Task of the shift was to continue the Chrocc power tuning while locked on CITF.

At the beginning of the shift the ITF was locked by the operator, after the usual first pre-alignment, he managed to lock the CITF and, for a quick test, it was possible to go on with the subsequent steps of the acquisition up to the step after 480 mW of arms power, then it unlocked during the jump to CARM null 3f. No phase or gain tuning was performed, but some fine tuning will be eventually needed.

At 7.45 UTC we started the planned activity: with the last Chrocc power and DAS settings set by Diana we locked the CITF in 1f signals by putting a positive (+) CARM offset of +3000, in order to perform an arms scan in this working point.

After a minor adjustment of some LSC gain and MICH/SRCL demodulation phase, we started the scan:

8.06.35 UTC: 1st Scan CARM set +38.000 Hz with 60 sec ramp: the scan worked, but rigth after reaching the final offset value the CITF unlocked. So for the subsequent scans, we decided to use a lower offset value, i.e. +35000 Hz.

8.14.00 UTC: 2nd scan +35.000 Hz 60 sec ramp.
8.16.57 UTC: CITF unlocked on the way back to +3000 Hz.

8.47.05 UTC: 3rd scan +35.000 Hz 60 sec ramp. This time the CITF unlocked during the ramp to 35.000 Hz offset.

results of the first two scans are reported respectively in Fig.s 1 and 2.

N.B. in the last lock, while we were in the scan condition (positive offsets) we experienced a general improvement of the figures of merit that we monitor during the lock, e.g. B4 DC reached 2.61 mW; the 12 MHz SB 0.138 mW.
Also the phase cameras on B4 showed a more circular and symmetric shape, while the ones on B1p got quite worse (see Fig.s 3 and 4)

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After these scan tests, we relocked the CITF in the usual working conditions (negative CARM Offset -3000 Hz), in order to start the Chrocc power tuning activity.

Phase cameras image at this instance (9.00 UTC is reported in Fig.5)

At 9.04.16 UTC: Diana made the first movement setting the Chrocc Voltage from 0.43 to 0.36 V.

9.19.00 UTC: we noticed a general worsening trend, as the aforementioned figures of merit were decreasing and the fringe was getting brigther: B4_DC from 2.61 to 2.54 mW; B4_12 to 0.135 from 0.138 mW (see again fig.3). In the meantime, B1p Phase cameras started to get worse a lot (especially the two e UPP and LWR 6 MHz sidebands, see Fig. 6).

9.57.26 UTC: CITF unlock.

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10.53.20 UTC: lock CITF -> 12 MHz 0.09 mW; B4_DC 1.95; B4 phase cameras had a very good circular shape, while B1p cameras kept getting worse (see Fig. 7). During this lock at first the BS was fine aligned by hand to improve the figures of merit, but we immediately noticed that the BS AA didn't engage as the QPDs shutters were closed, so we opened them by hand. The situation sligthly improved.

then at around 12.05 the CITF seemed to almost stabilize with constant figures of merit value, so we started to perform an other set of scans.

However, this last set of scan didn't succed, as everytime we unlocked as we were passing by the 56MHz LWR HOM2 mode, i.e. to the corresponding value of around 21500 Hz of CARM offset. We tried different ramp values, or adjusting MICH gain (as its correction sometimes saturated), but we never went past that value.

For reference the GPS of the scan attempts are the following:

12.05.45 CARM set 35.000 in 60 sec.
12.06.22 unlock CITF

12.12.16 UTC: scan 35000 Hz offset, 300 sec ramp;
12.15.10 unlock CITF at 21500 Hz offset;

12.18.00 UTC: scan 35000 Hz offset, 300 sec ramp;
12.20.57 unlock CITF at 21500 Hz offset;

12.27.59 UTC: scan 35000 Hz offset, 300 sec ramp;
12.30.54 unlock CITF at 21500 Hz offset; MICH correction saturated;

12.36.49 UTC: scan 35000 Hz offset, 300 sec ramp;  (this time with a lower MICH gain)
12.39.45 unlock CITF at 21500 Hz offset; This time MICH correction didn't saturate, but we unlocked anyway.

13.01.12 UTC: scan 35 kHz in 120 sec ramp.

13.02.27 UTC: unlock CITF at 21500 Hz offset;

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After these failed attempts, we relocked the CITF with the usual negative offset (-3000 Hz), in order to explore a new Chrocc power value.

At 13.39 UTC Diana started a Hartman measurement to take a reference.

At 13.47.53 UTC Chrocc Voltage has been set to a higher value: 0.4 V.

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In the following table are reported the different Chrocc settings performed in the past shifts, in terms of Power (W), Voltage (V) and Dioptres, together with the two equations to obtain the conversion from Voltage to Power and to Power to Dioptres.

Chrocc Power:

Dioptres:

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At the end of the shift we performed a scan while locked on beating signals, to evaluate the mismatch percentage. GPS of the scan are:

For the TEM 00 mode: 15.49.10 UTC

For the TEM 02 mode: 15.53.27 UTC.

Results are reported in Fig. 8., where a 3 % of mismatch has been found on North Arm.

We left the ITF locked on the IR, with the Chrocc left to the last voltage setting (i.e. 0.4 V).