This evening we locked the OMC in dark fringe.

ITF locked in CARM_NULL_1F at ~20h05 utc.

Enable AATuning servo at 20h20m53 UTC.

SDB1 high band width control enabled at 20h21m56 UTC.

Putting DARM offset 0.20 at 20h31m20 utc.

OMC scan at 20h34 utc:

Getting 20 uW on order 4, 13 uW on order 3, 8 uW on order 2, 10 uW On order 1, 5 uW on TEM00.

DARM offset increased at 0.3 at 20h43m25 utc > we found 3 uW on TEM00.

DARM offset at 0.5 at 20h49m40 utc > less than 1 uW on TEM00.

DARM offset at 0.0 at 20h54 utc > 22 uW on TEM00.

We keep a DARM offset of 0.0 to lock the OMC. We update the parameter darm_offset_rf = 0.0 in the ITF_LOCK.ini file.

OMC locked at 21h18 utc (see Fig.1). ITF unlocked at 21h18m29 utc (see Fig.2). This seemed to be caused by an oscillation of the DARM loop, presumably because the DARM gain was left too high in the ITF_LOCK.ini file. Fabio and Camilla reduced it from 0.07 to 0.04 (parameter darm_gain).

ITF locked again in CARM_NULL at 21h55 utc.

Enable AATuning servo at  21h56m32 UTC.

Enable high bandwidth control of SDB1_LC at 22h04m40 UTC

OMC locked at 22h10 utc (Fig.3). ITF unlocked at 22h10m37 utc. Again the same kind of unlock with an oscillation in LSC_DARM at about 20 Hz (Fig.4 and Fig.5).

Conclusions:

• We can lock the OMC in dark fringe with a DARM offset of 0.0.
• Despite the fact that the hand-off from RF to DC readout was commented, we systematically unlock while reaching the state LOCKED_OMC_DARM_B1_PD3. Having adjusted the DARM gain did not fix the problem.