Continuing in the investigation of the PRCL coupling and the attempts at applying a PRCL feedforward...

Since we have performed several injections into PRCL over the last few weeks, we can track the PRCL coupling. I made this plot comparing the DARM/PRCL transfer function from three different times: an injection performed before testing a new feedforward on Sept 16, an injection from the noise budget, and an injection done while updating the sensing matrix for SRCL and PRCL after rephasing POP9, Sept 23. The PRCL coupling is relatively stable from all of these tests.

However, the feedforward we have tried has not been working, which has been very confusing. I think I have discovered the reason why: in addition to the measure of the DARM/PRCL transfer function, there is a measure of the "preshaping", which includes the high pass filter we apply to the feedforward. Looking at my code, I found that I had been using the SRCL preshaping to calculate the PRCL coupling, which uses a different high pass filter (comparison of the three high pass filters). I should have been using the MICH preshaping measurement instead, since MICH and PRCL use the same high pass, and everything downstream from that point is the same. Sorry everyone for that mistake.

So, when using the proper preshaping, a PRCL feedforward fit should work. Since we already have this data taken, I ran a quick fit of the feedforward, and compared it to previous fits. Camilla's fit from July 11 was successful, and I can see that the low frequency gain and phase is very similar to the new fit. I believe one reason her fit no longer works is the coupling shape from up to about 70 Hz has changed. Meanwhile, the incorrect fits I tried recently are very different in magnitude and phase (shown in green as "old incorrect fit"), explaining why they failed.

This new fit requires about a factor of 5 more gain than Camilla's fit above 200 Hz, which I think should be ok. The PRCL injections see a large bandstop around 250 Hz, so we don't measure the coupling there. If we can turn off these bandstops we can probably get a much better measurement up to 1000 Hz, which will help this shaping.

Overall, the fit should subtract up to 10 times the noise from 10-30 Hz and on average 3 times the noise up to 100 Hz. Based on our recent noise budget projections, which show that PRCL is directly limiting DARM noise up to 30 Hz, this will have a positive effect on the low frequency sensitivity.

The new filter is placed in the PRCL FF bank in FM6, labeled as "new0926". Since we are in observing, I didn't load the model, just saved the filter. To test, load the filter bank, and engage with a gain of 1, along with the  FM10 highpass.

I recommend that this filter is tested along with the commissioning work for Thursday. It would be useful to have an injection before the filter is applied and after the filter is applied, each with enough averages to provide a good fit. If this feedforward filter doesn't work, we can refit. If it does, we can look at fitting iteratively for further subtraction. Please try to get about 30 averages for both measurements, and if possible, boost the injection strength above 50 Hz to get a bit better coherence.

If the feedforward doesn't work, please also additionally run a PRCL preshaping measurement. This means running with the PRCL FF input set OFF, with the high pass filter ON and a gain of 1. I think we have a template for this in the LSC feedforward folder. If not, the preshaping template of the MICH preshaping ("MICHFF") can be repurposed with the appropriate channels. This will help avoid the confusing mistake I previously made.