Jeff K., TVo, Hang

Before the computer crash last night we tried to went to nominal low noise last night. However, we could not go through the low noise ASC state as we kept encountering some CHARD YAW oscillations at 0.4-0.5 Hz.

Things did:

    1). As the TCS preloading changed from 50 W to 30 W, our SRC ASC locking point (using AS72) needed to be adjust. Thus we went to 20 W and tuned the SRM alignment by hand to optimize the POP buildups and then set the SRM locking point there by phasing all the demodulated 72 MHz signal at DC into the I phase and thus left the Q phase signal which we used as error signal 0. This offset set at 20 W seemed to be working fine at 2 W as well. It was a bit strange that we could make the POP18 signal significantly higher and quieter by moving SRM, and it seemed to indicate the cross-coupling from SRM to BS. Thus we might need to optimize the MICH ASC input matrix to try to better decoupling SRC.

    2). We tried to went through the low noise ASC state by hand and had a lot of troubles with CHARD YAW. First we could not reduce its gain to a UGF of 3 Hz as set previously. If we reduce the gain by this much an oscillation grew at ~ 0.4 - 0.5 Hz. Such an oscillation could be suppressed if we instead made the CH Y UGF 3.7 Hz by putting the DC gain to 1.25.

    3). As a result of higher UGF, we could not engage CH Y FM2 (ELP10) due to lack of phase margin. Instead, we shifted the cutoff freq up to 18 Hz (FM3 in CH Y, ELP18) and reduced the amount of attenuation. This of corse degraded our ASC noise but the loop's margin was already ~ 20 deg with this less aggressive filter.

    4). We tried to check the CH Y blending by dithering a CH Y line @ 8.125 Hz and then matching the refl path and the QPD path gain. Our setting of CH_Y_A (DC refl sensor path) gain 1.4 and CH_Y_B (AC TR QPD path) gain of 0.77 seemed to match the two paths well at 8.125 Hz. However, as indicated by the CH Y blending study (LHO:45922), the refl senors' response seemed to behave well at >~ 8 Hz. At lower frequency, the CH_Y OLTF measured using only refl sensors experienced mysterious phase delay relative to the OLTF measured with the QPDs. This might be due to the cross-couplings of many dofs (CH, PRC1/2, INP1...) in the refl WFS. Thus our way of matching the responses at 8 Hz might not guarantee the good matching between REFL WFS and TR QPD at 0.4 Hz. In fact, by increasing the ratio between the CH_Y_A to CH_Y_B we could shift the CH_Y oscillation freq from 0.4 Hz to 0.5 Hz, indicating the instability was likely to be related to the blending.

    5). While higher BW CH_Y gain suppressed the oscillation initially, we could see this 0.4-0.5 Hz thing come and go at ~ 10 min time scale, and eventually we lost lock at ~ hour timescale. Thus our current CH Y loop was very close to the stability margin...