Summary

Today I wanted to see if increasing the bandwidth of the SRC ASC loops could make the 90 Hz SRCL coupling into DARM go away or become more stationary. I cranked up the bandwidth of the AS_C loop, but saw no improvement. We should try cranking further, or else try cranking the bandwidth on the AS36I loop.

Details

I was able to bring the interferometer to dc readout at 10 W with a recycling gain of 39 W/W. To get there, I had to turn on the ITM oplev damping again to avoid 0.4 Hz instability during power-up (and sometimes during the last steps of the CARM reduction sequence). I was able to engage the MICH, SRCL, and dETM yaw boosts without issue.

Then I tried increasing the bandwidth of the AS_C pointing loops. I was able to increase the gain by a factor of 30 just fine (2 ct/ct to 60 ct/ct, pitch and yaw), which gave a ugf of about 250 mHz as measured by step response. This means that the original bandwidth was less than 10 mHz, since the loop is 1/f below the suspension resonances. Correspondingly, the low-frequency angular motion seen by AS_C is suppressed by a factor of 30 or so, and the rms angular motion seen by AS_C is reduced by a factor of 4 in pitch and some small factor in yaw. Based on the attached spectra, it seems like we would have to push the bandwidth up to several hertz in order to see any further improvement in the rms. Anyway, after doing this I did not see any improvement in the stationarity of the SRCL coupling aroung 90 Hz.

I attempted to repeat this exercise with the AS36I loops, since Gabriele had previously found that the pitch loop was the dominant signal modulating the SRCL/DARM coupling once dETM yaw was adequately suppressed. However, I could not increase the loop gains by more than a factor of 4 before they went unstable (this was the case even when I engaged the same HSTS compensation filter as used for the AS_C loops). This factor of 4 was not enough to give a noticeable improvement in the spectra of AS36I pitch and yaw, since these loops are also slow (<100 mHz, just based on watching the loops' responses to transients).