Keita, TVo, Hang

In the ASC related lockloss reported in Keita's morning alog (LHO:45969), we saw an oscillation grew first in DH Y at 0.8 Hz. See the first attached plot. After that, we were able to went to the nominal low noise state and stay there for > 4 hrs. However, locking at the ASC error points we saw an significant peak at 0.5 Hz in the ARM ASC loops. Both the 0.8 Hz and 0.5 Hz instabilities were due to my original boost filter which tried to partially invert the 10 W plant and thus put more gain at 0.3 Hz to suppress the microseismic motion. However, this boost seemed to make the loop too close to the stability margin and could not tolerate small changes in the ASC OLTFs due to the change in TCS preloading and the CHARD mysterious phase delay at <0.4 Hz for the REFL signal path.

Thus we gave up the original boost filters in both CH Y and DH Y, and simply replaced them with two low Q low height res g's. In the forth plot we showed the comparison of the old boost (blue) and the new boost (red). We gave up 10 dB gain at 0.3 Hz but the new filter made the loops much more robust. In the third plot we showed the ASC error signals after changing to the less aggressive boost.

With this filter modification, we were able to reduce the CHARD YAW loop gain by a factor of 2 (now DC gain 0.75) and engage a more aggressive LP filter than the one we used last night. A comparison between the LP filter we currently use (FM3 in CH Y) and the one we used in the past (FM2 in CH Y) was shown in the fifth attached plot. If we care only the noise above 20 Hz the current filters were fine but if we want to reduce the noise in the 10 - 20 Hz band we had to further shape the CH Y loop.

In the last plot we show the current CH Y loop's model at 10 W. We only need to consider a single power as we have the radiation pressure compensation.

With the modification in the YAW loops we were also able to reduce the DH P gain down. We first tried to decreased it all the way to the original value of -30 (3 Hz UGF) and engage the original, aggressive LP filter ELP10b (FM1 in DH P) . However, after 10 mins or so we saw an oscillation grew in DH P at ~0.5 Hz. Similar to our solution to the yaw loops we removed the DH P boost filter for now and increased the DC gain to -40. This would leave the upper UGF a low phase margin (~20 deg) with the original aggressive LP filter. Nonetheless this seemed fine for now as the loops are stable.