-COM (AdV commissioning (1st part) )

casanueva, nardecchia, mwas, bersanetti - 23:19 Tuesday 04 March 2025 (66303)

DAS tuning test

The reason of this shift is reported in #66243. The procedure followed during the shift is reported in this git issue.

At the beginning of the shift, after the unlock, Diego called Michal and Romain beacuse DET_MAIN node got stalled (66300).

At 16.05 UTC, the interferometer locked at LN2. At 17.35 UTC, the standard SSFS noise injection was performed.

After that, at 18.12 UTC, the DAS outer ring powers were adjusted differentially. In particular, the WI outer ring power was increased by 5%, while the NI outer ring power was decreased by 5%.

Reference	CH [W]	INNER DAS [W]	OUTER DAS [W]
(West) on the ITF	0.046	0.150	1.59 → 1.59(1+5/100)= 1.68*
(West) on the pick-off	0.284	0.025	0.26 → 0.27
(North) on the ITF	0.104	0.340	3.68 → 3.68(1-5/100)=3.5*
(North) on the pick-off	0.635	0.055	0.61 → 0.57

After one hour we started to see a growing oscillation at low frequency, in particular we checked that it was the usual interaction between PR and BS TX. SO we tried to move the crossing point of the two error signals. WE changed the weight from 0.67 to 0.82 and the oscillation got damped. After 2 hours we repeated the SSFS noise injections and Figure 1 shows the change on the coupling of the SSFS to DARM between before and after the DAS step.

Then we went to LN3 and we stayed there for 1 hour, and repeated the SSFS noise injection. After this injection Ilaria put back the initial DAS values (22.15 - 22.18 UTC).

Images attached to this report

Comments to this report:

mpinto - 10:31 Wednesday 05 March 2025 (66306)

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.

Images attached to this comment

casanueva - 11:35 Thursday 06 March 2025 (66312)

I have compared the measurement in LN3 during the shift (with the DAS step) and the one performed in LN3 on saturday, with the usual calibration. UNdortunately the measurement between 300Hz and 1kHz of saturday doesn't seem to have very good coherence and there is a clearly visible step, so I wouldn't trust it too much. ANd there were no injections performed on Monday,

I think it is already interesting since it shows that the coupling at high frequency hasn't changed at all and that at low frequencies it has slightly increased with the DAS step.

Images attached to this comment

66312_1741257182_250304_DARMprojection_LN3COmparison.png

mwas - 8:37 Monday 10 March 2025 (66333)

I have used the SSFS noise injection to fit the transfer function between 70Hz and 300Hz and use that for projection in the noise budget on the 10 minutes of data preceeding the SSFS noise injection.

Figure 1 shows the noise budget in LN3 after the DAS step, the 1/f^{2/3} noise is still explained by the usual level of noise, with a resonable match between the projection and the measured noise.

Figure 2 is the noise budget in LN3 the following morning, still inside the same lock, but after the DAS step was reverted and had ~12h to thermalize back to the normal steady state. The only difference is that the SSFS coupling has reduced in between, but that is not extremely informative as the BS TY loop may not have completely converged in the shorter LN3 state with the DAS step shown on figure 1.

Figure 3 shows LN2 data before the DAS step, compared to data taken in LN2 at the beginning of February the SSFS coupling was larger, but still an increase by a factor ~1.6 of the 1/f^{2/3} noise is needed to explain the sensitivity curve (in addition to the change in DARM optical gain due to the SR misalignment).

Figure 4 shows LN2 data after the DAS step. The frequency noise coupling had improved by about a factor 2, but there is no changes on the level of 1/f^{2/3} noise needed to explain the measured spectrum.

In conclusion the 1/f^{2/3} did not change with the DAS step neither in LN2 nor in LN3. The fact that the level changes by a factor ~1.6 was true for both DAS settings.

Images attached to this comment