Hirose, Tanaka
DRMI LSC oscillated at 70-80 Hz whenever POP90 and AS34 power decreased.
In pararell of Hirose-san's work in klog37320, I looked into the trend of build up sideband powers and the oplev signals. Fig.1 shows the time series of the build up sideband power (K1:LSC-POP_PDA2_R90_I_ERR_DQ, K1:LSC-AS_PDA2_R90_I_ERR_DQ), and mirror oplevs' signals (K1:VIS-{IMMT2,PRM,BS,SRM}_TM_WIT_{L,P,Y}_DQ). The drop rate of build up power seems to be about 0.1 Hz. It seems to be close to the Type B IM YAW resonant frequency. According to the time series, ITMY, SRM and BS seems to oscillate at around 0.1 Hz
I measured the coherences between POP90/AS34 power fluctuations and mirror alignment fluctuations. Thanks to the stable lock of DRMI, (almost 3 hours), we could measure it with long time. Fig. 2 shows the result. There seems to be large coherences around 0.1 Hz between AS34 and ITMY L, Y, SRM Y, PRM P, Y, IMMT2 P. Basically, SRM, PRM, IMMT2 are controlled by ADS. So it maybe fine. On the other hands, ITMY seems to has larger coherence than ITMX. So we need suppress the ITMY Yaw motion.
There seems to be some concerns.
- coherence around 1-2 Hz between PR3 alignments and AS34. I'm not sure of the reason why PR3 alignment has coherence with only AS34. One of the possiblities is the SRCL alignment is not good rather than the PRCL alignment.
- the coherence around 0.2-0.3 between SR3 and AS34.
- coherence peak around 0.45 Hz between AS34 and PR mirrors' motions and BS L (Type Bp resonant?)
- broad coherence bump from 7 Hz to 70 Hz between PRM L and AS34 (PRCL control?)
Anyway, we need to modify the suspensions' local controls
