We have been working for the last three days, trying to recover a more or less stable lock acquisition, but it is clear that we did not manage. Not only for the criticality of the OMC but also in general of the ITF. We had roblems with the WE LCs opening plenty of times, we have some problems with the PSTAB opening, we had some unlocks when going from DF to LN1, also we had some problems going from 0.1 to DF because the PRM was a bit misaligned and the automation prevent it from reaching DF, in this case it is enough to align it by hand (0.1-0.2 urad), Fast unlocks...
So basically the only thing we recommend to try to make it more robust is: stay some time on LN1 to allow the ITF to align completely and stay some time on OMC1 LOCKED before trying to lock the OMC2. What I would not recommend is to stay for a long time on DF, since all the alignment loops are opened, and that could misalign more than anything.
We wanted also to make a recap of the modifications we made on the code. They are not clean, but we expect to undo them when the new photodiode will be installed.
-LSC_Acl -> lines 130-141. These lines were added by Michal to ease the lowering of DARM offset. The target is that when we are using this new way of applying the offset/gain, the gain is calculated as a function of the offset, so the only thing we need to do is to lower the offset anf the darm gain is adapted as a consequence.
-So basically there are two regimes: standard regime where we have a DARM GAIN/SET and the new regime where we have a B1_DC_SET/GAIN. In the second regime, we only need to change B1_DC_SET since the gain will follow. Also notice that in this secod regime, we put the DARM_SET to zero to do not add 2 darm offsets.
-The change of regime is done at the end of the OMC1 lock (LOCKED_OMC1), and it is done without a ramp after recommendation of Michal. It is done in lines 3152-3158 in Metatron.
-Just after this change of regime, we decrease the offset until 4.5e-3, which is the lowest offset that kinf od works reliably. HOwever, notice that in the new regime, the offset is applied int erms of power on B1s2, so 4.5e-3 means 0.225. This darm offset reduction is done in lines 3178-3181. It is done in 70s, since we need to decrease the offset by a lot. While commissioning the lock acquisition we realized that the UGF was still too low, not well compensated, so at the same time we increase DARM_GAIN.
-Then after the OMC2 is locked, we decrease further the offset, until having 1.8mW in the B1 DC photodiode. This is done in lines 3352-3354. In this step we also decease slightly DARM_GAIN to reach normal UGFs.
-Finally at LN2 we adjust further the DARM_GAIN.