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AdV-COM (AdV commissioning (1st part) )
casanueva, vardaro, mwas - 10:05 Friday 12 July 2019 (46335) Print this report
Recap of the actions done / status of lock acquisition

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.

Comments to this report:
mwas, mantovani - 8:40 Monday 15 July 2019 (46369) Print this report

On Friday (July 12), to improve the stability of the lock acquistiion during OMC1 and OMC2 locks, we have done additional changes:

1) The transition between DARM SET/GAIN to B1 DC SET/GAIN is now done in a smooth way. This was done by adding B1s2_DC as one of the input choice to LSC_DARM_INPUT, and use it for the initial transfer from the RF error signal to the DC error signal. And reserve B1_DC_INPUT as the signal that has the offset from B1_DC_SET and the automatic gain adjustment. Thir resolve the issue with unlocks in ~50% of cases of the transition that was done instatenously before.

2) We increased the dark fringe offset from 0.225mW to 0.25mW on B1s2. In principle this is to much, as it corresponds to 3mW on the B1 photodiode and the fast shutter on SDB1 closes for 3mW on B1. But because 30% of the light on B1s2 is actually due to HOM and RF sidebands and not carrier TEM00. It actually correspond to ~2.1mW on B1 PD2 DC. This is also made possible as now the transition to remove B1s2 from the DARM error signal is done with a smooth ramp that decrease the dark fringe offset at the same time

3) The state of OMC1 locked with 0.25mW dark fringe offset is not very stable. At 0.225mW we have seen several times that gain of the DARM loop drops (and oscillates at 25Hz), but looking at the RF signal, it corresponded in the one case we looked at a zero crossing of the RF signal, so probably the B1s2 signal is to poluted by junk light and has a fluctuating real offset in meters for DARM

However, I wouldn't remove all the changes once we go back to 2 B1 photodiodes, the new lock acquisition is much more smooth. But it does need cleanup.

 

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