Last night, we again became unable to damp the roll modes (see previous experience in alog 17378) with the usual damping settings. After some random experiments, we became able to damp it with the usal damping settings for some reason. We have no idea why the mode occasionally behave in this way. Note that we use AS_WFS_A for damping them.
(The roll modes)
After the recycling gain study (alog 17645) and ASC study (alog 17646), we fully locked the interferometer with DC readout and 10 W. We immediately noticed that the DARM spectrum was extremely noisy which turned to be high roll modes saturating the OMC DC PDs at the ADC. Looking at the frequency of the peak in the DARM spectrum, we could idenfity the mode -- it was at 13.8 Hz which is the one from ITMX. The peak height was as high as 10-12 m/sqrtHz in the DARM spectrum with 0.1 Hz BW. We went back to ASQ in order to address the issue. The PSL power remained at 10 W. Evan tried different phases (e.g. +-60 deg) and even the negative sign in the damping gain, but none of them seemed to work. This is exactly the same situation as the one previously reported (alog 17378).
(Damping experiments)
- ITMX
Since I knew that it was mostly from ITMX, I first disabled the damping on ETMY in order to make the experiment straightforward. Then I narrowed the pass band on ITMX, which is nominally 1 Hz wide with a center frequency of 13.9 Hz, to 100 mHz passband with a center frequency of 13.8 Hz. They are 4th order butterworth filters. According to foton, this change causes an extra phase rotation of 30 deg, which I did not try to correct as this seemed small enough. Engaging the narrower butterworth, I was able to damp the mode with the same positive gain. This brought the peak height in the DARM spectrum to as low as 10-14 m/sqrtHz.
- ETMY
I then moved onto ETMY to propagate the same modification. ETMY also had a 1 Hz bandpass 4th order butterworth with a center frequency of 13.9 Hz. I tried a 100 mHz passband with a center frequency of 14 Hz. This again caused a 30 deg phase shift, but I neglected it. After engaging the narrower bandpass on both ITMX and ETMY, however the modes slowly started growing up. I tried different phases and negative damping gain on ETMY, but none of them helped. I also tried several configurations -- e.g. disabling ETMY and keeping ITMX, disabling ITMX and running ETMY, and etc. But I did not succeed in damping the modes. Moreover they kept growing on a time scale of a couple minutes.
- Ending up with the same old configuraion
After all, I switched the bandpss filters back to the 1 Hz passband ones to see if I can damp them. Yes, I was able to damp them. The decay time was approximately on a time scale of a couple minutes. No extra phases or sign flips were needed. Unsatisfactory (✖╭╮✖)
not a good idea to use the error signals for damping; using AS WFS requires that the roll to angle TF not have phase shifts at the ~30 deg level.
But the TF includes not only the roll -> angle mechanical TF, but also
(roll -> DC readout -> DARM OLG -> SUS actuators L2A -> WFS) +
(roll -> DC readout -> DARM OLG -> SUS actuators L2L -> WFS L2A)
so its complicated.
But also the seemingly straightforward way of using DARM_OUT that I support has issues since the roll -> DC readout TF changes with beam positioning. But if the spot positions are controlled, this way ought to be best as long as you always have a roll RG in the DARM loop.